Chapter
The first listing of chapters keeps you within the one large
file. The second listing of chapters (see below) links you to
individual chapters, so that if you wish to print, you
print just that one chapter, not the whole book.
How does one illustrate the descriptions and ideas of histology? The solution chosen here is to develop links to freely accessible (but copyrighted) Powerpoint diagrams and lists, most of which have now been done (June, 2001). Powerpoints are at the mercy of browser settings and projector light intensity, so that I am now making the backgrounds as black as possible. You can adjust via 'Format', 'Custom Background', 'Down arrow', 'Custom Background', your choice, 'Apply to the one slide/Apply to all slides', 'OK' (Then, reverse arrow, if you do not like the result)
Some of the Powerpoints are 'busy'. For projection, the content would be spread over two or more slides. They are in the condensed form so that they can be printed out six-to-a-page, then enlarged 120% for handouts.
For those seeking images of actual sections, here are links to some
Histology Websites with illustrations :Bergman, Afifi, & Heidger: U Iowa --- Heidger & U Iowa's histology slides - - - JayDoc HistoWeb - - - Vanderbilt Histology Lessons
Dr Alex Imholtz has more useful sites linked from his own site at Imholtz links
Histology is important for the medical student: it introduces her or him to
the broad range of cell types and molecules that let man live, and at which
therapy is aimed.
. Beyond this meet-the-family role, histology reveals time and again how
function is reflected in microscopically visible structural patterns of
organization: patterns that really mean something for detecting and curing
man's ailments and injuries.
. With many sources available for the actual visual images
of microscopic anatomy, I have used the space here to show the patterns of
knowledge as elements, structure, and sequence. For curricula where formal
lectures have been cut back, the layout preserves for the independent
student what the lecture offers by way of organization. (It does not yet offer
much of the stories, analogies, props, and jest, customarily used to bring the
subject alive.)
. The following storyline is sometimes helpful, and can be recognized later:
context - structures - tasks - means - mechanisms - molecules - malfunctions
The answers comprise a large body of knowledge graced in several ways. First,
histology is colourful. Secondly, almost everything seen is actually there;
which is not to say that what is not seen is absent. Third, one handles and
views actual slides - the source material for most of histology, not just
someone else's selected images. Fourth, the structures can be interpreted
as parts in developmental and functional sequences, and be fitted together
by satisfying accounts, for example, of how cells defend the body. So much is now known of the roles of cells and
structures that histology is both descriptive microanatomy, and an
introduction to function for the whole body. Powerpoint
.
Dead Living
(a)Section - a thin slice of Such preparations may be out of the
tissue or organ - on a glass slide body in a tissue culture system, or
or metal grid. living within the body but in an
(b)Smear on a glass slide - observable situation, e.g., a
suitable for suspensions, e.g., transparent chamber inserted into
blood, urine, mucus, cyst fluid, the ear or skin. The first need is
bone marrow, etc. to keep the preparation alive. This
(c)Spread sheet of tissue seriously limits the facilities for
stretched thin, e.g., areolar observation. For example, staining
connective tissue. is usually impracticable. Thus,
(d)Teased apart fibrous phase-contrast or interference-contrast
elements, e.g., muscle. microscopy has to be used in order
to overcome the poor contrast
between natural structures.
2 Steps needed to make and study a histological section
The only way to improve resolving power is to reduce substantially the
wavelength of the light. This is achieved by the electromagnetic beam of the
electron microscope. The beam is focused through the object suspended on its
metal grid, and is magnified before striking a fluorescent screen to be
transformed into a visible image (Chapter 30.K). [Caution!
the link takes you there , but 'back' brings you only to the start of the last
Chapter that you linked to.]
The resolutions so far achieved in biology with transmission electron microscopy
(TEM/EM) are of the order of 1 nm at a magnification of X 2 000 000. The other
forms of microscopy - phase-contrast, interference, fluorescence, confocal
scanning, atomic-force (and X-ray diffraction) - will be discussed in Chapter
30. in relation to the problems for which they are suited.
2 Microscopy for the student (may not apply in toto to the reader's use)
(c) Before plugging in the microscope, check to feel how the switch and
rheostat work. Plug in, switch on, and adjust the rheostat up one third of its
range to start.
(d) Otherwise, use artificial light provided by an electric bulb behind a
ground-glass screen to furnish a constant and reliable source. Light intensity
can be increased by bringing the lamp nearer to the mirror, if the lamp is not
built-in.
(e) If the condenser in use (nearly always), use the plane side of the mirror,
if the lamp is not built-in.
(f) Raise the condenser to very near the underside of the
stage, and open the iris diaphragm.
(g) Place a clean, stained slide on the stage and using the coarse and fine
focusing controls bring it into focus with X l0 objective.
(h) With the condenser racking knob focus the light source on the specimen.
This has happened when the specimen itself is in focus and some aspects of the
light source is also seen sharply defined, e.g., the bulb filament or
scratches on the frosted glass screen. If this feature of the light source is
obtrusive, now place the condenser very slightly out of focus. Do not
lower the condenser way out of focus as a means to reduce the light intensity.
(i) The iris diaphragm should now be closed just to the point where glare is
eliminated. Further closure will make the field too dark and reduce resolving
power.
(j) The microscope is now set up for use, but the requirements change for each
objective. Higher power objectives require more light thus the iris will need
to be opened and perhaps the lamp brought nearer to the mirror and the
condenser refocused.
(k) Note that the objective lenses are of different lengths, and they are not
always parfocal. Be careful when switching in a higher power lens that it
does not hit the slide because of its greater length. Clean the lenses only with
lens paper.
(l) If the X 44 objective will not focus to a clear image, check first that
the slide is not upside down on the stage.
(m) Use of the 'oil immersion' lens:
... (i) Select field of interest with the high dry lens (X40); centre
precisely the cell or object in the microscopic field; if X 95 lens is
already mounted go to (v).
... (ii) Raise the objective lens assembly and remove the low power (X4) lens.
... (iii) Place it in the container to be found on the door of the microscope
cabinet from which you have taken the oil immersion (X 95) lens.
... (iv) Screw the oil lens into the now vacant place on the objective nosepiece.
... (v) Place carefully one drop of immersion oil from the small bottle
issued on the area of the slide to be studied.
... (vi) Switch round the objective nosepiece to bring the oil immersion lens
into play.
... (vii) Very carefully lower the objective assembly with the coarse
focusing, until the tip of the oil lens touches the drop of oil. This
operation must be controlled by observing the descending lens from the
side. Do not yet look down through the eyepiece. Once the lens has touched
the oil raise it slightly, but not so far that the drop breaks away.
... (viii) Look in the eyepiece and focus down with the fine
focusing control very slowly and gently until the specimen comes into focus.
If you seem to have gone down a very long way without a clear image, again
check from the side that you have not overshot and the lens is not nearly on
the glass of the coverslip. If this has happened raise the lens slowly, while
looking for a focused image.
... (ix) The oil objective lens needs much light so that the iris diaphragm
may have to be opened.
... (x) As soon as you have finished using the oil lens, raise (remove) and
clean it. (Replace X 4 lens on the nosepiece and the oil lens in its box.)
Clean the slide of oil with lens or tissue paper. Do not allow oil to get on
to the other, dry, lenses.
(n) Other controls the class microscope may have include eyepiece focusing,
filter-holders, centring screws for the condenser, and a rheostat, lens and
light-stop for the light source. Ask for instructions in their use and for
help with any mechanical problem.
(o) Take care of the microscope, carry it only by its arm, protect it from dust by keeping it locked in its case, and do not stand it or boxes of slides near the edge of the bench. If lens paper alone is insufficient to clean a lens, use no solvents but consult a demonstrator. On no account exchange the lenses of your microscope for those of any other microscope.
Spectacle-wearers need not use their glasses in microscopy; if they do, they should beware of damaging their glasses, while trying to compensate for the narrowed field of view.
