These junctions influence the shape and folding of the epithelial tissue. In contrast with the tight and anchoring junctions, a gap junction forms an intercellular passageway between the membranes of adjacent cells to facilitate the movement of small molecules and ions between cells. These junctions thus allow electrical and metabolic coupling of adjacent cells. Epithelial tissues are classified according to the shape of the cells composing the tissue and by the number of cell layers present in the tissue.
Figure 4. Similarly, cells in the tissue can be arranged in a single layer, which is called simple epithelium, or more than one layer, which is called stratified epithelium.
Transitional describes a form of specialized stratified epithelium in which the shape of the cells, and the number of layers present, can vary depending on the degree of stretch within a tissue. Epithelial tissue is classified based on the shape of the cells present and the number of cell layers present. Watch this video to find out more about the anatomy of epithelial tissues.
Where in the body would one find non-keratinizing stratified squamous epithelium? The cells in a simple squamous epithelium have the appearance of thin scales. The nuclei of squamous cells tend to appear flat, horizontal, and elliptical, mirroring the form of the cell. Simple squamous epithelium, because of the thinness of the cells, is present where rapid passage of chemical compounds is necessary such as the lining of capillaries and the small air sacs of the lung.
This epithelial type is also found composing the mesothelium which secretes serous fluid to lubricate the internal body cavities. In simple cuboidal epithelium , the nucleus of the box-like cells appears round and is generally located near the center of the cell. These epithelia are involved in the secretion and absorptions of molecules requiring active transport. Simple cuboidal epithelia are observed in the lining of the kidney tubules and in the ducts of glands.
In simple columnar epithelium , the nucleus of the tall column-like cells tends to be elongated and located in the basal end of the cells. Like the cuboidal epithelia, this epithelium is active in the absorption and secretion of molecules using active transport.
Simple columnar epithelium forms a majority of the digestive tract and some parts of the female reproductive tract. Ciliated columnar epithelium is composed of simple columnar epithelial cells with cilia on their apical surfaces. These epithelial cells are found in the lining of the fallopian tubes where the assist in the passage of the egg, and parts of the respiratory system, where the beating of the cilia helps remove particulate matter.
Pseudostratified columnar epithelium is a type of epithelium that appears to be stratified but instead consists of a single layer of irregularly shaped and differently sized columnar cells. In pseudostratified epithelium, nuclei of neighboring cells appear at different levels rather than clustered in the basal end.
The arrangement gives the appearance of stratification, but in fact, all the cells are in contact with the basal lamina, although some do not reach the apical surface. Pseudostratified columnar epithelium is found in the respiratory tract, where some of these cells have cilia.
Both simple and pseudostratified columnar epithelia are heterogeneous epithelia because they include additional types of cells interspersed among the epithelial cells. For example, a goblet cell is a mucous-secreting unicellular gland interspersed between the columnar epithelial cells of a mucous membrane Figure 4. A stratified epithelium consists of multiple stacked layers of cells. This epithelium protects against physical and chemical damage. The stratified epithelium is named by the shape of the most apical layer of cells, closest to the free space.
Stratified squamous epithelium is the most common type of stratified epithelium in the human body. The apical cells appear squamous, whereas the basal layer contains either columnar or cuboidal cells. The top layer may be covered with dead cells containing keratin. The skin is an example of a keratinized, stratified squamous epithelium. Alternatively, the lining of the oral cavity is an example of an unkeratinized, stratified squamous epithelium.
They also contain communicating junctions gap junctions that allow allow communication between adjacent cells. Click here to find out about cell-cell junctions.
Cells in epithelia show a polarity along the axis between the external and internal environment, and their apical free surfaces often exhibit modifications. Selective diffusion - for example transfer of gases, nutrients and waste products between the blood and surrounding tissues.
These epithelia are usually simple squamous type, to provide the smallest barrier to diffusion. Click here to see some simple squamous epithelia. These epithelia are usually columnar, as the cells contain lots of endoplasmic reticulum, and golgi, for secretion, and they often have microvilli, to increase the surface area of the apical surface for absorption.
Click here to see some examples of columnar epithelia. Physical protection - for example, the skin protects underlying tissues from sunlight, heat, cold, abrasion, etc. Because the tissues form coverings and linings, the cells have one free surface that is not in contact with other cells. Opposite the free surface, the cells are attached to underlying connective tissue by a non-cellular basement membrane. This membrane is a mixture of carbohydrates and proteins secreted by the epithelial and connective tissue cells.
Epithelial Tissue Cells Cells Locations Function Simple squamous epithelium Air sacs of the lungs and the lining of the heart, blood vessels and lymphatic vessels Allows materials to pass through by diffusion and filtration, and secretes lubricating substances Simple cuboidal epithelium In ducts and secretory portions of small glands and in kidney tubules Secretes and absorbs Simple columnar epithelium Ciliated tissues including the bronchi, uterine tubes, and uterus; smooth nonciliated tissues are in the digestive tract bladder Absorbs; it also secretes mucous and enzymes.
Pseudostratified columnar epithelium Ciliated tissue lines the trachea and much of the upper respiratory tract Secrete mucous; ciliated tissue moves mucous Stratified squamous epithelium Lines the esophagus, mouth, and vagina Protects against abrasion Stratified cuboidal epithelium Sweat glands, salivary glands, and mammary glands Protective tissue Stratified columnar epithelium The male urethra and the ducts of some glands.
Secretes and protects Transitional epithelium Lines the bladder, urethra and ureters Allows the urinary organs to expand and stretch. Simple squamous epithelium. Allows materials to pass through by diffusion and filtration, and secretes lubricating substances. Simple cuboidal epithelium. Simple columnar epithelium. Ciliated tissues including the bronchi, uterine tubes, and uterus; smooth nonciliated tissues are in the digestive tract bladder. Pseudostratified columnar epithelium. Stratified squamous epithelium.
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