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 the skin, the oral cavity, and
many of the glands in 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 that 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 the 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 also 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. Epithelial tissue can be
classified based on their cell shape or their number of cell 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 finger like projections called micro villi, which is often termed the brush
border. Micro villi helps to increase the surface area for digesting and absorbing nutrients in the
intestine. Based on cell 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 a 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 a 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. And transitional
epithelium is found in the urinary tract. There is also what is termed keratinized stratified
squamous epithelium, in which the top superficial layers of cells are dead. As new cells are produced
in the basal bottom region, they migrate to the top and they produce a protein called keratin.
These cells fill with the keratin, which make them very strong, but they lose their organelles
and nuclei and die. The epidermis or outer layer of the skin consists of keratinized stratified
squamous epithelium. As you can see, epithelial tissue and it's cells have many different forms
that fit its many different functions in the body. And that be the basics on the epithelial tissue.