3 Differences between light and electron microscopy
l Chapters 2 and 3 deal with microscopic details of cells - cytology, for
which EM is better suited than LM. Table l gives some differences between the
two approaches. The detailed morphology revealed by EM may be called fine or
submicroscopic structure/ultrastructure.
2 The direct comparison of LM and EM images of a structure requires that the
magnifications be of the same order. Noting the magnification, on the 'scope
or in the figure legend, allows one to adjust one's expectations of what may
be seen, and should always be done.
3 A growing practice in histology and pathology is to fix and prepare
the tissue by EM standards, imbed in plastic and cut semi-thin (l µm) sections
for staining by modified LM methods. LM then reveals good cellular detail and
fewer artefacts.
4 Two other techniques yield anatomical images - fibre-optic endoscopy and
scanning EM, and are being digested by the anatomical texts. Endoscopy
from its low magnification is marginal to histology, but related in that
endoscopy is used to obtain biopsy specimens for histopathology.
SEM strengthens one's conception of microscopic structures, e.g.,
cilia, renal podocytes, bone under resorption, and effectively counters the
unavoidable impression of structures existing only in two dimensions. (From
hereon, EM is standard transmission electron microscopy.)
Table 2. Some differences between light and electron microscopy.
Light microscopy Electron microscopy
--------------------------------------------------------------------------
Image is presented directly to the Image is in shades of green on
eye. Image keeps the colours given the screen; photographically,
the specimen by staining. only in black and white.
Modest magnification to X 1500; High magnification, up to X 2,000,000
but a wider field of view and easier thus the range of magnification
orientation is greater
Resolving power to 0.25 µm. Resolving power to 1 nm (0.001µm.)
Frozen sections can yield an image Processing of tissue takes a day at
within 20 minutes. least.
Crude techniques of preparation High resolution and magnification
introduce many artefacts. demand good fixation (e.g. by
(Histochemical methods are better.) vascular perfusion), cleanliness
and careful cutting, adding up to
fewer artefacts.
Section thickness (1-30 µm) gives Very thin sections provide no
a little depth for focus for depth of focus, but 3-D information
appreciation of the third dimension. can be had from: (a) thicker sections
Serial sections can be cut, viewed by high-voltage EM; (b) shadowed
and used to build a composite image replicas of fractured surfaces; (c)
or representation. scanning electron microscopy (SEM).
Most materials and structures cannot Heavy metal staining gives a more
be stained and viewed at the same comprehensive picture of membranes,
time; stains are used selectively to granules, filaments, crystals, etc.;
give a partial picture, e.g. a stain but this view is incomplete and even
for mucus counterstained to show visible bodies can be improved by
cell nuclei. varying the technique.
Specimen can be large and Specimen is in vacuo. Its small size
even alive. creates more problems with sampling
and orientation.
Light microscopy Electron microscopy
2 Cells: chemical constitution and fixation
3 Cells: living properties and specialization
1 Properties of cells: (a) general - communication, respiration and
energy storage and release, synthesis, excretion, growth, differentiation,
reproduction; (b) specialized - irritability to stimuli (excitability),
motility, contractility, conductivity, absorption, phagocytosis, secretion.
2 During development from the fertilized oocyte, a great variety of cells
is formed in the mammal, each kind specializing in a certain function, e.g.,
secretion, but many activities, such as energy production, are common
to all cells. The cells of the four primary tissues - epithelial,
connective, nervous and muscular - are divided along lines of
specialized function, e.g., muscle for contractility and excitability.
4 Cell morphology
Powerpoint
1 Cells performing a given function have a characteristic size, form and
fine structure adapted to that task. However it may help at this stage
to think in terms of a composite cell having all the features the various
cells of the body display.
2 The cell is defined as a distinct entity by having a thin skin or
plasmalemma/cell membrane separating off from the outside a soft, viscous,
almost fluid cytoplasm, in which are suspended a number of firmer,
recognizable structures - organelles and inclusions - and one or more nuclei.
The nucleus, likewise, is a mass of material enclosed in nuclear membranes.
5 Cell components
1 Cytoplasm: the so-called soluble phase of the cell, consisting
mostly of water, dissolved solutes, and larger molecules in suspension tending
to link repetitively with covalent bonds giving the cytoplasm a dense, viscous
colloidal sol or gel consistency.
2 Cell or plasma membrane/plasmalemma
3 Nuclear membrane
(a) Is doubled with a 20 to 25 nm perinuclear space between the two
membranes. Each is of unit membrane with a 'trilaminar' nature similar to that
of the plasma membrane.
(b) Many of the apparent interruptions or pores/fenestrations through
the doubled membrane are covered by a very thin 'diaphragm', actually
granular and fibrillar in nature.
(c) Transport between nucleus and cytoplasm takes place at assemblies of
proteins at the pore complexes.
(d) The outer nuclear membrane sometimes leads on out to a membrane system
existing in the cytoplasm.
4 Organelles
Structures 1-8, below, are organelles - actively participating
cytoplasmic bodies of characteristic structure and behaviour.
Points to note: (a) their morphology in light microscopy; (b) morphology
in EM; (c) multiple or single; (d) any special location in the cell; and
(e) functions.
1. Endoplasmic reticulum
(a) Is the name given to the cytoplasmic membrane system of many parallel
membranes and tubules in communication with one another. This system of closed
channels (cisternae) leads towards the Golgi complex.
(b) Two varieties of endoplasmic reticulum (ER) are seen.
....(i) Granular/rough/GER has fine granules, ribosomes of ribonucleo-
... protein (RNP), l5 nm in diameter, in clusters studding the outer surface
... of the parallel membranes.
...(ii) Agranular/smooth lacks the ribosomes and is more tubular. This
... kind is associated with cholesterol metabolism among other things, whereas
... the granular variety is related to protein synthesis, e.g., enzyme
... formation.
2. Ribosome particles: may lie free in the cytoplasm in small clusters
(polyribosomes/polysomes) unrelated to membranes. This is noted
particularly in growing cells.
3. Golgi body/complex/apparatus
(a) This usually takes one area near to the nucleus and often in a specific
place, e.g., supra-nuclear in cuboidal epithelial cells.
(b) It consists of a complex of stacked smooth cisternae-enclosing lamellae,
tubules, and vesicles of various sizes.
(c) It is more disorderly in appearance than the smooth endoplasmic reticulum
and has more closed sacs or vesicles, often with dark staining material within
them.
(d) In LM after special silver staining, the Golgi apparatus may be seen as a
tangled network. With routine haematoxylin staining in certain cells, e.g.,
active osteoblasts, the juxta-nuclear vacuole reveals the site of the
Golgi structure as a pale negative image.
(e) Its tasks are the concentration and preparation for storage of proteins,
and completing the synthesis of complex sugars.
4. Microtubules: with a diameter of 20-25 nm; seen lying in the
cytoplasm only with EM or fluorescent tagging; they give form to the cell,
and are responsible for much intracellular transport and some movements.
5. Filaments: fine threads visible in EM, but may be aggregated into
thicker fibrils visible in LM; in muscle cells they are a very
important contractile element; in all cells they furnish a flexible skeleton
articulated by the cell itself.
6. Mitochondria
(a) The many to be found in almost all cells may or may not have a special
location within the cell.
(b) In LM they can be stained with special methods and appear as coloured rods
or granules. In EM their shape tends to be tubular or spheroid.
(c) They are hollow bodies enclosed in two unit membranes; the inner
membrane projects inwards as plates or tubules called cristae, studded
with small 9 nm wide elementary particles - rounded bodies on stalks.
In the matrix of the intercristal space inside the mitochondrion, granules
may be found.
(d) Mitochondria are very rich in the enzymes associated with the storage and
release of energy, and with respiration, and fatty acid metabolism, etc. They
may also store calcium.
7. Lysosomes
(a) Are round, single-membrane-limited, darkly staining bodies without
cristae and containing another class of enzyme, hydrolytic or
digestive.
