An Introduction to Epithelial Types
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Epithelium is a surface tissue.  By covering or lining an organ or other tissue, one side of the tissue is attached to the underlying substance of the body while the outer surface is free or unattached.  The shape of mature cells is always seen next to the Free Surface.  The attached surface is connected to the tissues so that the epithelium forms a continuous barrier or exchange surface.  This attachment is through a mix of fine fibers (from the underlying tissue) and a gel produced by the epithelium.  This thin - acellular (without cells) - layer is the basement membrane.  Epithelium does NOT have blood vessels in it.  Therefore, its thickness is limited by the diffusion of nutrients from blood vessels to epithelial cells.  Epithelium is always very thin indeed.

Epithelium is classifed according to three criteria: (1) the numbers of cell layers between free surface and basement membrane.  One cell layer = SIMPLE, multiple layers = STRATIFIED; (2) the shape of mature cells at the free surface.  Thin, flat cells (like pancakes) = SQUAMOUS.  Cells roughly as deep as wide = CUBOIDAL.  Cells deeper than wide = COLUMNAR.  Sometimes, modifications of surface cells require additional classification.  For example, some tissues have cilia at the free surface.  These are designated as ciliated columnar epithelium.  Certain types of squamous (flattened) cells have a layer of dried out, dead cells covering the epithelium.  This is referred to as CORNIFIED or KERITINIZED stratified squamous.

You need to know how to identify each of the tissue types and to know examples of where such tissues are found or used in the body.


To see examples of Epithelia or to practice your microscope skills, click on the MAJOR HEADINGS BELOW:


Unlabeled Pictures of Simple Epithelium - Good for Practice
Unlabeled Pictures of Stratified Epithelium -
Labeled Pictures of both types - for orientation
Dynamic Views as you see them through the microscope!

Some Basic Information on Epithlium
and where it is found

Simple Epithelium
Simple Squamous   serous membranes, endothelium, lung alveoli
Simple Cuboidal  glands, thyroid, sweat, kidney nephron
Simple Columnar  lining the gut
Ciliated Simple Columnar   small bronchi, oviduct
Pseudostratified Ciliated Columnar  lining trachea, upper bronchi

Stratified Epithelium

Stratified Squamous   lining of mouth, esophagus, urinary tracts, lining  of anus
Cornified Stratified Squamous Skin
Stratified Cuboidal RARE - Complex Sweat Glands
Stratified Columnar RARE - Nasal Passages, immature forms of epithelium
Transitional Urinary Bladder