Histology Guide

virtual microscopy laboratory

Chapter 2 - Epithelium

Epithelium forms continuous sheets of cells that line internal surfaces and cover the external surface of the body. It is a selective barrier that protects tissues and is often involved in absorption or secretion. A basement membrane separates an epithelium from the underlying connective tissue.

Epithelia are classified based on three criteria:

Epithelial cells are polarized:

An epithelium does not contain blood vessels and receives nourishment via diffusion from the underlying connective tissue.

Glands are formed by the down growth of an epithelium into the underlying connective tissue (discussed in Chapter 12 - Exocrine Glands).

It is not necessary to learn the names of specific tissues for this chapter, but rather learn to recognize variations in epithelia.

Simple Squamous Epithelium

Simple squamous epithelium consists of a single layer of flattened cells. The thinness of these cells facilitates the transfer of materials (e.g., gases, fluids or nutrients) across the epithelium.

Simple Cuboidal Epithelium

Simple cuboidal epithelium consists of a single layer of cuboidal cells. This epithelium is often associated with absorption, secretion, or excretion of waste matter.

Simple Columnar Epithelium

Simple columnar epithelium consists of a single layer of cells that are taller than they are wide. This epithelium is often associated with absorption or secretion.

Pseudostratified Columnar Epithelium

Pseudostratified epithelium appears to be stratified because the nuclei of the epithelial cells are at different levels. However, every cell is in contact with the basement membrane, but not all cells reach the lumen.

Stratified Squamous Epithelium

Stratified squamous epithelium has multiple layers of cells becoming flattened as they move from the basal layer to the apical layers. It provides protection from abrasion and is keratinized on the external surface of the body.

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(H&E)
(keratinized and non-keratinized)
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(H&E)
(non-keratinized)
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(H&E)
(non-keratinized)
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(H&E)
(keratinized)
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(H&E)
(keratinized)
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(H&E)
(keratinized)
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(H&E)
(non-keratinized)

Stratified Cuboidal Epithelium

Stratified cuboidal epithelium has multiple layers of cells with an outermost layer of cuboidal cells. Limited distribution - found in the lining of larger ducts.

Transitional Epithelium

Transitional epithelium (urothelium) is adapted for extensibility and is restricted to the urinary tract. It has multiple layers of cells with an outermost layer of much larger, dome-shaped cells (umbrella cells) that change shape during contraction and distention.

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(H&E)
(relaxed and stretched bladder)