William A Beresford MA, D Phil ©
Professor of Anatomy
Anatomy Department, West Virginia University, Morgantown, USA
1 In the early development of the embryo the rapidly multiplying cells
lie in layers. From each of these three germ layers a class of tissue
develops that persists to maturity as a tightly packed layerof cells,
contrasting with the connective tissues where the cells are spaced out in
an extensive extracellular matrix. The class is the epithelial kind of
tissue. Epithelia cover the outside of the body and line the spaces and
tubes within the body. Powerpoint
2 Epithelium is more than an inert covering or lining: it works. Examples of
its activities are:
This list indicates some of the many epithelial functions, for which there
are several types of epithelium. The last function listed, secretion,
often entails a gland.
- The tongue has receptors for touch and taste in its protective
- In the gut, digested foods, but not unwanted and toxic materials have to
be absorbed into the blood.
- In the skin, heat is transferred from underlying blood vessels to the air,
and skin is a surface on which sweat spreads and evaporates: mechanisms for
cooling the body.
- In the eye, epithelia serve both transparency and light
- In many epithelia the cells are secretory. They take raw materials
from blood and build up complex materials for release from the cell as a
secretion. Epithelial secretions protect, lubricate, digest, or, in the case
of hormones, control. For instance, in the GI tract and airway, epithelial
cells make anti-bacterial defensive chemicals.
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.
B EPITHELIUM: SUPPORT AND NUTRITION
l Support of the cells of a simple epithelium or of the bottom layer
of a stratified epithelium is by their lying on, and attachment to, a
glycoprotein sheet reinforced by fine filaments - the basal lamina (BL).
This is anchored by collagen fibrils to the denser fibres of a supporting
connective tissue lamina propria.
2 The basement membrane (BM) seen in LM is the basal lamina, fibrils
and connective-tissue ground substances.(The term 'basement membrane' is often
used for just the basal lamina.) Of principally epithelial origin, the
basal lamina comprises interacting macromolecules: special glycoproteins,
e.g., laminin, nidogen (a sulphated glycoprotein), and collagen
type IV and others, and also heparan sulphate proteoglycan.
The principal epithelial grip is by cell-membrane integrin to laminin.
Seen with TEM, the basal lamina is subdivided into two or three layers - a
pale lamina lucida next to the epithelium, a lamina densa, then
the deeper lamina fibroreticularis (less consistently visible).
BMs differ by location, and experience various pathological changes
- thickening, breaks, duplication, autoimmune attack, etc.
3 The lamina propria has collagenous and elastic fibres, other matrix
materials, fibroblast cells, blood and lymphatic vessels, and wandering
defensive cells to protect it and the epithelium.
4 The nutrition of epithelial cells is by indirect exchange
through the BL and matrix substances with blood in the capillaries of the
5 Tunica mucosa (abbreviated to mucosa)/mucous membrane
comprises an epithelium, its BL, and the lamina propria, including
structures such as glands lying in it. The exceptions are the skin
(epidermis on a dermis), the mesothelium-covered serous membranes where
tunica serosa is applied, and the endothelium-lined tunica intima of
6 In glands, the working epithelial cells constitute the parenchyma
. The supporting connective tissue and other elements make up the
C MORPHOLOGICAL VARIETIES OF EPITHELIA
l Simple and compound epithelia
- The primary classification is based upon the layering: one cell thick
is simple, two or more cell layers thick constitutes stratified/
compound. Cell shapes give the secondary classes.
- Simple epithelia, in general, are adapted to absorptive and
secretory roles, while compound epithelia protect against damaging
mechanical and chemical actions.
- Compound epithelia frequently, and simple sometimes, have several types of
cell present. Cells lying basally on the BL are mitotically active and migrate
upwards, differentiating to replace cells lost from the surface, or cells that
have destroyed themselves by apoptosis.
- Epithelia shed cells continually. Such cast-off or desquamated cells
may be examined in smears of the appropriate fluid - sputum, gastric,
uterine cervical - for signs of malignant change and/or chromosomal
abnormality in their epithelium of origin: the technique of exfoliative
2 Simple epithelia
- Cuboidal and
- (a) Cell shape is indicated approximately by the name; most
epithelial cells are really polyhedral with many sides or faces.
- (b) Cells stand one cell high, although their nuclei may lie at
slightly different levels.
- (c) Cells are fastened and sealed at the top of their sides by
encircling junctional complexes.
- (d) Cells have three surfaces: free/luminal, lateral and basal;
each may have membrane specializations, e.g., cilia at the free, occluding
junctions and desmosomes at the lateral, and infoldings at the basal surfaces.
- (e) Note the position and shape of the nucleus, and special locations of
organelles and inclusions that also indicate the cell's polarization.
4 Pseudostratified columnar
- (a) Very flattened cells presenting a minimal barrier to the passage of
materials, e.g., oxygen, through them.
- (b) Cytoplasm is very hard to see with LM.
- (c) The very similar endothelium and mesothelium line blood and lymph
vessels, and serous cavities, respectively.
3 Stratified/compound/layered epithelia
- (a) Nuclei lie at different levels suggesting stratification, but all
cells are in contact with the BL.