(b) The material that they digest may be:
... expended organelles of the cell itself;
... extracellular matter engulfed by phagocytosis and membrane-enclosed in
a phagosome;
... endocytotic vesicles.
Material resistant to digestion may persist as a residual body.
8. Centrosome/diplosome/division body/centriolar complex
(a) A body which lies alongside the nucleus at the cell centre or
cytocentrum and is just visible as dots with LM. EM reveals that it consists
of two centrioles lying at right-angles to one another. Each is an
open-ended cylindrical body with a wall composed of nine bundles, each of
three microtubules. These tubules help to initiate and control the
aster, also microtubular, during cell division.
(b) A similar cylindrical structure is seen at the base of each cilium and is
called a basal body/kinetosome. One way basal bodies and cilia develop
is by a multiplication of centrioles.
5 Cell nucleus
(a) Nuclear membranes have been discussed in 5.3.
(b) Chromatin granules/karyosomes seen in the sap during the interphase
period are chromosomes composed of deoxyribose-nucleic acid molecules (DNA)
not fully uncoiled. The sex chromatin of female cells is an extreme example of
not uncoiling, where one of the female's two X chromosomes continues in a condensed
heterochromatin, rather than the dispersed euchromatin, form,
although a few genes on that chromosome escape inactivation.
(c) The nucleolus seen as a dense body in most cells' nuclei with LM.
is a condensation of protein and RNA and DNA, comprising granules supported by
microfilaments. The dense branching strand is the nucleolonema.
(d) There is great variety in the size of the nucleus, its shape, the
densities of staining of chromatin and nucleolus, and in the position of the
sex chromatin, if present.
(e) In general, a large nucleus, with much pale euchromatin and a prominent
nucleolus, is very active in the control of protein synthesis by transcribing
mRNA from chromatin DNA. The nucleolus is mostly pre-ribosomal RNA. Nucleoli
are prominent in nerve and Sertoli cells.
6 Cell inclusions
(a) Non-living, non-participating, poorly structured cell elements, very
rarely seen in an intra-nuclear position; usually cytoplasmic.
(b) Examples:
7 Dynamic nature of the cell
The cell is not a static entity in life. Its chemical constitution and
morphology are in continuous flux. Its complement of organelles is altering,
with wearing-out and replacement, i.e., the cell is having to synthesize its
own material. The cell itself represents a system of activities isolated to
partial extent from an extracellular environment. Within the cell things are
constantly being altered, moved around and joined up within the membranes. The
membranes define temporary compartments separated from the cytoplasm, where
particular activities can be confined and controlled by enzymes attached to
the extensive membrane surfaces. Dynamic aspects of the cell's existence are
partly deduced from a study of its morphology in specimens fixed in various
states, partly from microscopical observations of living cells, and from
histophysiological experiments outlined in Chapter 30.
8 Cell staining
l Cellular material contains a lot of water and has little natural colour.
It was discovered that various dyes intended for textiles would selectively
stain different structures in the cell. These stains may be discussed
for how their chemical nature relates to the particular things in the cell
with which they react and stain: for more details see Chapter 30.B.3.
2 Routine staining of human material usually employs haematoxylin,
which can be made to react preferentially with nucleic acids and acidic groups
of proteins. The nucleic acids are concentrated in the nucleus and the rough
endoplasmic reticulum. The colour is usually some shade of blue or brown
depending on the particular method of haematoxylin staining used.
3 A few cells have a lot of granular ER in their cytoplasm, which gives a
strong staining reaction, and the cell is said to be basophilic/
basophil - liking basic stains and haematoxylin. This term depends on how the
cytoplasm reacts: it is taken for granted that the nucleus will react
positively with basic stains.
4 Most cells do not have such a concentration of GER that their cytoplasm
stains significantly with haematoxylin. Consequently, another cytoplasmic
stain is needed. Such a general stain is eosin, acidic in nature, which stains
the cytoplasm of most cells red or pink. Some structures stain bright red.
These are said to be strongly eosinophilic or acidophilic/acidophil -
liking an acidic stain.
5 Three points should be mentioned.
9 Cytological description of an individual cell
In light microscopy involves: (l) relative and absolute size; (2) shape;
(3) number of nuclei; (4) shape and size of nucleus/nuclei; (5) intensity of nuclear
staining; (6) amount of cytoplasm; (7) staining affinity of cytoplasm, e.g.,
basophilic, acidophilic (eosin), argentophilic (silver stains), or chromophobe
(liking no stain); (8) granular cytoplasm; (9) nature of any inclusions, for
instance, melanin pigment, fat, carbon, bacteria, zymogen granules, glycogen,
mucus; (l0) specializations of the cell membrane, e.g., cilia, a brush/striated
border (many microvilli); (ll) distinctive organelles in cytoplasm and their
position, e.g., prominent Golgi complex, many fibrils, numerous orderly
mitochondria giving another striated effect, Nissl substance (GER) in nerve
cells; (l2) whether the cell is in some phase of mitosis or meiosis; (l3) the
cell's surroundings; (l4) manifest properties of the living cell, e.g.,
motility, phagocytosis, contractility.
10 Aspiration cytology
The above cellular detail discriminates pathological, e.g., malignant, change
in cells. These can be obtained, single and clumped, from any tissue or organ
of the body, where a needle can be introduced to suck out cells to make a
smear for fixing and staining - fine-needle aspiration cytology (FNA).
11 Cell division
l Review from biology what happens to (a) the centrioles and spindle, (b)
the nuclear chromatin and nucleolus, (c) the nuclear membrane, (d) the cell
membrane, and (e) the cytoplasm and its organelles and inclusions,
2 during the various phases: pro-, meta-, ana-, telo-phase
and interphase.
3 During interphase the chromatids duplicate themselves by an
exact replication, when DNA has to be synthesized.
4 Pursue biochemists and molecular biologists for their accounts of the
molecular controls on cell division, progress through the cell cycle Powerpoint, and the
continue working-proliferate-die decisions, and the medical relevance.
12 Apoptosis
1 The orderly or programmed death of cells is needed to balance cell
proliferation in mature renewing tissues, such as blood and epithelia. Also,
one strategy of development is to overproduce cells, then select, e.g., for
the survival of correctly connected neurons, or of lymphocytes reactive to
non-self antigens. Thirdly, if an organ cannot work properly, e.g., a gland
has a blocked main duct, or its hormonal drive stops, many cells die by
apoptosis. Apoptosis
2 For apoptosis, endonucleases are activated which break up the
chromatin, chopping up the DNA, transcription slows and stops, organelles
clump, and the GER dilates. Caspases digest relevant cellular
materials and structures. The cell and internal membranes bleb out.
Finally the shrunken cell is phagocytosed by macrophages.[Caspases - cysteinyl
aspartate-specific proteinases]
3 With LM, only the increasing nuclear density and cell shrinkage are
noticeable, unless special cytochemical methods to detect apoptotic events
(2 above) are used.
4 Apoptosis is intentionally unobtrusive, to remove single cells without
provoking inflammation or upsetting tissue function.
5 Quite often, the inclination of cells is to die, and they need survival signals not
to undergo apoptosis.
13 Stem cells
For a stable population, the corollary to cell death is cell renewal.
This requires:
..(i) the proliferation of cells;
..(ii) an enduring population of stem cells;
..(iii) controls (+ & -) that promote division of stem cells to
maintain their numbers - self-replication;
..(iv) controls that cause differentiation of certain of the stem
cells to become the determined/committed precursors of the mature cells
of the tissue;
..(v) factors to promote division of the precursors/progenitors and their
further differentiation. The controlling factors include cytokines (Chapter 8.F).
More is known about the ensuing progenitor cells than about the stem cells. Although not essential to the concept of stem cells, at step (iv) above, stem cells usually give rise to more than one lineage of differentiated cells, in order to furnish the needed diversity of cell types in blood and most epithelia. One mechanism for this is the asymmetric cell division, wherein the daughter cells of a mitosis differ.
The next chapter (Cytology II) reviews in more detail the structures of the cell, emphasizing their functions.
