It forms boundaries between different environments
Many functions : protection, absorption, filtration, excretion, secretion, and sensory reception
Special characteristics of epithelium
Cellularity : composed of tightly packed cells
Specialized contacts
: cells are bound together at many points by tight junctions, desmosomes and other types of lateral contacts
Polarity
: all epithelia have a apical and basal surface
Basement membrane
: basal lamina – between epithelium and the connective tissue deep to it. It’s a selective filter and a scaffolding. Reticular lamina is deep to the basal (but still superficial to the connective tissue).
Innervated but avascular
: "lots o’ nerves; no blood vessels."
Regeneration
: high regenerative capacity
Classification of epithelia
Simple : single layer of cells; found where absorption and filtration occur
Stratified
: two or more layers of cells; common in high abrasion areas
All epithelial cells are roughly six-sided, but they have different heights
Squamous
: flat, scale-like. The nucleus appears as a disk.
Cuboidal
: about as tall as they are wide. The nucleus is spherical.
Columnar
: column shaped, taller than they are wide. The nucleus is elliptical.
Classes of epithelia
Simple layers
Simple squamous : flat, sparse cytoplasm. Simplest of the epithelia. Found in alveoli, kidney glomeruli, lining of the heart, blood vessels, lymphatic vessels, serosae. It’s designed to allow quick diffusion of materials.
Simple cuboidal
: located in kidney tubules; ducts and secretory portions of small glands, ovary surfaces. Its functions are to secrete and absorb.
Simple columnar
: some bear cilia on apical surface, may contain goblet cells. Found in lining of the digestive tract, gallbladder, excretory ducts of some glands. Ciliated variety is found in small bronchi, uterine tubes, some regions of the uterus. Several functions including absorption, mucus secretion, enzyme secretion.
Pseudostratified epithelium
: single layer of cells with differing heights. Many may have goblet cells and/or cilia. Found in large gland ducts, parts of male uethra, ciliated type in upper trachea and upper respiratory tract. Its main function is secretion.
Stratified layers
: two or more layers of cells. Each is named for the dominant cell type.
Stratified squamous
: mostly squamous, but basal cells are cuboidal or columnar. The apical surface cells may be keratinized (and dead). Functions as protection, especially in high abrasion areas such as the skin, moist lining of the mouth and esophagus.
Transitional
: no defined layers. It is almost a cross between stratified squamous and stratified cuboidal. It is a stretchy tissue and is found in the ureters and urinary bladder.
Rarely found are stratified cuboidal and stratified columnar