Epithelial Tissue

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Bailey, Regina. "Epithelial Tissue." ThoughtCo, Feb. 3, 2017, thoughtco.com/animal-anatomy-epithelial-tissue-373206. Bailey, Regina. (2017, February 3). Epithelial Tissue. Retrieved from https://www.thoughtco.com/animal-anatomy-epithelial-tissue-373206 Bailey, Regina. "Epithelial Tissue." ThoughtCo. https://www.thoughtco.com/animal-anatomy-epithelial-tissue-373206 (accessed September 26, 2017).
Ciliated Epithelial Cells
This colored scanning electron micrograph (SEM) of a section through the wall of a trachea (wind pipe) shows ciliated epithelial cells. Credit: Steve Gschmeissner/Science Photo Library/Getty Images

What Are Tissues?

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. Let's 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 continuous 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 body's first line of defense against 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.

    Animal Tissue Types

    To learn more about animal tissues, visit:

    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, kidneysskin, 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 cardiovascular system and lymphatic system structures. Endothelial cells are epithelial cells that form a thin layer of simple squamous epithelium known as 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 continuous 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 and 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 Regulation

      Endothelium 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 Response

      Blood 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 Lymphangiogenesis

      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 Regulation

      Endothelial 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 (http://www.cancer.gov/cancertopics/understandingcancer/angiogenesis/AllPages)