1 Although looking trilaminar in routine EM, it behaves as if it comprises a
double layer of lipid molecules, in which proteins are distributed
asymmetrically in a mobile mosaic pattern. Thus, some proteins span the width of the
membrane, and may vary rates of transport by changes in their conformation.
Others, as enzymes, receptors, or adhesion molecules, etc. have active domains
at the surface held in correct position by intramembranous domains imbedded in
the lipid layer, and intracellular domains to engage in events inside the cell.
A sometimes fuzzy-looking coat of glycoprotein - glycocalyx - sticks to the external face of the membrane.
2 Functions of the membrane are:
3 Molecules Wherever such actions are described, special molecules
are acting, by binding to each other, changing their conformation, or some
other means. Examples are:
.. (a) Spectrin/fodrin provides a subplasmalemmal skeleton attached to the
cell membrane by ankyrin, and to actin of the cytoskeleton, to permit control
of the membrane's shape and movement.
.. (b) Cell adhesion molecules (CAMs) allow cells to attach to only
certain cell types or substrates.
.. (c) Integrins are cell-surface-membrane dimeric molecules (an
alpha with a beta), by which cells choose to which extracellular matrix (ECM)
components they wish to fasten, e.g., laminin.
.. (d) Connexins are proteins that combine as hexamers to form
connexons - the gap-junction channels, allowing ions and small molecules to
pass between cells. Connexins and the transports allowed vary among liver
cells, neurons, etc.
.. (e) Occludins are responsible for the seal preventing passage of
materials past inter-epithelial tight junctions.
l Nuclear membrane is a doubled membrane with pores that separates off genetic material and influences its degree and kind of interaction with the cytoplasm. The membrane sometimes has deep infoldings. (The rare annulate lamellae are parallel membranes with pores, and closely resemble nuclear membranes, but are stacked in the cytoplasm.)
2 Granular endoplasmic reticulum (ergastoplasm) is a membrane system providing some communication with the nucleus via the latter's outer membrane. Both have ribosome particles studding their outer sufaces, i.e., those facing outwards towards the cytoplasm. The membranes and ribosomes, in association with the nucleus, are concerned with protein synthesis. Some proteins pass into the sacs or cisternae, formed between the double membrane layers, for folding and processing. Membrane vesicles holding the protein bud off and travel along microtubules to the cis (receiving) face of the Golgi complex.
. Endosomes are the natural transport vesicles moving from the donor
membrane compartment to the acceptor compartment. Endosomes require active
mechanisms for: membrane budding, separation, transport, targetting &
sorting, docking, and fusion. The donor and acceptor compartments may be the
plasmalemma (via endocytotic vesicles), the Golgi complex, lysosomes, or the
endoplasmic reticulum.
Some materials, e.g., receptors, are in a constant cycle between cell membrane, early
endosomes, and late endosomes, with branch points for materials to come or go to the
Golgi complex, lysosomes, or elsewhere in the cell.
[Note that microsomes are small bodies formed during separation by biochemical
cell fractionation techniques: morphologically they are artefacts. They are
derived from broken parts of the smooth or granular ER whose membranes 'heal'
to form small vesicles, with or without ribosomes.]
3 Golgi apparatus serves as a hub for traffic out of, into, and around the cell. It is a complex transit region (there may be more than one in large cells) occupied by smooth-surfaced tubules, sacs and flatter chambers varying considerably in size. It concentrates, modifies and packages certain secretory products to await transport to the cell membrane for release, or application to some intracellular purpose. Vesicles depart from the trans or releasing face. Also, the Golgi is where glycoconjugates are finished by adding the remaining sugars (using glycosyltransferases), e.g., in cartilage cells and mucus-secreting goblet cells. Glyco
4 Many inclusions - structures not actively participating in the metabolism of the cell at the time of observation - are products bound in membranes by the Golgi apparatus, e.g., melanosomes, zymogen granules. Other inclusions, e.g., glycogen granules, form in the cytoplasm without any enclosing membrane.
5 Smooth endoplasmic reticulum resembles the granular form in
sometimes having systems of parallel membranes following the curvature
of nearby structures, but it usually exists in tubular and vesicular
forms. Its functions are varied and include:
6 Mitochondria
l These are complex bodies with a double membrane, the inner
membrane extending inwards as sheets or tubules called cristae.
2 The inside of the mitochondrion is occupied by a matrix in which dense
bodies may sometimes be found.
3 Enzymes of oxidation and energy-release, and for some syntheses,
are present; some associated with the crista membranes or with the external/
outer limiting membrane, others and coenzymes are in the matrix.
4 Mitochondria of steroid cells are distinctive in having tubular
cristae.
5 Mitochondria are able to reproduce themselves. Also, they contain circular DNA for
13 respiratory-chain proteins. One rare mitochondrial genetic disorder of this
DNA causes, for example, a muscle disease with mitochondrial inclusion bodies.
6 Apoptosis involves mitochondria. Increased permeability of the outer mitochondrial
membrane allows the release of inter-membrane factors, e.g., cytochrome c, that help
start the caspase enzymatic cascade.
7 Lysosomes
l They are roughly spherical with a single enclosing membrane.
2 The storage/primary form is derived from the Golgi apparatus and contains
hydrolytic enzymes,
3 whose access to other intracellular materials is controlled by the
enclosing membrane and processes of membrane fusion. The stability of the
membrane can be influenced by vitamin A and glucocorticoid hormones.
4 Lysosomes fuse with endosomes, phagosomes, surplus
secretory granules or expended organelles, which they destroy.
Multivesicular bodies contain distinct vesicles, inside a limiting membrane.
The vesicles seem to be endosomes on their way to meet lysosomes, or for storage
as a way to keep membranes and membrane proteins intact for redeployment.
5 The form of lysosomes changes from round and fairly homogeneous to varied
secondary kinds including myelin figures and residual bodies.
Some residual bodies become yellow or brown lipofuscin/lipochrome
granules.
6 In autophagy, the lysosome fuses with the autophagosome, consisting of a double membrane wrapped around the target organelles and cytoplasm to be broken down. . Autophagy PowerPoint
7 Lysosomal enzymes are also used in the turnover of extra- and
intra-cellular materials. In lysosomal deficiency diseases, the inherited
absence of an enzyme causes the massive accumulation of the material, e.g.,
glycogen, normally broken down. (Excess storage, hence "storage disease/disorder",
e.g., Glycogen storage disorder II from a lack of a-glucosidase.)
8 A devastating storage disease is Hurler's, where a deficiency in lysosomal
a-L-iduronidase causes intra- and extra-cellular excess accumulations of dermatan-sulphate
and heparan-sulphate glycosaminoglycans. Aside from the dwarfism and mental retardation,
the many cardiovascular defects bring about early death.
8 Peroxisomes/Microbodies are widespread, but particularly in hepatocytes and renal proximal tubule cells. They have a dense matrix enclosed by a single membrane, and hold enzymes involved in the beta and alpha oxidations of certain fatty acids, and some phospholipid synthesis. Catalase is a useful marker enzyme for peroxisomes. The congenital lack of peroxisomes - deficient 'biogenesis' of the organelle - causes fatal syndromes with brain, liver and kidney dysfunctions. Zellweger's syndrome is the best known. If the peroxisomes have formed, but only one enzyme is genetically at fault the damage is less severe and may permit life.
The three components of the cytoskeleton - actin-myosin, microtubules, and intermediate filaments - are functionally linked as the dynamic organizer of the cellular domain, controlling cell shape, cell locomotion, where materials move in the cell, and hence cell polarity.
l Filaments
2 Microtubules Each microtubule is built of 13 tubulin filaments. The dimers constructing the filaments confer a repeated polarisation along the tubule so that one end is 'plus', the other 'minus' - a difference that tells the attached microtubule motor proteins which end to head for.
2 Protein synthesis is the primary cell activity influenced by the nucleus. Proteins, as enzymes, transcription factors, receptors, and so on, are the key to the particular cell's character (Chapter 32).
