The Function and Cell Types of Epithelial Tissue

The word tissue is derived from a Latin word meaning to weave.  Cells that make up tissues are sometimes woven together with extracellular fibers. Likewise, a tissue can sometimes be held together by a sticky substance that coats its cells. There are four main categories of tissues: epithelial, connective, muscle and nervous. Lets take a look at epithelial tissue. Epithelial Tissue Function Epithelial tissue covers the outside of the body and lines organs, vessels (blood and lymph), and cavities. Epithelial cells form the thin layer of cells known as the endothelium, which is contiguous with the inner tissue lining of organs such as the brain, lungs, skin, and heart. The free surface of epithelial tissue is usually exposed to fluid or the air, while the bottom surface is attached to a basement membrane.The cells in epithelial tissue are very closely packed together and joined with little space between them. With its tightly packed structure, we would expect epithelial tissue to serve some type of barrier and protective function and that is certainly the case. For example, the skin is composed of a layer of epithelial tissue (epidermis) that is supported by a layer of connective tissue. It protects the internal structures of the body from damage and dehydration.Epithelial tissue also helps to protect against microorganisms. The skin is the bodys first line of defense aga inst bacteria, viruses, and other microbes.Epithelial tissue functions to absorb, secrete, and excrete substances. In the intestines, this tissue absorbs nutrients during digestion. Epithelial tissue in glands secrete hormones, enzymes, and other substances. Epithelial tissue in the kidneys excrete wastes, and in the sweat glands excrete perspiration.Epithelial tissue also has a sensory function as it contains sensory nerves in areas such as the skin, tongue, nose, and ears.Ciliated epithelial tissue can be found in areas such as the female reproductive tract and the respiratory tract. Cilia are hair-like protrusions that help propel substances, such as dust particles or female gametes, in the proper direction. Classifying Epithelial Tissue Epithelia are commonly classified based on the shape of the cells on the free surface, as well as the number of cell layers. Sample types include: Simple Epithelium: Simple epithelium contains a single layer of cells.Stratified Epithelium: Stratified epithelium contains multiple layers of cells.Pseudostratified Epithelium: Pseudostratified epithelium appears to be stratified, but is not. The single layer of cells in this type of tissue contain nuclei that are arranged at different levels, making it appear to be stratified. Likewise, the shape of the cells on the free surface can be: Cuboidal - Analogous to the shape of dice.Columnar - Analogous to the shape of bricks on an end.Squamous - Analogous to the shape of flat tiles on a floor. By combining the terms for shape and layers, we can derive epithelial types such as pseudostratified columnar epithelium, simple cuboidal epithelium, or stratified squamous epithelium. Simple Epithelium Simple epithelium consists of a single layer of epithelial cells. The free surface of epithelial tissue is usually exposed to fluid or the air, while the bottom surface is attached to a basement membrane. Simple epithelial tissue lines body cavities and tracts. Simple epithelial cells compose linings in  blood vessels,  kidneys,  skin, and the  lungs. Simple epithelium aids in  diffusion  and  osmosis  processes in the body. Stratified Epithelium Stratified epithelium consists of epithelial cells stacked in multiple layers. These cells typically cover exterior surfaces of the body, such as the  skin. They are also found interiorly in portions of the  digestive tract  and  reproductive tract. Stratified epithelium serves a protective role by helping to prevent water loss and damage by chemicals or friction. This tissue is constantly renewed as  dividing cells  on the bottom layer move toward the surface to replace older  cells. Pseudostratified Epithelium Pseudostratified epithelium appears to be stratified but is not. The single layer of cells in this type of tissue contain  nuclei  that are arranged at different levels, making it appear to be stratified. All cells are in contact with the basement membrane. Pseudostratified epithelium is found in the respiratory tract and the  male reproductive system. Pseudostratified epithelium in the respiratory tract is  ciliated  and contain finger-like projections that help to remove unwanted particles from the  lungs. Endothelium Endothelial cells form the inner lining of the  cardiovascular system  and  lymphatic system  structures. Endothelial cells are epithelial cells that form a thin layer of simple squamous epithelium known as  the endothelium. Endothelium makes up the inner layer of vessels such as  arteries,  veins, and  lymphatic vessels. In the smallest blood vessels,  capillaries  and sinusoids, endothelium comprises the majority of the vessel.