This epithelial tissue has many cell layers it forms the outer layer of the skin

Epithelial tissues cover all external and internal surfaces of the body. Think about it as a shrink wrap that continues into body cavities. If the endoscope can go there without puncturing the skin or mucosal membranes and “drawing blood”, the surface is covered by epithelium. Skin is covered by epithelium (that’s the epidermis layer), lips are covered by epithelium, the mouth cavity is, as well as the esophagus, stomach and intestines, you get the picture. Another organ system that branches off from the oral cavity is the respiratory system; so the trachea, bronchi, and all the way to the alveoli in the lungs, are covered by epithelium. Don’t forget that epithelium also covers places like the cornea, sclera of the eye, and the tympanic membrane in the ear.

In several places, epithelial sheets roll up into tubes and form the ducts of exocrine glands. Some epithelia form the majority of the organ, as seen in the thyroid gland and the liver. I bet you have never thought of the liver as a specialized epithelial tissue.

One particular type of epithelium, known as endothelium, lines all blood vessels, including capillaries and the chambers of the heart. After all,  blood vessels are hollow organs filled with blood. This epithelium has one important characteristic, it has anti-coagulation properties. The blood never sticks to it.

All epithelial tissues have these common characteristics:

1. They form sheets of tightly bound cells or roll into tubes.
2. Epithelial cells lie on the basement membrane.
3. Epithelial cells have two different “sides”—apical and basolateral.
4. The apical side always faces out of the body (outside or into a lumen).
5. There is a small amount, or an absence of, extracellular matrix.
6. They are avascular (meaning, they don’t have blood vessels) and get their nutrients via diffusion.
7. They have a high regenerative potential.

In addition to these common characteristics, epithelium has differences based on its location and function in the body. It is either thicker or thinner, it might secrete mucus, or it can attain special surface modifications to help its purpose. Surfaces that experience abrasive forces, such as the skin or the cornea, have multiple-layered epithelium, while single-layered epithelium is sufficient to cover internal organs. Epithelium in the upper respiratory system has cilia, membrane extensions that resemble tiny hairs. The epidermis of the skin maintains a layer of keratin fibers on top.

Epithelial tissues differ in the following ways:

1. Number of layers in the epithelial sheet.
2. The shape of the cells.
3. Apical surface modifications.
4. Additional functions (the primary function is always to create a barrier).

4.2.1 Classification of epithelia – number of cell layers 

Based on the number of layers, epithelium can be classified as:

• Simple
• Stratified
• Pseudostratified

Simple epithelium has one layer of cells lying on the basement membrane. Stratified epithelium has two or more layers of cells stacked on top of one another, usually to increase the strength of the barrier. Simple epithelia are found in places where there is not much mechanical impact on the surface, while stratified epithelia are located in places subject to more mechanical force.

There is also an exception, a single layered epithelium that appears multilayered under the microscope because the nuclei of the cells are at different levels. It is called pseudostratified (as in fake-looking stratified).

4.2.2.Classification of epithelia – shape of the cells

Based on shape, epithelial cells can be classified into one of 3 groups:

• Squamous
• Cuboidal
• Columnar

Squamous epithelial cells are flat and look like fried eggs with a nucleus that projects a little higher than the rest of the cytoplasm. Squamous epithelium can be single-layered (called simple) or multilayered (called stratified), see the classification based on the number of layers above. Squamous epithelium’s function is lining and covering.

Cuboidal epithelial cells are as wide as they are tall.  They can be single-layered or have a maximum of 2-3 layers. Cuboidal epithelium usually has some secretory function and is found within glands.

Columnar epithelial cells are taller than than they are wide. They are primarily simple columnar, arranged in a single layer. Stratified columnar epithelia are rare.  Columnar epithelia are located in organs with high secretory activity and often have apical surface modifications.

Each epithelium is therefore described in two words:  number of layers and the shape of the cells. There is, for example, simple squamous epithelium, simple cuboidal, simple columnar or stratified squamous, etc. As a rule, we place the word that denotes  the number of layers before the word that denotes shape. However, there are two exceptions to the rule. Pseudostratified is used as one word because it denotes the number of layers (one) and the shape of the cells (columnar but not really) at the same time. There is also another type of epithelium that defies the rules. It is transitional epithelium, a multilayered, kind of cuboidal, but different enough that it deserved its own name. It will be discussed later in the chapter together with the function.

4.2.3. Classification of the epithelia – function

Based on their function, epithelial cells can be divided into:

• Covering and lining epithelium.
• Absorptive epithelium.
• Secretory (a.k.a glandular epithelium).

Covering and lining epithelia

The principal function of all epithelia in the body is to cover the surface or line organs. The “covering and lining epithelia” name is reserved for the epithelia that hardly do anything else in addition to creating a barrier between the exterior and interior of the body. The epithelial barrier protects the organism from losing water (think about a scraped knee and the yellowish serum droplets on the surface when the barrier is broken), invasion of microbes, and from trauma. Simple epithelium is enough on the inside of the body. It is located on the surface of each organ (such as reproductive epithelium on the surface of an ovary, inward facing side of the cornea, or in the lungs). In places where the body is exposed to more mechanical stress, epithelium becomes stratified. Stratified epithelium forms the external part of the skin (and is called the epidermis) or the outside, externally facing side of the cornea and sclera of the eye.

4.2.4. Location of the epithelia

SIMPLE SQUAMOUS
location	special features/notes	image
innermost layer of the blood vessels	a.k.a. endothelium
lining of the heart chambers	a.k.a. endocardium
innermost layer of the lymphatics
external surfaces of internal organs	a.k.a. mesothelium
walls of alveoli (lungs)