Types of surface epithelium
Surface epithelia cover the external and internal surfaces of the body
epithelial tissues, epithelial tissue, integument, columnar epithelium, squamous epithelium, cuboidal epithelium, mucous membrane, keratinised, non-keratinised, single layer, stratified, ciliated epithelium, endothelium, absorptive surface, keratinous layer, callus, human, animal, biology
Non-keratinized stratified squamous
Keratinized stratified squamous
The cells that make up the simple squamous epithelium are flat and irregularly-shaped; they are arranged in one layer on the basement membrane. This thin and vulnerable type of tissue is found in the body where mechanical protection is not needed and where absorption, secretion and filtration occur. Simple squamous epithelium is found for example in the lining of the alveoli in the lungs and in the inner lining of the walls of blood vessels (where it is called the endothelium).
The cells that make up the simple cuboidal epithelium are nearly cube-shaped and arranged in one layer on the basement membrane. Nuclei are located in the center of the cells. This type of epithelium is found in simple cuboidal epithelia on the surface of the ducts of certain glands, in the lining of renal tubules and on the surface of the ovaries.
The cells that make up the simple columnar epithelium are cylindrical; their elongated nuclei are located near the base of the cell. This type of epithelium is typically found in the epidermis of invertebrates. In the human body, simple columnar epithelium is found in the lining of intestines, where the epithelial cells are covered with microvilli, and in the inner lining of oviducts, where the epithelial cells are ciliated.
Cellular nuclei of the pseudostratified ciliated columnar epithelium are found in several layers, but the cells form a single epithelium with all the cells resting on the basement membrane. Goblet cells that secrete mucus are typically found in this type of tissue.
This tissue lines most of the lower respiratory tract: the mucus and debris are continuously moved up towards the pharynx by the rhythmic movement of cilia. As nicotine paralyzes the cilia, the respiratory tract in smokers is not cleaned properly, which makes them cough.
In the non-keratinized stratified squamous epithelium, only the lowest layer of cells rests on the basement membrane. Cells become flattened towards the top. This tissue provides more protection than simple epithelia. As it does not keratinize, it does not prevent cutaneous respiration. It is therefore typically found in the epidermis of fish. In the human body, it is found where mechanical protection is needed: in the mouth cavity and the pharyngeal cavity, in a part of the esophagus, around the anus and in the vagina.
The keratinized stratified squamous epithelium is the most resistant type of epithelial tissue. The elongated cells of the lowest layer divide, producing new epitelial cells that are pushed towards the surface. Meanwhile, they become flattened and keratin protein accumulates within them. They undergo apoptosis (or programmed cell death) and a keratinous layer is formed on the surface.
The thick keratinous layer plays an important role in life on dry land, as it decreases water loss by evaporation and increases tissue resistance. However, it prevents cutaneous respiration and it is therefore typically found in animals with well-developed lungs: reptiles, birds and mammals. The epithelium of amphibians is not fully keratinized and does not prevent cutanewous respiration.
Keratinized stratified squamous epithelium constitutes the external layers of our skin. Its thickness depends on the amount of mechanical stress: physical work often causes calluses on the skin of the palm of our hands.
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