Simple epithelium is found generally on secretary and absorptive surfaces. It seldom covers surfaces exposed to mechanical or chemical abrasions. It is formed of a single layer of cells resting on the basement membrane. There are various types of simple epithelium:
Simple squamous epithelium.
Squamous epithelium consists of cells, which are thin, flat and polygonal and with prominent round or oval nuclease. The cell have irregular boundaries that fit closely into those of neighboring cells. For this reason, such an epithelium is also called as pavement epithelium. The main function of squamous epithelium is protection of the underlying tissue. It forms the inner-lining of lung alveoli, blood vessels and peritoneum of body cavity. The outermost layer of skin of fog is also made up of squamous epithelium.
Cuboidal epithelium is made up of cells, which are polygonal in out line but appear cubical or cuboidal in vertical section. In addition to protection, these cells participate in secretion (gastric juice, hormones, etc), excretion and absorption. The cells of cuboidal epithelium in absorption surfaces often bear microvilli on their free ends. This type of epithelium is found in proximal tubules of kidneys, lines of small salivary glands, pancreatic ducts thyroid follicles and ovary.
Columnar epithelium is characterized by the presence of tall pillar like cells, which resemble polygonal columns. The oval nucleus is generally present at the base of the cell. The function of columnar epithelium is secretion or absorption and it is found in the inner surface of the intestine, stomach and gall bladder. A columnar epithelium also occurs in gastric and intestine glands. The intestinal mucosa in lined by columnar epithelium, whose free (luminar) ends are thrown into tiny finger like projections (microvilli). This type of projections, which are formed to increase the absorptive surface, are often referred to as brush border.