Epithelial Tissue - What Is Epithelial Tissue - Functions Of Epithelial Tissue - Epithelial Cells

In this video we look at what is epithelial tissue, some of the functions of epithelial tissue, and the different types of epithelial cells.
Epithelial tissue

What is epithelial tissue, where is it located, and what are its functions structure?

Epithelial tissue is one of the four major types of tissue in the body and it can be found throughout many parts of the body. It lines many of the structures of the respiratory tract such as the trachea, bronchi, bronchioles and alveoli, which are the tiny air sacs in the lungs. It also lines most of the digestive tract, the epidermis of skin, the oral cavity and many of the glands of the body.

Epithelial tissues have some common characteristics. It is comprised of tightly packed cells, with very little extracellular space. They also have an apical, or free surface that is exposed to an internal body space, or the external environment, and a basal, or deep surface that is attached to a thin basement membrane, which is connected to connective tissue, as you can see here.

They also lack blood vessels, which is called avascular, so they get nutrients from their apical free surface or by diffusion across their basal surface from the underlying connective tissue. Epithelial tissue is also innervated, which means they have a rich supply of nerves, and epithelial cells can reproduce themselves, which is important because they go through severe wear and tear, such as in the skin, and respiratory and digestive tracts.

Epithelial tissue has several important functions. It provides protection, as the skin protects the internal body from bacteria and other harmful substances. It provides sensory functions in the skin, nose, eyes and ears. Epithelial cells are important in secretions, as they secrete hormones, sweat, digestive juices and mucus. And epithelial tissue is important in absorption, such as nutrients in the gut, and the exchange of gases in the lungs.

Cell structure
Epithelial cells are classified based on their shape or their number of layers.

Based on shape they can be squamous, which are flat, wide and a bit irregular, these are found in the air sacs in the lungs. They can be cuboidal, which are similar to a cube, about as tall as wide, and have a spherical nucleus. Columnar cells are taller than they are wide, and have an oval shaped nucleus. Pseudostratified cells, as you can see here, have varying heights, as not all of them reach the apical or top surface, but they all do connect to the basement membrane.

Some epithelial cells may contain goblet cells, which secrete mucin, which forms mucus that helps with lubrication and protection, and some cells have extensions called cilia, which are sensory organs and also provide movement of mucus away from the lungs and toward the mouth. And some epithelial cells have tiny fingerlike projections called microvilli which is often termed the brush border. Microvilli helps to increase the surface area for digesting and absorbing nutrients in the intestine.

Based on layers, epithelial cells are classified in three different ways, simple, stratified, and pseudostratified. Simple epithelium is only one cell layer thick, and all cells are in direct contact with the basement membrane. There is simple squamous, simple cuboidal, and simple columnar epithelium.

Stratified epithelium have two or more layers of cells. Only the deepest basal layer cells have direct contact with the basement membrane and their formation looks like a brick wall. This formation provides better structural support and protection from wear or tear. Again, there are stratified squamous, stratified cuboidal, and stratified columnar epithelium.

And pseudostratified epithelium is comprised of pseudostratified cells we discussed earlier.

Two other notes regarding epithelial tissue. There are also transitional epithelium, which can be in a relaxed or stretched state. In a relaxed state, they have umbrella looking rounded cells at the top apical surface, and the bottom basal cells appear cuboidal. In a stretched state, the top cells flatten and appear squamous. Some of these cells will also have two nuclei. Transitional epithelium is found in the urinary tract.

Timestamps
0:00 Epithelial tissue in the body
0:22 Common characteristics of epithelial tissues
1:11 Functions of epithelial tissue
1:45 Epithelial cells based on shape - squamous, cuboidal, columnar, pseudostratified
2:42 Epithelial cells based on layers - simple, stratified, pseudostratified
2:52 Simple epithelium
3:03 Stratified epithelium
3:32 Transitional epithelium
3:55 Keratinized stratified squamous epithelium

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