Epithelial tissues are sheets of cells that cover the surface of the body and the outside and inside of many internal organs. The main functions of epithelial tissues include protection, secretion, absorption, and filtration. Cells within epithelial tissues are tightly packed with little extracellular space. Epithelial tissues have an apical surface and a basal surface. The apical side is a free surface not in contact with other cells, and the basal surface is attached to the underlying connective tissue via a basement membrane, an extracellular layer that anchors epithelial tissue to surrounding tissue. The basement membrane contains two noncellular layers made up of proteins and carbohydrates, the basal lamina and the reticular lamina. The basal lamina is secreted by the epithelium, and the reticular lamina is secreted by the underlying connective tissue. The basement membrane strengthens the epithelium and prevents it from tearing apart from the connective tissue.
The apical surface of epithelium in the lumen, or inside, of some tubular organs may contain microvilli or cilia. Microvilli are fingerlike extensions of the plasma membrane that increase the surface area of the cells. The increased surface area from microvilli in the small intestine allows an increase in the rate of nutrient absorption. Cilia are hairlike extensions that aid in motility across the surface, such as moving mucus out of the respiratory system. Epithelial tissue is avascular (no blood supply), relying on diffusion from the blood capillaries in the adjacent connective tissue for the exchange of nutrients, wastes, and gases.
Epithelial tissues are classified based on the cell shape of the apical cells and the number of layers of cells. Cell shape can be squamous, cuboidal, or columnar. A squamous cell is a type of cell in epithelial tissue that has a flat, thin shape. A cuboidal cell is a type of cell in epithelial tissue that has a box-like shape. A columnar cell is a type of cell in epithelial tissue that is tall and has a cuboid shape. A special case is made for transitional epithelial tissue found in the wall of the urinary bladder. The cell shapes change based on how much the organ is stretched. As the bladder fills, a combination of cuboid, columnar, and dome-shaped cells stretch and flatten to look squamous.
The number of cell layers in epithelial tissue determines whether the tissue is classified as simple or stratified. Simple epithelium is epithelial tissue that is one cell layer thick. Stratified epithelium is epithelial tissue that contains two or more layers of cells. In epithelial tissue with many layers, such as in the skin and alimentary canal of the digestive system, the basal cells undergo cell division to replace exfoliated apical cells.
|Type of Epithelial Tissue||Functions||Representative Locations|
|Simple squamous||Material exchange via diffusion and filtration across membranes||Lung alveoli, kidney glomeruli, blood and lymphatic vessels, membranes|
|Simple cuboidal||Secretion and absorption||Glands, ducts, kidney tubules|
|Simple columnar||Secretion and absorption; ciliated cells propel secreted mucus across the apical surface, and absorptive cells contain microvilli to increase surface area.||Alimentary tract of the digestive system, respiratory tract, uterus, ducts, gallbladder|
|Pseudostratified columnar (appears stratified because the height of cells in the single layer varies)||Secretion||Respiratory tract, ducts|
|Stratified squamous||Protection of other tissues from abrasions||Skin, lining of mouth, esophagus, vagina|
|Stratified cuboidal||Secretion of sweat, saliva, milk, and some hormones||Ducts of large glands|
|Stratified columnar||Mucus secretion||Anus|
|Transitional||Stretching, protection from harsh conditions of urine||Urinary bladder|