Epithelial Tissue Histology
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Direct answer
Epithelium is one of the four basic tissues: closely apposed cells with little intercellular material, resting on a basement membrane, covering surfaces, lining cavities and forming glands. It is classified by number of layers — simple, stratified, pseudostratified — and by cell shape — squamous, cuboidal, columnar — with transitional epithelium reserved for the urinary tract. Avascular and nourished by diffusion, it renews itself continuously, which makes it the commonest seat of cancers.
What you must remember
- Simple squamous: flattened plates lining alveoli, blood vessels (endothelium), serous cavities (mesothelium) and Bowman capsule — built for diffusion and filtration.
- Simple cuboidal: small ducts, thyroid follicles and renal tubules; the proximal tubule bears a brush border for absorption.
- Simple columnar: stomach, intestine and gallbladder; the small intestine adds goblet cells and absorptive cells with microvilli; uterine tubes are ciliated.
- Pseudostratified ciliated columnar with goblet cells: the respiratory epithelium of trachea and bronchi — all cells touch the basement membrane, only some reach the lumen; also lines the male urethra.
- Stratified squamous: non-keratinised in oral cavity, oesophagus and vagina; keratinised in epidermis; resists abrasion.
- Transitional (urothelium): up to six layers in the relaxed bladder, with dome-shaped umbrella cells whose membrane plaques permit stretch without leakage; found from renal pelvis to urethra.
- Glands: unicellular (goblet) or multicellular; exocrine secretion is merocrine by exocytosis (pancreas), apocrine with apical pinching (mammary gland) or holocrine with cell death (sebaceous gland); endocrine glands are ductless.
- Surface specialisations: microvilli with actin cores form brush or striated borders for absorption; stereocilia are long microvilli of the epididymis; cilia have a 9+2 microtubule axoneme with dynein arms and 9+0 basal bodies, immotile in Kartagener syndrome.
Common confusion
Pseudostratified epithelium looks multilayered but is a single layer, since every cell rests on the basement membrane — the nuclei merely sit at different heights. Transitional is misfiled as stratified squamous: it has no keratin and dome-shaped surface cells. Endothelium and mesothelium are both simple squamous, separated only by location — vascular lining versus serous sac lining.
Exam-focused takeaway
Histology practicals ask identification of epithelia on slides of skin, trachea, intestine, bladder and thyroid with site and function. Theory questions cover the classification with one example each, modes of glandular secretion, and differences between cilia and microvilli. In the viva, explain why epithelium is avascular and how periodic acid-Schiff stain demonstrates the basement membrane.
Frequently asked questions
What distinguishes pseudostratified from stratified epithelium?
In pseudostratified epithelium every cell touches the basement membrane though only some reach the surface; in stratified epithelium only the deepest layer rests on it.
Where is transitional epithelium found and why?
In the urinary tract from renal calyces to the urethra; its dome-shaped superficial cells with membrane plaques let the bladder distend without leaking.
Name the three modes of glandular secretion with examples.
Merocrine secretion by exocytosis in the pancreas and salivary glands, apocrine secretion with loss of apical cytoplasm in the mammary gland, and holocrine secretion by cell breakdown in sebaceous glands.
How do microvilli differ from cilia?
Microvilli are static projections with actin cores that increase absorptive surface; cilia are motile with a 9+2 axoneme and dynein arms and move mucus or ova along the surface.
What are endothelium and mesothelium?
Both simple squamous epithelium: endothelium lines the heart, vessels and lymphatics, mesothelium lines the pericardial, pleural and peritoneal cavities.