3 Nuclear constituents are:
4 Protein synthesis (also Chapter 32)
5
Cell fusion techniques, using the Sendai virus or chemicals, to produce hybrid
cells have revealed that the cytoplasmic environment can alter the activity
of the nucleus. Thus, a dense and inaccessible chromatin in a nucleus may be
transformed by new cytoplasmic surroundings to one that is looser, paler and
more reactive, as is seen in the small lymphocyte after antigenic stimulation.
Cell fusion can also be between a nucleated cell and one made nucleus-free - a
cytoplast.
Structureless at the EM level, but of vital biochemical and physiological importance with its water, electrolytes, soluble proteins, amino acids, sugars, enzymes, etc. It is in continuous interaction with the filamentous and membrane-enclosed systems. Unwanted or defective proteins to be destroyed are conjugated with ubiquitin , which takes them to chemically structured concentrations - proteasomes - of proteolytic cytoplasmic enzymes Powerpoint.
2 Epithelium is more than an inert covering or lining: it works. Examples of its activities are:
3 A gland is a structure formed to increase greatly the epithelial working surface, without occupying too much space in the body. The several ways of doing this are presented as a separate topic - Glands (Chapter l6). Here, the interest is the epithelia that lie more stretched out in a covering or lining attitude.
4 The embryological origin, in terms of the three germ layers, of epithelial and other tissues does not correspond with the morphological divisions set out in Section C, and is clinically significant only as a basis for certain terms such as 'mesenchymal tumour'. Should you need them, details of the origins are tabulated in older histology textbooks.
2 Simple epithelia
7 Stratified squamous
8 Keratinized/cornified stratified squamous
9 Transitional/urinary
4 Sites of occurrence, examples
(a) Simple - l, cuboidal, kidney tubules; 2, columnar,
gall-bladder, gut, uterus (ciliated); 3, squamous, Bowman`s capsule in
kidney, lining of lung alveoli; 4, pseudostratified columnar,
epididymis, trachea (ciliated);
(b) Stratified - 1, cuboidal, sweat gland's duct; 2, columnar,
penile urethra; 3, squamous, oesophagus, vagina; 4, keratinized
squamous, skin; 5, transitional, urinary tract.
2 Attachments: function and observation
l Attachments provide for:
2 Something (glycocalyx + ?) appears as a black line between cells treated with silver nitrate and sunlight. This outlines well the individual cells, e.g., in a stretched mesothelial sheet. Otherwise, cell membranes are not easily seen in LM except in the kidney collecting tubules. Elsewhere, the nuclei and their spacing are often the only guides to the number and shape of the cells and their layering. Even so, the pseudostratified epithelia show that this guide is fallible.
2 Mesenchymal cell
l Has a similar appearance to a small, young fibroblast, but is far more
multipotential in what cell types it can turn into.
2 In adult tissues, two views are:
...(a) a few are present and can explain such findings as the formation of
ectopic (out of its expected place) bone in soft CT, otherwise difficult to
account for unless differentiated cells such as fibroblasts can dedifferentiate and change their role;
...(b) mesenchymal cells all differentiate early in life and thereafter are
not present, and fibroblasts or other cells can de- and redifferentiate and
become osteoblasts.
3 Macrophage/histiocyte
l An ovoid or spheroid cell, which may change its shape while lying alongside
fibres, or when extending pseudopodia to move and ingest materials.
2 Phagocytoses dead cells, cell debris, live and inert foreign bodies.
3 Coordinates the inflammatory response and healing by means of
signalling peptides and proteins - cytokines, e.g., IL-1, TGF-b (Chapter 8.F).
4 Nucleus is smaller and more condensed than that of the active fibroblast.
5 Cytoplasm is pale with little GER, but has many lysosomes, when
digesting phagocytosed material.
6 Macrophages may fuse to become foreign-body giant cells with many
nuclei, when faced with a large object for digestion. More on macrophages.
4 Macrophage/reticuloendothelial/mononuclear phagocyte system (MPS)
l Comprises cells related directly to blood monocytes, or derived from
the same precursor in marrow.
2 A tentative division of the macrophage-system cells recognizes:
...Phagocytic antigen-presenters (Chapter l9.B.l)
... (a) Macrophages of connective tissues and serous cavities.
... (b) Alveolar macrophages/lung dust cells.
... (c) Macrophages of lymph nodes, spleen and bone marrow.
... (d) Kupffer sinusoid-lining cells of liver.
.........................................
... Weakly phagocytic antigen-presenters
... (e) Dendritic and interdigitating reticulum cells of lymphoid tissues.
... (f) Langerhans cells of epidermis and other epithelia.
.........................................
... Specialized (Some not phagocytic? Some not antigen-presenters?)
... (g) Foreign-body giant cells.
... (h) Microglia cells of CNS.
... (i) Synovial A cells lining joints.
... (j) Osteoclasts resorbing bone.
3 The phagocytic group (i.e., the original reticulo-endothelial series) can be revealed by vital injection (into the living animal) of colloidal or particulate coloured matter, e.g., Trypan blue or India ink, which the phagocytic cells of the system preferentially accumulate in their cytoplasm, thereby identifying themselves. Nowadays, MPS cells are distinguished by their cell-surface glycoprotein profiles, e.g., CD antigens.
5 Mast cell
l A `watchdog' cell starting the inflammatory response to noxious intruders.
2 From the German verb, mästen, it meant a `fattened' cell.
3 Spheroid or ovoid with a small central nucleus, and its cytoplasm packed
with dense basophilic granules.
4 Granules give a metachromatic staining reaction with thionine or
toluidine blue, i.e., a reddish-purple colour, because they contain a
sulphated polysaccharide - heparin.
5 Heparin is an anticoagulant for blood, first obtained from the liver
(hepar), but it also inhibits vascular smooth muscle proliferation and
some immune complement reactions. As a polyanion, it can complex materials,
e.g., the trypsin-like enzyme, tryptase, in the granules.
6 Histamine, increasing capillary permeability, is also present in
the granules. The chemokines also released can then more easily attract
white blood cells out of the vessels.
7 Many stimuli (e.g., antigens and agents released by lymphocytes during an immune
response) activate a release of the granule contents, from this `mobile-pharmacy'
cell, with its many chemical mediators.
8 Mast cells favour positions in CT close to veins (MCt subtype), and at dermal
and mucosal interfaces with the hostile environments of the skin, airway,
and gut (MCtc subtype).
9 The mast cell subtypes in man differ in the proteases that they contain:
...MCt cells have mast-cell tryptase and are involved directly with defence.
...MCtc cells contain chymase, cathepsin G, and
other proteases, in addition to mast-cell tryptase, and are more concerned with adaptive and remodelling
responses of blood vessels and CT.
6 Fat cell/adipocyte
l A genuinely fattened cell, initially resembling a fibroblast with a few
droplets in the cytoplasm.
2 For the white or yellow unilocular fat seen in adult man, the
droplets (mainly glycerides of fatty acids) coalesce and more fat is added,
3 until the nucleus is bulged to one side of a spheroid cell up to 200 µm
in diameter, distended by a huge droplet.
4 Cytoplasm, with a Golgi complex, ER and mitochondria, is present as an
attenuated peripheral shell.
5 The cell is static, but its content is not. The stored fat is participating
in the body's carbohydrate and fat metabolism.
6 Fat in the usual wax-imbedded section is dissolved out, but with osmium
tetroxide fixation it remains and is black. Some dyes will colour it, if it is
preserved by frozen sectioning.
7 Besides a number of adipocyte-specific enzymes for fat metabolism, fat cells secrete
leptin, which helps control energy balance and body fat mass.
7 Melanophore/CT pigment cell/CT melanocyte
l A process-bearing cell with melanin pigment granules in its cytoplasm.
2 Found in the skin`s dermis, brain's pia matter and the scleral and choroid
coats of the eye.
