It is formed of a single layer of cells, resting on the basement membrane. Simple epithelium occurs mainly on secretory and absorptive surfaces.
- Squamous Epithelium consists of a layer of thin, flat, scale-like cells with prominent nuclei. The cells have irregular boundaries that loosely into those of neighbouring cells. It forms the inner lining of lung alveoli and is involved in functions like forming a diffusion boundary and prevents underlying parts from injury.
- Cuboidal Epithelium has cells which are polygonal in outline, but appear cuboidal in vertical section. It covers salivary and pancreatic ducts and thyroid vesicles.
- Columnar Epithelium is characterised by the presence of tall elongated shaped like polygonal columns. Columnar epithelium covers the inner surface of the intestine, stomach and gall bladder. It also occurs in gastric and intestinal glands. Its function is secretion or absorption. The intestinal mucosa is lined by Brush-Bordered Columnar Epithelium which is highly absorptive.
- Ciliated Epithelium consists of columnar or cubical cells bearing cilia on their free surfaces. The function of the cilia is to move particles, free cells or mucus in a specific direction over the epithelial surface. Ciliated epithelium lines the inner surfaces of some hollow organs such as Fallopian tubes, bronchioles and small bronchi. Ciliated columnar epithelium lining the ventricles of brain and spinal canal is called as ependyma. Cilia is of two types (i) Kinocilia are motile cilia with 9 + 2 organisation, (ii) Stereocilia — Basal granule absent, non-motile, 9 + 2 organisation is absent. Stereocilia are found in some parts of the male reproductive tracts such as the epididymis and Vas deferens.
- Pseudostratified Epithelium covers the inner linings of trachea and large bronchi. Although made up of a single layer of columnar cells, it appears two-layered, because some cells are shorter than the others and have their nuclei at a different level. The shorter cells lack cilia and secrete mucus which traps particles on the epithelial surface. The longer cells are ciliated. The ciliary movement propels the mucus and the particles towards the larynx.