All body surfaces except the teeth and the gliding surfaces of joints are normally covered with epithelium.

Epithelium is tightly-joined cells on top of a layer of collagen (the "basement membrane") which marks its border with connective tissue.

This extends to the tiny surfaces of glands. Even the glands whose ducts are lost during embryonic life (i.e., the endocrine glands) have epithelium as their business cells.

Here is how to name a surface epithlium:

Count the layers:

Simple: There's only one layer

Stratified: Looks like several layers, and is.

Pseudostratified / Transitional: Looks like several layers, but all the cells contact the basement membrane

What cell is on the top layer?

Squamous: Flat (scaly)

Cuboidal: About as wide as it is tall

Columnar: Much taller than it is wide.

This is a cluster of ducts in the pancreas. You can see the cell borders easily here.

Don't worry about what's at the far right -- it's a curious little type of duct slipping into a secretory unit.

With your study partners, find:

• simple cuboidal epithelium

Hotshots: We think that too much attention goes to memorizing where you're supposed to see "tall cuboidal" vs. "low columnar".

You do need to know:

Simple squamous: For permeability. Inside of the cardiovascular system ("endothelium"), lung air spaces, and body cavity inner linings ("mesothelium", on pleural, peritoneal, and pericardial cavities)

Stratified squamous: For protection. Skin, mucous membranes.

Transitional: For stretch. Urinary bladder.

Simple cuboidal: For ion pumping, i.e., ducts.

Simple columnar: High-volume secretion.

The presence of contractile myoepithelium as a second layer around the epithelium does not turn simple into stratified.

United States Pronunciations:

• Squamous: SKWAY-muss. (Not SQUAW-muss, please.)

Roots:

• Squamo-: scaly

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Kansas City University of Medicine and Biosciences