The Histology Guide

Epithelia: How to classify them.

Why do we need to classify epithelia?

The cells in epithelia have different shapes, and different types of epithelia have different numbers of layers of cells (from one to many). The shape of the cells, and their organisation are important to the particular function of each type of epithelia.

How do we classify epithelia?

There are three main criteria for classifying epithelia:

  1. How many layers of cells are there?
  2. What is the cell shape?
  3. Are there any specialisations on the free (apical) surface?

If a specialisation is present, then the type of specialisation present is included in the classification.

Read the classification rules below, then have a go yourself.


How many layers?

One layer - Simple Epithelium

diagram of simple epithelium

one layer of cells

Find out for more about simple epithelium.

Two or more layers - Stratified epithelium

diagram of stratified epithelium

two or more layers of cells.

Find out more about stratified epithelium

Pseudostratified epithelium

diagram of pseudostratified epithelium

one layer of cells, but the nuclei are at different heights, so it looks as though it is more than one layer. These epithelia usually have goblet cells present.

Find out more about Pseudostratified epithelium.

Cell Shape

When sectioned at right angles to their basement membrane the cells in different epithelia have a variety of shapes:

Squamous:

diagram of squamous epithelium

cells are flat, width is much greater than height.

Cuboidal:

diagram of cuboidal epithelium

cells appear approximately square.

Columnar:

diagram of columnar epithelium

height is greater than width.

Transitional

diagram of transitional epithelium

A special form of epithelium, in which the cells can alter their shape. When the epithelium is relaxed they appear cuboidal but when stretched they appear squamous.

Specialisations

Read the summary below, then find out more on the Epithelium Specialisations page.

Microvilli

diagram of cell with microvilli

Fine, finger-like projections which contain a central core of microfilaments. Increase the apical surface area in cells. Most developed in cells specialised for absorption (e.g. intestinal cells).

Cilia:

diagram of ciliated cell

Long, motile projections of the apical surface. These are longer than microvilli. They contain a core of microtubules and beat synchronously. They are found on cells lining the upper respiratory tract for example, where their rhythmic beating moves mucus upwards in the respiratory tract.

Keratin:

diagram of keratinised epithelium

is found in areas susceptible to abrasion and water loss (i.e. skin). Layers of the intermediate filament protein keratin are found on the apical surface.

Goblet cells.

diagram of goblet cell

Sometimes epithelia are specialised for secretion - and these cells are called Goblet cells. Goblet cells secrete mucus onto the apical surface. So, they are really 'glandular' epithelia.