Epithelium covers body surfaces, lines cavities & ducts, forms glands.

Properties: cellular, polar (apical/basal), avascular, sits on basement membrane, regenerates well.

Classification by SHAPE:
• Squamous (flat).
• Cuboidal (cube-like).
• Columnar (tall).

Classification by LAYERS:
• Simple = one layer (e.g., simple cuboidal — kidney tubules).
• Stratified = many layers (e.g., stratified squamous — skin, oesophagus).
• Pseudostratified = looks like layers but all touch BM (trachea, ductus deferens).
• Transitional = changeable — bladder.

Glandular epithelium = exocrine (with duct, e.g. salivary) + endocrine (no duct, e.g. thyroid).

EPITHELIAL TISSUE — COVERINGS, LININGS & GLANDS

Epithelium is one of the four basic tissues. It is highly cellular (very little intercellular substance), avascular (no blood vessels — diffusion from underlying CT), supported by a basement membrane, and POLARISED (apical free surface vs. basal anchored surface). Strong regenerative capacity.

FUNCTIONS: Protection, absorption, secretion, excretion, sensation, gas exchange, filtration.

CLASSIFICATION:

I. By Number of Layers + Cell Shape:

SIMPLE EPITHELIA (single layer):
• Simple Squamous — flat plates. Locations: alveolar epithelium (lungs), endothelium of vessels, mesothelium of body cavities, parietal layer of Bowman's capsule, lining of ducts. Function: passive transport.
• Simple Cuboidal — cube cells with central nucleus. Locations: thyroid follicles, kidney tubules, small ducts of glands, surface of ovary (germinal). Function: secretion + absorption.
• Simple Columnar — tall cells with basal nucleus. Locations: lining of stomach, intestines, gall bladder, uterus. May have microvilli (intestine — 'striated border') or cilia (uterine tube). Function: secretion + absorption.
• Pseudostratified Columnar — all cells reach BM but nuclei at different levels → looks layered. Often with cilia + goblet cells (trachea + bronchi — 'respiratory epithelium'). Also lines ductus deferens & epididymis.

STRATIFIED EPITHELIA (multiple layers):
• Stratified Squamous Non-keratinised — wet surfaces; oesophagus (especially in ruminants & horse — long oesophagus), oral cavity, vagina, cornea (anterior).
• Stratified Squamous Keratinised — dry surfaces; epidermis of skin, lining of forestomachs (rumen, reticulum, omasum) of ruminants.
• Stratified Cuboidal — rare; large duct walls (sweat gland duct).
• Stratified Columnar — very rare; large excretory ducts, conjunctiva.
• Transitional (Urothelium) — unique to urinary tract (renal pelvis → bladder → upper urethra). Cells change shape with stretch — domed when relaxed, flat when stretched. Top cells often binucleate. Special tight junctions to keep urine in.

II. SURFACE SPECIALIZATIONS:
• Microvilli — finger-like projections; multiply absorptive area (intestinal 'brush border', renal proximal tubule).
• Cilia — motile (move mucus / oocyte / sperm); 9+2 microtubule pattern.
• Stereocilia — long microvilli (despite the name!); on epididymis & ductus deferens.
• Keratin — protective protein layer on dry surfaces.
• Glycocalyx — sugar coat for protection + cell recognition.

III. GLANDULAR EPITHELIUM:
Glands form by epithelial invaginations into underlying connective tissue. Two categories:
1. Exocrine glands — keep their duct; secrete to a surface or lumen.
– By shape: tubular, alveolar (acinar), tubuloalveolar.
– By branching: simple vs compound.
– By secretion product: serous (watery, e.g. parotid), mucous (thick, e.g. sublingual), seromucous (mixed, e.g. mandibular).
– By mode of secretion:
• Merocrine — exocytosis (most glands, e.g. salivary).
• Apocrine — apical part of cell pinches off (mammary lipid, sweat in some glands).
• Holocrine — entire cell breaks down (sebaceous gland).
2. Endocrine glands — duct lost; secretion (hormone) goes to blood. e.g. thyroid follicles, pituitary, adrenal.
Some organs are BOTH exocrine + endocrine — pancreas (acini are exocrine; islets are endocrine), gonads.

BASEMENT MEMBRANE:
• Thin, non-cellular sheet between epithelium + underlying CT.
• Two layers: basal lamina (made by epithelium) + reticular lamina (made by CT).
• Stains by PAS (Periodic Acid-Schiff).