8 Plasma cell
l Many tissues, particularly those lining tracts open to outside the body,
are not immunologically virgin, but have been exposed to foreign organisms
that have provoked immune responses by local CT plasma cells and lymphocytes.
A lamina propria may have many of both and some eosinophils, e.g., in the gut.
2 Plasma cells are ovoid, roughly l0 µm in length, with an
eccentrically placed nucleus having its denser chromatin granules
clumped regularly around the nuclear membrane (clock-face appearance).
3 Cytoplasm is deeply basophilic from the rich GER, except for a pale
central region where the Golgi complex lies.
4 Proteins synthesized by plasma cells in lymphoid organs reach the plasma
as immunoglobulins/ antibodies, inactivating foreign invaders, e.g.,
viruses.
5 Plasma cells in CT make antibodies for local use, e.g., in the airway or
gut, to counter toxins and control microbial populations.
9 Reticular/reticulum cells
l Immunocytochemistry, EM, and enzymatic analysis distinguish at least three
kinds of reticular cell: fibroblastic, and two phagocytic kinds -
interdigitating (T-zone:) and dendritic (B-zone:
antigen-presenting).
2 The supporting reticular fibres of lymphoid tissues and bone marrow are
presumed to be produced by the fibroblastic variety.
3 Caution! The principal reticular cell in the thymus is an epithelial kind,
although extending cell processes to build a reticulum.
[Any time you hear 'reticular cell', ask for the type meant.]
5 Collagen staining
(a) Collagen (type I) often is present in bulk, and is stained selectively by:
aniline blue in Mallory's method, light green in Masson's, or red acid fuchsin
in van Gieson's. (Eosin stains it orange.)
(b) Mallory's, Masson's and van Gieson's trichrome methods distinguish
collagen from muscle, and also react with the nuclei and cytoplasm of other
cells.
6 Caution for rat CT spreads. Preparations of rat subcutaneous tissues may be contaminated by hairs. The segmentation of the medulla of a hair gives a crossbanding effect in LM. Collagen fibril crossbanding is visible only in EM.
7 Collagen types: extended from the classification of Prockop DJ & Kivirikko KI. Ann Rev Biochem 1995;64:403-434
Fibril-forming I II III V XI Network-forming IV VIII X Beaded filament-forming VI Anchoring filament-forming VII FACIT IX XII XIV XVI XIX Nonsecreted transmembrane XIII XVII* XXIII XXV XV XVIII Basement membrane zone XV XVIII* transmembrane collagen XVII is a component of hemidesmosomes. An autoimmune reaction to it can cause poor epidermal adhesion and hence skin blistering in humans.
2 Reticular fibres
l Collagen fibres, running parallel to one another, do not join up with others
running differently. Such an arrangement is seen, however, with reticular
fibres, which form a network or reticulum.
2 Reticular fibres stain black with reduced silver methods, hence their
other names - argyrophil or argentophil. H and E and some trichrome stains
leave them unstained.
3 X-ray diffraction and EM show them to be like fine collagen fibres, having
the same 67 nm-repeating crossbanding. Furthermore, they appear first at many
sites, as in mesenchyme and healing wounds, where collagen fibres will later
form. Thus reticular fibres are an immature, fine kind of collagen fibre,
mostly of type III collagen.
4 They persist into the adult in several organs, where a fine fibrous
support is needed that does not interfere with a close relation between
fixed cells and blood or lymph, e.g., in endocrine glands.
5 Reticular fibres fasten to the underside of basal laminae of epithelia
and endothelium, and bind and secure muscle and nerve fibres, using their
external laminae.
3 Elastic fibres
l May be fine, single and branching in areolar CT, or thick and parallel in
elastic ligaments. Walls of blood vessels have incomplete elastic membranes.
2 The elastic nature of the fibres is shown by the spiralling and kinking of
their recoiled broken ends, in spread preparations.
3 Elastic fibres and membranes, if thick, stain pink with eosin, or red with
Masson's method; otherwise, they remain unseen, unless elastic stains, e.g.,
orcein or Verhoeff's, are used.
4 In bulk, unstained, they appear yellow to the naked eye.
5 Formation and nature - fibroblasts and vascular smooth muscle
cells form and release two components: (a) fine protein microfibrils
thought to orient (b) tropoelastin as it polymerizes into amorphous
elastin. With little structure in EM, elastin is a network of long protein
chains held in a springy arrangement crosslinked by desmosines, each
derived from four lysines of the protein amino-acid chains.
2 Nature - large negatively charged proteoglycan molecules
(polyanionic macromolecules) bind to a varying degree water, electrolytes,
and other macromolecules, such as collagen, and the glycoproteins,
fibronectin and tenascin.
3 Proteoglycan chemistry - from a long protein backbone
molecule, many long sugar side chains stick out, because negative charges along
each chain repel adjacent chains and each other. The chains are composed of
repeating pairs of sugar/saccharide units. Each pair has an hexosamine
and a uronic acid. The loss of hydrogen ions from the many acids in the
chain of glycosaminoglycans (GAGs) leaves negative charges, only some
of which are neutralized by counterions such as Na
4 Nomenclature - the many linked sugars of the side-chains
are polysaccharides, hence with the protein backbone the general name -
'protein-polysaccharide'. However, this also describes glyoproteins, for
example, mucoproteins and mucopolysaccarides. Proteoglycans differ from
glycoproteins in: their core proteins; the use of fewer species of sugar; lack
of branching of the sugar chains; and usually their longer sugar chains, and
more acidic/negative character
5 Proteoglycan varieties - dependent on the specific sugars, and the
sites of sulphation, if any: 6 Staining - the failure of counterions to neutralize all anions
leaves regions of high negative charge density. If the proteoglycan is
prevented from dissolving out, its reactions are:
7 Physical properties - the high negative charge:
8 Overview of proteoglycans (PGs) and glycoproteins in connective tissues
1 The large PG monomer molecules may be aggregated by being strung
along a hyaluronate backbone, by means of a link protein for
the core protein-HA attachment.
2 The glycosaminoglycan side chains of proteoglycans vary in number, nature and
length. Combinations of sulphated and non-sulphated hexosamines, and
relatively tissue-specific core proteins, yield a diversity of PGs, crudely
classifiable by molecular size into large and small:
LARGE
SMALL
3 Non-collagenous glycoproteins of connective tissues include:
Fibronectin, Tenascin, Thrombospondin, Bone sialoprotein/BSPII,
Osteopontin/BSPI, Osteonectin/Bone Gla protein, Cartilage-matrix protein,
Alkaline phosphatase, Chondronectin, and Fibrillin.
One clinical aspect is their use as urinary or serum markers of
excessive turnover, e.g., Gla protein for bone disease.
4 Fibronectin and Tenascin
5 For more on vulnerabilities from the cellular and ECM-molecular interactions see
ECM
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l Areolar tissue
2 White adipose tissue
3 Brown adipose tissue
4 Reticular tissue
5 Elastic tissue
6 Dense fibrous (collagenous) tissue
7 Loose fibrous (collagenous) tissue
8 Mucous/mucoid/primitive connective tissue
Physiological factors controlling connective tissues are listed in
Chapter 8.E.
A specialized CT to resist compression, and provide modest rigidity with
flexibility, by having its cells, chondrocytes, produce a firm
resilient matrix of ground substances, and fibres or fibrils.
The rapid growth of cartilage is used to assist the growth of bones and
the repair of fractures. Based on the composition of the matrix, three
kinds are distinguished: hyaline, elastic, and fibrocartilage.
Powerpoint.
2 Matrix, apparently amorphous with HE staining in LM, contains:
3 Chondrocytes or cartilage cells are large and rounded, each lying
in a space - lacuna - enclosed by matrix. Cells often are grouped in
nests of 2, 4, or 6 as a result of mitoses and restricted cellular
movement. EM reveals cells to have short stubby processes, fat droplets,
glycogen and the GER and Golgi complex appropriate for secretion of the
matrix components: proteoglycans, type II collagen [with the
homotrimeric molecule a1(II)3], and glycoproteins.
