Epithelial Tissue

Epithelium does not do one job; it does a family of jobs. The textbook splits the tissue itself by function into four groups — covering, glandular, sensory, contractile (myoepithelial). The first two are the giants of this unit; the other two are footnotes for later. Read the table and notice that the same kind of cell, by rearranging itself, can either line a surface or sink into the connective tissue to form a gland. Glands are not a different tissue; they are epithelium that has dug in.

Overall epithelial functions: protection · secretion · absorption · excretion · sensory reception · contractility.

When the body's only job at a surface is to let things across, it picks the thinnest cell it can build — a flat, single-layered pancake. That is simple squamous epithelium. Stand it on its edge in a section and you see a fine pink line with the occasional bulge where a nucleus sits; flatten it surface-on and you see irregular tiles with jigsaw borders, like crazy-paving. Each cell is so thin that a red blood cell crossing an alveolar capillary is only microns from the air it must collect.

Two named varieties of this same epithelium have their own exam-worthy labels — endothelium when it lines the lumen of the cardiovascular and lymphatic system, mesothelium when it lines the great serous cavities (pleura, peritoneum, pericardium). Examiners love these two terms because they trip up the student who has memorised “simple squamous = thin” without learning the proper noun for the location. Get them cold.

• Surface view: irregular cells with serrated (jigsaw) borders fitted together (silver-stained).
• Vertical section: a single layer of flattened cells with a flattened, ellipsoid nucleus that bulges at the cell centre; cytoplasm extremely thin.

Take that flat pancake and pile in more cytoplasm and organelles until the cell is roughly as tall as it is wide — you now have a cube. Why a cube? Because the cell is no longer just letting things drift across; it has work to do. It is pumping sodium, reabsorbing glucose, secreting thyroglobulin. That work needs mitochondria, ribosomes and a respectable Golgi, and they all need room.

The give-away on a slide is the nucleus: a single, round, centrally-placed nucleus in each cube. Find that pattern and you are nearly always looking at a renal tubule or a thyroid follicle, with small ducts as the third common haunt. Compare and contrast with simple columnar in the next subtopic — the nucleus position alone usually decides the answer.

• Surface view: polygonal cells. Vertical: single row of cube-shaped cells; spherical, centrally-placed nucleus.
• Distribution: renal tubules, thyroid follicles, small ducts. Function: secretion & absorption.

Stretch that cube vertically and you get a column. The extra height is not for decoration — it is more cytoplasm for more secretory and absorptive machinery, plus a long apical border on which to plant microvilli. This is why the entire luminal length of the gut from stomach to rectum chose simple columnar: there is a lot to absorb and a lot to secrete, and you want to do both per square millimetre.

The histology trick is the nucleus — oval, lined up in the basal third of every cell, like a row of soldiers along the bottom. Above the nuclei, you often see goblet cells (pale, mucus-filled balloons) and an apical brush border made of microvilli. Brush border = absorptive turbo-boost; goblet cells = lubrication and protection. Together they are the signature of the small intestine.

• Vertical: single row of tall column cells; oval nucleus in the basal third, all aligned at the same level.
• Often bears apical microvilli (striated/brush border) and interspersed goblet cells (intestine).
• Distribution: GIT (stomach→intestine), gall bladder, uterus. Function: secretion & absorption.

Here is the trap. You look down the microscope at the trachea and see nuclei scattered at different heights — some near the base, some halfway up, some near the surface. Your first instinct is “stratified, several layers.” Wrong. Every cell here actually touches the basement membrane; some are short and never reach the lumen, others are tall and do. The nuclei sit wherever the cell's widest part happens to be, which gives the illusion (“pseudo”) of layers. It is one layer of cells of unequal height, not multiple layers.

The functional cast is mixed: tall ciliated columnar cells dominate, goblet cells pump out mucus, and short basal cells sit on the floor as a stem-cell reserve. The result is the mucociliary escalator — mucus traps dust and bugs, cilia sweep the loaded mucus upward toward the throat to be swallowed. Smoking paralyses the cilia, the escalator stops, and the airways drown in their own secretions. That is the structural story behind the smoker's cough.

• Cell types: ciliated columnar cells, goblet cells (mucus), short spindle (basal) cells, conical (basal) cells.
• Distribution: respiratory passages (trachea, bronchi). Function: protection + mucociliary transport (cilia sweep trapped particles + mucus upward).

