HISTOLOGY FULL-TEXT

William A Beresford MA, D Phil ©
Professor of Anatomy
Anatomy Department, West Virginia University, Morgantown, USA

Chapter 4 EPITHELIA

A INTRODUCTION

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
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2 Epithelium is more than an inert covering or lining: it works. Examples of its activities are:

  1. The tongue has receptors for touch and taste in its protective epithelium.
  2. In the gut, digested foods, but not unwanted and toxic materials have to be absorbed into the blood.
  3. 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.
  4. In the eye, epithelia serve both transparency and light screening.
  5. 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.
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.

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 lamina propria.
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 blood vessels.
6 In glands, the working epithelial cells constitute the parenchyma . The supporting connective tissue and other elements make up the stroma.

C MORPHOLOGICAL VARIETIES OF EPITHELIA

l Simple and compound epithelia
  1. 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.
  2. Simple epithelia, in general, are adapted to absorptive and secretory roles, while compound epithelia protect against damaging mechanical and chemical actions.
  3. 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.
  4. 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 cytology.

2 Simple epithelia

  1. Cuboidal and
  2. Columnar
3 Squamous/pavement
4 Pseudostratified columnar
3 Stratified/compound/layered epithelia
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

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.

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' folded membranes.
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:

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.

E METAPLASIA

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.
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:
  1. Capillaries - very rarely; only in cochlear stria vascularis.
  2. Nerve axons - common in skin, oral mucosa; less common elsewhere.
  3. Neural crest derivatives - as melanocytes, and accessory glial-type cells associated with receptors.
  4. Lymphocytes - common in gut and airway; less common elsewhere.
  5. Langerhans cells - contributors to immune defence in stratified squamous epithelia.
  6. Globular leucocytes - a special granular leucocyte of some epithelia.

William A Beresford, Anatomy Department, School of Medicine, West Virginia University, Morgantown, WV 26506-9128, USA - - e-mail: -- wberesfo@wvu.edu -- wberesfo@hotmail.com -- beresfo@wvnvm.wvnet.edu -- fax: 304-293-8159