4 Growth occurs in two ways:
Growth is vulnerable to X-rays, poor nutrition, and disturbed blood supply,
for example, from fractures at the growth plate.
5 Territories Most noticeably in articular cartilage there are:
6 Nutrition - cartilage is avascular and no blood vessels
serve the matrix directly, but cartilage canals may carry vessels
through the matrix to non-cartilaginous regions, e.g., secondary
ossification centres. Therefore, nutriment and wastes must diffuse
through the matrix for the cells to stay alive and perform their slow
turnover of the matrix macromolecules. The diffusion may break down and
various degenerations then occur, e.g., calcification. This last is prompted,
organized and made use of in the process of endochondral ossification.
Bone is a hard CT with cells, osteocytes, in much matrix, and
serves for support, attachment, leverage, protection and mineral storage.
l To obtain great strength and rigidity with some elasticity, the
matrix is composed of densely packed collagen fibrils infiltrated with
bone mineral as fine crystals of calcium salts resembling
hydroxyapatite crystals. Mineral constitutes about 65 per cent of the
dry weight of bone. The densely packed collagen fibrils are primarily
type I. There are small amounts of distinctive non-collagenous proteins, e.g.,
calcium-binding osteocalcin and bone sialoproteins (Chapter 5.C.8).
2 Matrix is strong but dense, thus nutritive fluids cannot diffuse freely
through it. Osteocytes therefore have to differ from chondrocytes in having
many long processes extending through canaliculi (narrow passages)
and making contact with one another and, indirectly, with blood vessels.
The cell body lies in a cavity, a lacuna, in the matrix.
3 Throughout life, for mineral homeostasis, and for its special
problems of growth, bone is subject to an unending turnover, with
selective destruction and replacement - the remodelling process.
Powerpoint
2 Based on the presence or absence of lamellae (layers) and
osteons/Haversian systems:
Woven bone's matrix has disorderly fibrils, whereas in lamellar bone the
fibrils of a lamella share a predominant orientation. Note that a particular
bone will have areas of woven and lamellar bone, depending on how far
remodelling has involved all regions.
2 Dense cortical bone. Where wide, e.g., femoral shaft, this layering
is often present:
3 Cancellous medullary bone whose trabeculae are lined by a thin
cellular endosteum and have some lamellae, but can be sustained by
marrow blood vessels without the need for Haversian canals.
4 Marrow cavities lie between trabeculae, inside the tubular shaft,
or in the diploic spaces of flat skull bones.
2 Osteocyte
3 Osteoclast
4 Bone cell dynamics
O-LINKED GLYCOPROTEIN an example
|Core protein
AA Fu Side chain is short (1-20 sugars) & branching
| |
AA Ga - Gln - Ga Wide variety of sugars
| / ^
T - Gln Uses one of several sugar core^ types
| ^ \ for attachment to the protein
AA Gun - Gln - Na
| ^ Na - sialic acid, Fu - fucose
TYPICAL STRAIGHT PROTEOGLYCAN SUGAR SIDE-CHAIN
-------->
| Repeating disaccharide pair
AA ___|___
| | |
S - Xy - Ga - Ga - Ua - Gln - Ua - Gln - Ua - Gln - Ua - Gln
| - - - Unsatisfied negative
AA charges
|Core protein, with serines (S) & threonines (T)
... Hyaluronate - soft connective tissues; synovial fluid; vitreous humour;
... Dermatan sulphate (chondroitin sulphate B) - skin and corneal CT;
... Keratan suphate - cartilage matrix;
... Chondroitin-4-sulphate (A) - cartilage matrix;
... Chondroitin-6-sulphate (C) - cartilage matrix:
... Heparin (also sulphated) - granules of mast cell and basophil.
PG aggregation produces huge molecules extending over micrometres, and visible
with conventional TEM. Proteoglycans amenable to such
assembly are aggrecans, susceptible to breakdown by aggrecanase.
However, the chemical nature and heterogeneity of monomers and their
aggregates make study of these important matrix constituents difficult.
Note that proteoglycans are also kept within some cells to work with other molecules.
Chondroitin-6-sulphate, skeletal keratan sulphates - Cartilage
Versican/Fibroblast PG - Soft CTs
Cell-surface-associated, e.g., the membrane-attached PGs syndecans, with
heparan-sulphate and chondroitin-sulphate chains, and the HSPGs
- glypicans - on epithelial and other cells
Basement-membrane heparan-sulphate PGs - basement membranes,
e.g., perlecan
Decorin/PGII (chondroitin/dermatan sulphates) - extracellular matrix
Biglycan/PG-S1 ( " ) - associated with a variety of cells including non-CT ones
Fibromodulin (keratan sulphate)
Dermatan sulphate
Small bone proteoglycans I & II
They interact with other macromolecules and influence cell behaviour.
Fibrillin is a crucial component of elastic fibres and other
structures in CTs; and genetic defects in its formation result in
the weak arterial walls, poorly suspended eye lens, lax ligaments,
etc. of Marfan's syndrome.
D TYPES OF CONNECTIVE TISSUES
Based upon: (a) the density and order of fibre packing; and (b) the
predominant cell and fibre types.
l Loose textured with a mixture of all cell and fibre types (but seldom
pigmented cells).
2 Rich in ground substances which fill the spaces or areolae, and confer
physical properties and control transport.
3 Locations - the lamina propria of the gut, under the skin, around
joints, muscles and some viscera, and other sites needing some freedom of
movement; the eye's choroid coat serving a more nutritive role also has
pigment cells.
4 Areolar tissue merges with the somewhat denser CT of D.6. Both types may
be regarded as belonging in one broad loose category.
5 Serous membranes are similar to areolar tissue but also have a layer
of simple squamous mesothelium (sometimes two layers).
6 Milky spots on serous membranes are dense accumulations of the
macrophages and lymphocytes present to protect serous body cavities.
l Comprises primarily fat cells enclosed in basal lamina, and held on
a framework of reticular fibres in association with many blood
capillaries.
2 Fibrous CT encloses the tissue, subdividing it with septa.
3 Found subcutaneously in the hypodermis (in the child, a panniculosus
adiposus), and in the mesentery, omentum, and retroperitoneal area.
4 Padding fat in palmar, plantar and intraorbital sites is not so
freely available as an energy store, and can survive starvation.
5 Adipose deposits in the hips, buttocks, and breasts are especially under
the control of female sex hormones, but many hormones control fat metabolism.
6 Functions - energy store; insulation; padding; steroid conversions.
l Cells have many separate (multilocular) fat droplets, relatively more
cytoplasm, and are smaller than white fat cells.
2 Found around the thorax and kidneys of animals naturally
exposed to severe cold, particularly hibernators.
3 Brown fat is a thermogenic organ providing a prompt and direct
source of heat to maintain the temperature of vital organs. Uncoupling
protein 1 lets mitochondria divert energy in this otherwise unwanted
thermal way by uncoupling respiration from ATP formation.
4 Seen in the human newborn; in adults BAT is detectable after
adrenergic stimulation. Brown fat might dissipate surplus energy from
overeating.
l Has the reticular fibre as the supporting fibre, and phagocytic fixed
macrophages.
2 The fibres are made by some of the stellate reticular cells acting as
fibroblasts.
3 Reticular tissue also contains parenchymal cells (the main
working cells) held by the fibres, e.g., hepatocytes or lymphocytes.
l Elastic fibres or membranes are the predominant element.
2 The fibres may be:
(a) thick or very thick (l0-l5 µm) and orderly as in the elastic ligaments,
e.g., ligamentum nuchae (in the neck of heavy-headed grazing animals),
vertebral ligamentum flavum, penile suspensory ligament, and in the
vocal chords; or
(b) finer and mixed with membranes in elastic arteries. The lung and
airway also have many elastic fibres.
3 In the ligaments, elastic fibres are formed by fibroblasts and held
together by reticular fibres, proteoglycan, and glycoproteins.