Now the body needs a tougher surface — something that can shrug off rubbing, chewing, scraping, walking. The answer is to stack many layers and let the top layers be sacrificial. New cells are born at the bottom by mitosis, climb upward as fresh ones push them along, flatten as they go, and finally die and shed from the top. The whole epidermis turns over every 4–6 weeks.

Two flavours: keratinised, where the surface cells die hard — they fill with keratin, lose their nuclei, and become a dry waterproof crust (skin); and non-keratinised, where the surface cells stay nucleated and moist (mouth, oesophagus, vagina, cornea). The choice depends on the local climate: dry external surfaces need waterproofing, internal wet surfaces do not. Either way, the engine is the basal layer — the cuboidal/low-columnar cells sitting on the basement membrane, dividing constantly. Wound healing, regeneration, and (sadly) basal cell carcinoma all start here.

Many layers; named for the flat surface cells. From base up: basal layer (single row of cuboidal/low-columnar, mitotically active) → several polyhedral layers → flattened/spindle layers → flat surface cells.

A rare type — the body almost never wants tall columnar cells in layers. Where it does appear, only the surface layer is columnar; the deeper layers are polygonal and short. Think of it as a stratified epithelium that put a row of columnar cells on top as the working face. You will see it lining the conjunctiva, parts of the male urethra, and the larger excretory ducts of glands. Its job is protection in places that need slightly more substance than a single columnar row could provide.

• Deep layer(s) of polygonal cells; surface layer of columnar cells. Relatively rare.
• Distribution: conjunctiva, parts of the male urethra, large excretory ducts. Function: protection.

The bladder has an engineering problem the rest of the body does not: it has to triple in volume several times a day and then shrink back, while keeping a fiercely cytotoxic urine on the inside and the body fluids on the outside. No fixed-shape epithelium can do that. The answer is transitional epithelium — a stratified epithelium whose surface cells (the famous “umbrella cells”) are large, dome-shaped, often binucleate, and have a thick apical plaque membrane built specifically to resist urine.

When the bladder is empty, the cells pile up — you count 6–8 apparent layers, with the umbrella cells bulging into the lumen. Fill the bladder, the wall stretches, and those same cells slide over each other and flatten until you can only see 2–3 layers. It is the same epithelium changing shape, not changing cell number. That single sentence is worth a mark every time the question appears.

• Distribution: renal calyces, ureter, urinary bladder, upper urethra. Function: protection + accommodates volume changes; barrier against hypertonic, cytotoxic urine.

Every epithelium sits on a thin extracellular mat that separates it from the connective tissue below. That mat is the basement membrane, and despite its modesty on H&E (a faint pink line, easily missed), it is one of the busiest structures in the body. It anchors the epithelium, filters what passes between epithelium and CT, and instructs epithelial cells how to migrate and differentiate during repair. Stain it with silver and it blackens; stain it with PAS and it turns magenta — both tricks exploit its rich carbohydrate content (type IV collagen, laminin, perlecan, entactin).

By electron microscopy, the basement membrane resolves into two layers. The upper basal lamina (lamina lucida + lamina densa) is laid down by the epithelial cells themselves. The lower reticular lamina (reticular/type III collagen fibres) is made by fibroblasts of the underlying connective tissue. Two tissues collaborating to build one membrane — remember the division of labour, examiners ask it directly.

• Functions: structural support & attachment of epithelium; a selective (semi-permeable) molecular filter; and a scaffold that guides epithelial migration, proliferation and differentiation during repair.

If the apex needs surface area for absorption, the base sometimes needs surface area for active transport. The trick is the same but reversed — instead of pushing membrane outward (microvilli), the basal membrane folds inward deep into the cytoplasm. Wedged between the folds are mitochondria, packed like batteries against the ion pumps that have to be supplied with ATP. This is the structural signature of the renal proximal and distal convoluted tubules and of the striated ducts of salivary glands — cells that move huge volumes of sodium and water with high efficiency.

Anchoring the cell to the basement membrane is a different job, done by the hemidesmosome — literally “half a desmosome.” A desmosome has two cytoplasmic plaques (one in each neighbouring cell) connected through the gap; a hemidesmosome has only the cell-side plaque, with integrins reaching down into the basal lamina. Keratin filaments anchor into the plaque from inside; the integrins grip from outside. Result: the cell cannot be lifted off the basement membrane by ordinary shearing. Lose these and you get a blistering disease (bullous pemphigoid).

• Hemidesmosome: “half a desmosome” — a plaque on the basal membrane that anchors the epithelial cell to the underlying basement membrane (keratin filaments insert into one plaque only).