Chapter 1: Epithelial Tissue

Epithelium is one of the four basic tissue types, and it's usually the first thing you're asked to name on a slide. The trick to identifying it quickly is to answer two questions in order: how many layers, and what shape are the surface cells?

Step 1 — count the layers

• Simple: a single layer of cells, all resting directly on the basement membrane. Found wherever diffusion, secretion, or filtration needs to happen fast (capillaries, alveoli, kidney tubules).
• Stratified: two or more layers. Found wherever the tissue needs to resist abrasion (skin, esophagus, vagina).
• Pseudostratified: a common trap. It looks like multiple layers because nuclei sit at different heights, but every single cell actually touches the basement membrane — it's technically simple. Classic location: the respiratory tract, often with cilia and goblet cells.
• Transitional (urothelium): unique to the urinary tract. Looks stratified cuboidal when relaxed, but the dome-shaped surface cells flatten out as the bladder or ureter distends.

Step 2 — name the surface cell shape

• Squamous — flat, scale-like. Simple squamous lines vessels (endothelium) and body cavities (mesothelium); stratified squamous covers skin (keratinized) and lines the esophagus/oral cavity/vagina (non-keratinized).
• Cuboidal — roughly as tall as wide. Lines kidney tubules and small gland ducts.
• Columnar — taller than wide, often with a clear apical/basal polarity. Lines the GI tract (with microvilli for absorption) and, in its pseudostratified ciliated form, the airway.

Step 3 — check for a surface specialization

Layers and shape are usually enough to identify a slide, but a complete description adds a third detail: is the surface plain, ciliated, or keratinized/cornified? Chapter 5 covers this axis in depth, along with the junctions that hold epithelial sheets together, microvilli and stereocilia, and how epithelium folds into glands.

Putting it together

A slide description like "simple cuboidal epithelium" or "stratified squamous, non-keratinized" is a complete, testable answer — layers first, then shape. If you see cilia, think respiratory tract or fallopian tube. If you see goblet cells, think a mucus-secreting surface (airway or gut).

Key teaching point

Epithelium is always avascular — it gets its nutrients by diffusion across the basement membrane from the underlying connective tissue. That basement membrane is also the boundary pathologists watch for invasion: a carcinoma that hasn't crossed it is still "in situ."