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Epithelium: Support And Nutrition
  1. 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 (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, and 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 (diffusion) 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.
Morphological Varieties of Epithelia

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 cytology.
  1. Simple epithelia
Cuboidal and Columnar
  1. Cell shape is indicated approximately by the name; most epithelial cells are really polyhedral with many sides or faces.
  2. Cells stand one cell high, although their nuclei may lie at slightly different levels.
  3. Cells are fastened and sealed at the top of their sides by encircling junctional complexes.
  4. 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.
  5. The position and shape of the nucleus, and special locations of organelles and inclusions that also indicate the cell’s polarization.
  1. Squamous/pavement
  1. Very flattened cells presenting a minimal barrier to the passage of materials, e.g., oxygen, through them.
  2. Cytoplasm is very hard to see with LM.
  3. The very similar endothelium and mesothelium line blood and lymph vessels, and serous cavities, respectively.
  1. Pseudostratified columnar
  1. Nuclei lie at different levels suggesting stratification, but all cells are in contact with the BL.
  2. Two or more cell types are present: short basal, tall columnar.
  3. Stratified/compound/layered epithelia
  1. Stratified cuboidal, and
  2. Stratified columnar
  3. Stratified squamous
    1. Many cells thick.
    2. Surface cells are flat plates and flake off as squames. :
    3. Basal-most cells are cuboidal or columnar and divide.
    4. Cells above the base become polyhedral and are held together by many desmosomes to resist the abrasive forces on this peotective epithelium.
    5. Underside of the epithelium is indented by vascular papillae of connective tissue, except in the cornea.
  4. Keratinized/cornified stratified squamous
    1. 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.
  5. Transitional/urothelium
  1. Several cells thick, but the surface cells are large, rounded, alive and sometimes binucleate, with spare cell membrane in vesicles.
  2. No connective tissue papillae indent the epithelium.
  • Definiition Dorland Medical Dictionary): Modified columnar epithelium also called as sensory epithelium
  • Found in the special sense organs like tongue (Gustatory), Nose (Olfactory), Ear (Vestibular & Cochlear) - Gray’s Anatomy
  • Found in the Eye (Retina) - Ross Histology (? controversial)
A derivative of Monocytes like Kupffer cell (Liver) and Microglia (Brain).
As many as 50 monocytes precursors join together and hence osteoclast is a Giant multinuclear cell.
Osteoclast is a phagocyte (bone eater) cell and works under PTH (Parathormone). But the control is indirect since osteoclasts have no receptors for PTH.
Osteoclastoma leads to pathological fractures due to osteolytic lesions and gives Soap Bubble Appearance in X-Ray. Associated hypercalcaemia is also observed.
Sites of occurrence, examples
  1. Simple -1, cuboida (kidney tubules (PCT); 2, columnar, gall-bladder, gut uterus (ciliated); 3, squamous, Bowman’s capsule in kidney, lining of lung alveoli; 4, pseudostratified columnar, epididymis trachea (ciliated).
  2. Stratified-1, cuboidal, sweat gland’s duct; 2, columnar, penile urethra; 3, squamous, oesophagus, vagina; 4, keratinized squamous, skin ; 5, transitional, urinary tract.
Cell Attachments
Devices for attachment
These are used to attach not only epithelial cells but, with some modification, those of the other tissue, e.g. muscle, osteocytes, neurons. To be seen clearly or at all, EM is needed.
  1. Desmosome (the macula/spot/punctate kind of adhaerens attchment): 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 aere distinct plaque and demosomal membrane proteins.
  2. Hemi-desmosome: for better adhesion of the basal cell membrane to the basal lamina; includes a plaque and tonofilaments.
  3. Gap Junction/nexus: where two cell’s 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 cell’s activities.
  4. Tight junctions (resembles a zonula occludens but is not always belt-like): outer parts of two cell’s membeanes are fused together thereby occluding the intercellular cleft.
  5. Plication/folding and interdigitation of the adjoining cell’s folded membranes.
  6. Glycocalyx in the usual 20 nm cleft existing between membranes where specialized attachment are absent.
  7. Cells bridges with true cytoplamic continuity: seen only rarely, e.g., between spermatids.
  8. Fascia adhaerens: at intercalated discs of cardiac muscle.

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