Two kinds:
...(a) Regular, e.g., tendon, ligament, aponeurosis, fascia, with
collagen fibres oriented to take stress principally in one direction. (The
dense corneal stroma has very orderly collagen for transparency as well as
strength.)
...(b) Irregular, e.g., dermis, organ capsules, periosteum, perichondrium,
epitendineum, with irregular, interwoven bundles of collagen.
l Although 6(b) and 7 have fibroblasts and collagen fibres as the
principal elements, reticular and elastic fibres and other cells are
present to a lesser degree, together with blood and lymphatic vessels
and nerves.
2 An example of a loose fibrous tissue is the lamina propria of the
urinary bladder, looser than dermis, denser than that of the gut. Indeed, the
gut's lamina propria is so given over to defence and defensive cells that it
is hardly recognizable as a CT.
. However, fibrous CTs form a continuum from dense, regular to areolar,
making implausible any assignment to rigid categories.
l Very rich in proteoglycans and water, has some fine collagen fibres
and widely separated young fibroblasts.
2 As Wharton's jelly of the umbilical cord it encloses and cushions
the vessels; the ocular vitreous and young dental pulp also fit tolerably
well in this class.
E FUNCTIONS OF CONNECTIVE TISSUES
1 Mechanical and protective - supporting, restraining, binding,
separating, directing and padding.
2 Transport of nutrients, metabolites, and signalling factors.
3 Storage of energy-rich lipids, water and electrolytes.
4 Defence against pathogenic organisms.
5 Repair of damage to itself, and organs supported or
enclosed, by fibrosis - the formation of irregular collagenous scar
tissue.
6 Thermogenesis (brown fat) and insulation (white fat).
Chapter 6 CARTILAGE
A HYALINE CARTILAGE
l Occurs fused with bone or as discrete pieces, looking hyaline/translucent
(glass-like) to the unaided eye. Most surfaces, except joint/articular ones, are
covered by a nutritive CT perichondrium/capsule with collagen and elastic
fibres, fibroblasts and blood vessels. It merges gradually via a chondrogenic zone
with the cartilage proper.
.. (i) the chondron - the chondrocyte and the pericellular matrix
immediately around it;
.. (ii) proteoglycan-rich territorial matrix outside the chondron;
.. (iii) interterritorial matrix, lying between the territorial matrices.
The matrix of the chondron has its own profile of special collagens,
proteoglycans, and cartilage glycoproteins, whereas the differences between
territorial and interterritorial matrices are more quantitative, and related to
collagen fibril thickness and orientation.
B ELASTIC/YELLOW CARTILAGE
l Is more opaque and flexible than the hyaline kind, but the cells are
similar in appearance and distribution; and it occurs as separate pieces
with a perichondrium.
2 Matrix is permeated by many elastic fibres that can be selectively
stained by stains such as orcein or Verhoeff's. The matrix is not prone
to degeneration and calcification.
C FIBROCARTILAGE
l In the intervertebral (IV) disc, fibrocartilage at first appears to have a rather
disorderly matrix with many thick collagen fibres, amongst which are dispersed
only a few chondrocytes in lacunae. However, the fibres are orderly in their alternating
orientations and layering, like the burst-resisting fibres of an old-style bias-ply car tyre.
2 The matrix gives the staining reaction of collagen, mostly type I,
except for close around the cells where proteoglycans are abundant.
3 Lacks a perichondrium and is not seen as discrete pieces; rather it is a
strong tension-resistant, but flexible transitional tissue located between
tendon and bone, bone and bone, hyaline cartilage and hyaline cartilage.
4 In the IV disc, the enclosed central nucleus pulposus is not cartilage, but
nevertheless has collagen type II, which diminishes in the innermost layers of the annulus
fibrosus as it is replaced by type I.
D DISTRIBUTION OF THE THREE CARTILAGE VARIETIES
l Hyaline - articular surface of most synovial joints; costal
cartilages; nasal and respiratory tract cartilages; basis of most of the
fetal skeleton; fracture callus, Chapter 31.E.1
2 Elastic - external ear, pharyngotympanic tube, epiglottis, and some
laryngeal and bronchiolar cartilages.
3 Fibrocartilage - intervertebral disc's annulus fibrosus (around a
nucleus pulposus of notochordal origin, present until late in life); pubic symphysis; femoral ligamentum teres; many tendon
insertions into bone; and the articular surface of some joints, e.g.,
temporomandibular.
Chapter 7 BONE
A PARTS OF A BONE
See Chapter 8.B for terms, e.g., diaphysis, epiphysis, etc.
B CLASSIFICATIONS OF BONE
l Based on the size of the spaces within the bone, and its trabecular
(lattice-like) or dense nature:
.. (a) Cancellous/spongy/trabecular
.. (b) Compact/dense
.. (a) Woven/primitive
.. (b) Lamellar/Haversian
C HAVERSIAN BONE
l An Haversian system is roughly cylindrical and arranged around one or two
small vessels in a central Haversian canal.
2 Osteocytes and bone lamellae making up the system are disposed in 4-20
concentric rings centred on the canal.
3 A lamella is the territory formed and maintained by the osteocytes
lying in a ring when seen in a cross-section. From the orderliness of the fibrils,
lamellae can be distinguished in polarized light, but it is only in a smaller
unit, the domain, that SEM reveals the fibrils to be aligned in the same
direction.
4 Haversian canals branch and join up with others. Their vessels originally
entered the bone from the periosteum or marrow via Volkmann's canals,
around which osteocytes are not especially ordered.
D MATURE HUMAN BONE
Studied from the outside working inwards has:
l Periosteum of dense CT divisible into:
In practice, some areas of dense bone remain woven or primary and are not
replaced by this classic lamellar architecture.
E BONE CELLS
l Osteoblast
l Lies on the surfaces of bone, in a one-cell thick layer, as most of
the endosteum and inner periosteum.
2 May be in two states:
3 Forms the collagen, glycoproteins, and proteoglycans of the matrix, and
controls the deposition of mineral crystals on the fibrils.
l Osteoblast becomes an osteocyte by forming matrix around itself and
becoming buried or immured.
2 Young osteocyte thus resembles an active osteoblast; older ones have
smaller, flattened bodies.
3 Processes extending from the body down the canaliculi are not
visible by LM; but EM shows that osteocytes, like osteoblasts, remain
connected by gap junctions.
4 The mature osteocyte is involved in maintaining the matrix of its territory.
SEM evidence puts into doubt the proposal that osteocytes can resorb bone
by osteolysis. Lacunae empty of osteocytes indicate dead bone.
l Large, multinucleated cell, with a pale acidophilic cytoplasm.
2 Lies on the surface of bone, often in an eaten-out hollow
- Howship`s lacuna.
3 Cell surface is attached to the bone by podosomes to create a sealed
compartment against the bone, in which the moving long cell processes of
the ruffled border can agitate the resorbing - bone-destroying -
materials.
4 Cytoplasm has vacuoles and lysosomes, since the mechanism of bone resorption
is partly an enzymatic digestion, by cathepsins and collagenase, and
also from acid made by an osteoclastic proton pump.
5 In dense bone, many osteoclasts act together to erode resorption
tunnels, which are later partially filled in with lamellar bone to
become osteons.
l Skeletal growth, changes of shape, and the physiological responses of bone
need changes in the populations of 'blasts and 'clasts.
These rely on a
proliferation of osteoblasts or a precursor, while osteoclasts come from the
fusion of blood-derived monocytes, which also partipate indirectly as macrophages
in the bone resorption.
2 The osteoprogenitor cell is a small, organelle-poor cell on the
surface or lying just behind the osteoblasts. It might be just an inactive
osteoblast: that it is more of a stem cell is shown by its occasionally
becoming chondroblastic, e.g., in tumours and fracture repair.
F HISTOLOGICAL METHODS FOR BONE
Special techniques are needed because of the difficulty of cutting such hard
material into sections thin enough for microscopy.