​ Blood vessel  endothelium  is contiguous with the inner tissue lining of organs such as the  brain,  lungs,  skin, and  heart. Endothelial cells are derived from endothelial  stem cells  located in  bone marrow. Endothelial Cell Structure Endothelial cells are thin, flat cells that are  packed closely together to form a single layer of endothelium. The bottom surface of endothelium is attached to a basement membrane, while the free surface is usually exposed to fluid. Endothelium can be continuous, fenestrated (porous), or discontinuous. With continuous endothelium,  tight junctions  are formed when the  cell membranes  of cells in close contact with one another join together to form a barrier that prevents the passage of fluid between the  cells. Tight junctions may contain numerous transport vesicles to allow the passage of certain molecules and ions. This can be observed in the endothelium of  muscles  and  gonads. Conversely, tight junctions in areas such as the  central nervous system  (CNS) have very few transport vesicles. As such, the passage of substances in the CNS is very restrictive. In  fenestrated endothelium, the endothelium contains pores to allow small molecules and  proteins  to pass. This type of endothelium is found in  organs  and glands of the  endocrine system, in the intestines, and in the  kidneys.   Discontinuous endothelium  contains large pores in its endothelium and is attached to an incomplete basement membrane. Discontinuous endothelium allows  blood cells  and larger proteins to pass through the vessels. This type of endothelium is present in the  sinusoids  of the liver,  spleen, and  bone marrow. Endothelium Functions Endothelial cells perform a variety of essential functions in the body. One of the primary functions of endothelium is to act as a semi-permeable barrier between body fluids (blood  and lymph) and the  organs  and tissues of the body. In blood vessels, endothelium helps blood to flow properly by producing molecules that prevent blood from clotting and  platelets  from clumping together. When there is a break in a blood vessel, endothelium secretes substances that cause blood vessels to constrict, platelets to adhere to injured endothelium to form a plug, and blood to coagulate. This helps to prevent bleeding in damaged vessels and tissues. Other functions of endothelial cells include: Macromolecule Transport RegulationEndothelium regulates the movement of macromolecules, gases, and fluid between the blood and surrounding tissues. The movement of certain molecules across the endothelium is either restricted or allowed based on the type of endothelium (continuous, fenestrated, or discontinuous) and physiological conditions. The endothelial cells in the brain that form the blood-brain barrier, for instance, are highly selective and allow only certain substances to move across the endothelium. The  nephrons  in the kidneys, however, contain fenestrated endothelium to enable the filtration of blood and the formation of urine.Immune ResponseBlood vessel endothelium helps cells of the  immune system  exit blood vessels to reach tissues that are under attack from foreign substances such as  bacteria  and  viruses. This process is selective in that  white blood cells  and not  red blood cells  are allowed to pass through the endothelium in this manner .Angiogenesis and LymphangiogenesisThe endothelium is responsible for angiogenesis (creation of new blood vessels) and lymphangiogenesis (new lymphatic vessel formation). These processes are necessary for the repair of damaged tissue and tissue growth.Blood Pressure RegulationEndothelial cells release molecules that help to constrict or dilate blood vessels when needed. Vasoconstriction increases blood pressure by narrowing blood vessels and restricting blood flow. Vasodilation widens vessel passages and lowers blood pressure. Endothelium and Cancer Endothelial cells play a critical role in the growth, development, and spread of some  cancer cells. Cancer cells require a good supply of oxygen and nutrients to grow. Tumor cells send signaling molecules to nearby normal cells to activate certain  genes  in the normal cells to produce certain  proteins. These proteins initiate new blood vessel growth to tumor cells, a process called tumor angiogenesis. These growing tumors metastasize, or spread, by entering blood vessels or lymphatic vessels. They are carried to another area of the body via the  circulatory system  or the  lymphatic system. The tumor cells then exit through the vessel walls and invade surrounding tissue. Sources: Alberts B, Johnson A, Lewis J, et al. Molecular Biology of the Cell. 4th edition. New York: Garland Science; 2002. Blood Vessels and Endothelial Cells. Available from: (http://www.ncbi.nlm.nih.gov/books/NBK26848/)Understanding Cancer Series. Angiogenesis. National Cancer Institute. Accessed 08/24/2014