Frontispiece · Ordo Epithelia — the linings

The ground plan

Which way is up

Every epithelium in the body is doing the same job: it is a border. One side faces an opening — the inside of a tube, a sac, a vessel, or the outside world. The other side is anchored to the tissue underneath. Find those two sides on a slide and the names stop being a list to memorise; they become a description of what you are looking at.

Fig. 1 · anatomy of a lining

The four surfaces, the membrane, and the space the whole thing faces.

The five words

• Lumen — the open space the tissue faces: the inside of the gut, the airway, the bladderA muscular organ that stores urine before excretion., the blood vessel. On a slide it is the empty-looking part. Find it first; it tells you which way is up.
• Apical surface — the free edge, facing the lumenThe inside space of a hollow organ or structure.. Cilia, microvilliTiny projections on the surface of epithelial cells that increase surface area for absorption. and secreted mucus all turn up here, because this is the side that meets whatever is passing through.
• Lateral surfaces — the sides, where one cell is joined to the next. The junctions along here are what make the sheet leakproof.
• Basal surface — the bottom edge, planted on the basement membrane.
• Basement membrane — a thin protein sheet the epithelium secretes and anchors into. It is the boundary between epithelium and connective tissue. Epithelium has no blood vessels of its own, so everything it needs crosses this membrane by diffusionPassive movement of molecules from areas of high to low concentration. from the connective tissue below — which is also why an epithelium can only be so thick.

Half of identifying a tissue is deciding which edge is the top. Do that before you say anything else out loud.

First question

What shape are the cells?

Take a cell at the free surface and compare its height with its width. That is the whole test — and the answer is the second half of the tissue’s name.

Fig. 2 · the three shapes

One layer in every panel. Only the shape of the cell changes.

• Squamous — flat and wide, like floor tiles seen edge-on. The nucleusThe control center of the cell that contains DNA and directs cellular activities. is flattened along with the cell.
• Cuboidal — about as tall as it is wide. Round nucleus, parked in the middle.
• Columnar — taller than it is wide, standing on end. Oval nucleus, usually sitting low in the cell near the basement membrane.

Judge the shape from the cellsThe basic structural and functional units of life. at the apical surface. In a layered tissue the ones at the bottom are a different shape, and they are not the ones the name is talking about.

Second question

How many layers?

Count the rows of nucleiClusters of neurons in the CNS responsible for processing information. stacked between the basement membrane and the lumen. One row, or more than one.

Fig. 3 · simple and stratified

Count the rows of nuclei. That is the test.

• Simple — one layer. Every cell touches the basement membrane and reaches the lumen. One row of nuclei. Built for things to cross it: absorption, secretionThe process of moving substances from the blood into the nephron tubule to be excreted in urine., filtrationThe process by which fluid moves out of capillaries into surrounding tissues due to hydrostatic pres, diffusion.
• Stratified — two or more layers. Only the deepest row touches the basement membrane; only the top row meets the lumen. Several rows of nuclei. Built to take abuse — the surface cells get worn off and are replaced from below.

A stratified tissue is named for the shape of its surface cells, not its basal ones. Stratified squamous sits on a row of cuboidal cells, and nobody calls it stratified cuboidal.

The naming key

How to read a name

Put the two answers together, layering first and shape second, and you have said the name. The Latin on each plate is built the same way, which is the only reason it is there.

Worked both ways

• Simplex cubicus → simple cuboidal → one layer of cells as tall as they are wide.
• Stratifex squamosus → stratified squamous → many layers, flat cells on top.
• You saw one row of nuclei and tall narrow cells → simple columnar → Simplex columnaris.

Two that break the rules

The exceptions

Two of the eight will not sit still for that procedure. They are worth knowing precisely because they break the rule you just learned — and both of them are reliably the ones that go wrong on a practical.

Fig. 4 · the exceptions

Left: looks layered, isn’t. Right: won’t hold a shape.

Keying out a specimen

Four questions, in this order

This is the procedure. It works on a slide you have never seen before, which is the only kind that matters on a practical.

1. Find the lumen. Which edge is free and which is anchored? Everything below depends on getting this right.
2. Count the rows of nuclei. One row → it is simple; skip to 4. More than one row → go to 3.
3. Does every cell touch the basement membrane? If yes, it only looks layered: pseudostratified. If no, it really is stratified.
4. Name the shape of the cells at the apical surface — flat, cube, or column. Say the layering, then the shape.

One override: if the surface cells are big and domed and you are in the urinary tract, stop and call it transitional.

The plates

The eight specimens

Six of them follow the rules; the last two are the exceptions. Each plate gives you the drawing, where the tissue lives in the body, what to look for at the scope, its common look-alikes, and a slide to work on yourself.

YOur missions

Field Notes for Future Missions

Before you go to the plates, try the key on a specimen you have not been told the name of.

Field notes

Why a field guide

A list of eight tissue names is something you can hold in your headRounded proximal end that fits into the acetabulum of the hip bone. until Thursday. A procedure — find the lumen, count the rows of nuclei, check the basement membrane, name the shape — is something you still have in April, and it works on a slide nobody has labelled for you.

That is exactly the task at the microscope: looking at something unnamed and talking yourself to an answer out loud. So the eight pages that follow are written as specimen accounts rather than definitions. The plates are the specimens. This page is the key.