- (b) Two or more cell types are present: short basal, tall columnar.
- (c) More details: Chapters l2.B.5. and 22.B.2.
5 Stratified cuboidal, and 6 Stratified columnar
(a) Surface cells of 5 and 6 resemble those of 2.1 and 2, but between them
and the BL is a layer of cuboidal basal cells.
7 Stratified squamous
- (a) Many cells thick.
- (b) Surface cells are flat plates and flake off as squames.
- (c) Basal-most cells are cuboidal or columnar and divide.
- (d) Cells above the base become polyhedral and are held together by many
desmosomes to resist the abrasive forces on this protective epithelium.
- (e) Underside of the epithelium is indented by vascular papillae of
connective tissue, except in the cornea.
8 Keratinized/cornified stratified squamous
- (a) Similar in its basal and middle layers to 7, but the uppermost
epithelium has granular cells concerned with forming special, dead cells
solidly packed together as a surface keratin layer for greater protection.
- (b) More details: Chapter 21.A.
- (a) Several cells thick, but the surface cells are
large, rounded, alive and sometimes binucleate, with spare cell membrane
- (b) No connective tissue papillae indent the epithelium.
- (c) More details: Chapter 23.B.I.
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.
D CELL ATTACHMENT
l Devices for attachment
These are used to attach not only epithelial cells but, with some modification
, those of the other tissues, e.g. muscle, osteocytes, neurons. To be seen
clearly or at all, EM is needed.
l Junctional complex of: the girdle-like zonula occludens and
zonula adhaerens/belt desmosome, below which is a ring of maculae
adhaerentes/ spot desmosomes. Filaments of the terminal web in each cell's
apical cytoplasm fasten to the complex. Something of the complex was seen as
the terminal bar of LM.
2 Desmosome (the macula/spot/punctate kind of adhaerens attachment):
disc-like structures scattered on cell's surface; each is contributed to by
membranes of two cells; cytoplasmic tonofilaments (keratin intermediate
filaments) converge on and insert into dense subplasmalemmal plaques. There
are distinct plaque and desmosomal membrane proteins.
3 Hemi-desmosome: for better adhesion of the basal cell membrane to
the basal lamina; includes a plaque and tonofilaments.
4 Gap junction/nexus: where two cells' membranes come closely
together with only a 2 nm gap bridged by 'connexons' allowing ions,
nucleotides, and amino acids to pass from cell to cell for coupling and
coordination of many cells' activities.
5 Tight junction (resembles a zonula occludens but is not
always belt-like): outer parts of two cells' membranes are fused together
thereby occluding the intercellular cleft.
6 Plication/folding and interdigitation of the adjoining cells'
7 Glycocalyx in the usual 20 nm cleft existing between membranes where
specialized attachment are absent.
8 Cell bridges with true cytoplasmic continuity: seen only rarely,
e.g., between spermatids.
9 Fascia adhaerens: at intercalated discs of cardiac muscle.
2 Attachments: function and observation
l Attachments provide for:
- mechanical strength;
- barriers to the indiscriminate passage of materials, and thus the
possibility of selective transport;
- formation of intercellular compartments, e.g., bile canaliculus;
- communication for migration, differentiation, function and exfoliation.
But, for repair and normal cell replacement, cells move relative to one
another and must break their attachments. Also, white blood cells can pass
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.
l Metaplasia in any tissue is a change (usually abnormal) from one
distinctive tissue to another, at a definite site after development is over.
It implies a change in cell type - a transdifferentiation. Metaplasia
is noted in epithelia, for example:
2 The term is applied neither to normal differentiation, e.g., from
basal to ciliated cell, nor to a change to an abnormal tumour cell for
which 'neoplasia' is used. Although, having become neoplastic, a cancer cell
may then change its identity in a metaplastic manner.
- (a) a change from pseudostratified columnar to stratified squamous after
repeated chemical or thermal insult, say, smoke in the larynx and trachea;
- (b) from oesophageal stratified squamous to gastric or intestinal
simple columnar (Barrett's oesophagus), after repeated acid reflux;
- (c) from columnar, cuboidal or transitional to cornified stratified
squamous in severe vitamin A deficiency.
- (d) from transitional to stratified squamous in bladders with a stone, or
infested with worms. Image - R
3 Epithelioid is a term for a non-metaplastic epithelium-like
appearance of non-epithelial osteoblasts as a layer on bone, or secretory
cells of muscular origin as in the kidney's juxta-glomerular apparatus.
4 Oncocytes are large, eosinophilic, mitochondrion-rich epithelial cells
with small dark nuclei, occurring with increasing age in glandular and
lining epithelia, and constituting a kind of metaplasia.
F SPECIAL ELEMENTS IN EPITHELIA
Non-epithelial structures sometimes occur within an epithelium:
- Capillaries - very rarely; only in cochlear stria vascularis.
- Nerve axons - common in skin, oral mucosa; less common elsewhere.
- Neural crest derivatives - as melanocytes, and accessory
glial-type cells associated with receptors.
- Lymphocytes - common in gut and airway; less common elsewhere.
- Langerhans cells - contributors to immune defence in stratified
- Globular leucocytes - a special granular leucocyte of some