epithelium
pl. epithelia [Gr.] the cellular covering of internal and external surfaces of the body, including the lining of vessels and other small cavities. It consists of cells joined by small amounts of cementing substances. Epithelium is classified into types on the basis of the number of layers deep and the shape of the superficial cells. Standard classifications include ciliated (cilia attached), columnar (taller than wide), cuboidal (same height as width), pseudostratified (single layer but because of varying cell height appears to be more than one), simple (one cell layer), squamous (flattened, plate-like cells), stratified (more than one layer), transitional (variable number of layers apparent). Other types are listed below.
anterior epithelium
the thin layer of stratified squamous cells that form the outermost layer of the cornea.
corneal epithelium
see anterior epithelium (above).
germinal epithelium
thickened peritoneal epithelium covering the gonad from earliest development; formerly thought to give rise to germ cells.
glandular epithelium
that composed of secreting cells.
lens epithelium
cuboidal epithelium covering the lens.
pigmentary epithelium, pigmented epithelium
that made of cells containing granules of pigment.
posterior epithelium
the single layer of epithelial cells on the back of the cornea, between stroma and aqueous humour; the corneal endothelium.
retinal pigment epithelium
see retina.
sense epithelium, sensory epithelium
see neuroepithelium (1).
corneal epithelium
The outermost layer of the cornea consisting of stratified epithelium mounted on a basement membrane. It is made up of various types of cells; next to the basement membrane are the basal cells (columnar in shape), then two or three rows of wing cells and near the surface are two or three layers of thin surface squamous cells (or superficial cells). The outer surfaces of the squamous cells have projections (called microvilli and microplicae), which extend into the mucin layer of the precorneal tear film and are presumed to help retain the tear film. The epithelium in humans has a thickness of about 51 μm. Some dendritic cells of mesodermal origin are also normally present. Epithelial stem cells are located at the limbus; they give rise to the basal cells. The corneal epithelium receives its innervation from the conjunctival and the stromal nerves. The life cycle of epithelial cells is about a week (Fig. C20). See Langerhans' cell; epikeratoplasty; mitosis; pachometer; palisades of Vogt.
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Fig. C20 Diagram showing the various layers of the corneal epithelium
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