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: as classified by 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* 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.
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).