surfaces of the cells that are in contact with alumen (i.e., kidney tubule cells and glandular duct cells) or external environment (i.e., surface cells of the epidermis).
surfaces of cells that are locked together by special junctional complexes with neighboring cells as well as the surface opposite the apical domain that is in contact with the basal lamina.
Epithelial Tissues may be derived from:
Ectoderm: I.e., the skin epidermis and glands
Endoderm: I.e., the lining of the gastrointestinal tract (including glands)
Mesoderm: I.e., the lining of blood vessels, mesothelium, Bowman’s capsule
Epithelial Cell Characteristics
Cells typically have a relatively uniform geometricshape when viewed from the side. Cells are tightly bound together. Tissues have relatively little intercellular matrix. Cells exhibit polarity Epithelial tissues do not contain blood vessels. Epithelial tissues sit on top of a basement membrane. Epithelial tissues line body cavities and cover body surfaces. Epithelial tissues form secretory and excretory parts of glands. Epithelial tissues may be innervated.
Classification of epithelial tissues
According to number of cell layers: Simple (one layer) vs. Stratified (more than one layer) According to the shape of the most superficial layer of cells as seen in a side view: Squamous (flattened), Cuboidal, Columnar According to the surface modifications on the apical domains of the outermost cell layer: Cilia, Stereocilia, Microvilli (brush border) According to the presence orabsence of keratin Lightly keratinized in esophagus and vagina Heavily keratinized in epidermis
Special categories of epithelium
Simple squamous epithelium
Lines lumina of ducts, vessels, and other tubular structures
Forms walls of alveoli, Bowman’s capsules and inner surfaces ofmembranous labyrinth and tympanic membrane
Simple Cuboidal epithelium
In side view, cells may be low or high (approaching almost squamous or almost columnar).
May have a brush border (kidney tubules) on apical surface composed of microvilli
Found on surface of ovary, pigmented epithelium of retina, kidney tubules,glands and ducts, terminal bronchioles, choroid plexus, and anterior capsule of lens of eye.
Simple Columnar Epithelium
May have a striated border (intestinal cells) on apical surface composed ofmicrovilli
Ciliated columnar epithelium is found in the upper respiratory tract, uterine tubes, uterus, paranasal sinuses, and the central canal of the spinal cord.
Non-ciliated columnar epithelium is found in the digestive tract beginning with the cardia of the stomach, the gall bladder, and in parts of the excretory ducts of glands.
Stratified Squamous Epithelium
Heavily keratinized stratified squamous epithelium is found in the epidermis with connective tissue papillae and in the cornea without connective tissue papillae.
Lightly or non- keratinized stratified squamous epithelium is found in the esophagus, vagina, lining of the mouth, tongue, and part of the epiglottis.
Stratified Cuboidal Epithelium
Seldom found, but may occur in small areas of the anal mucosa, large excretory ducts of some glands, and part of the male urethra.
Stratified Columnar Epithelium
Seldom found; occurs in the ducts of adult sweat glands, fornix of the conjunctiva of the eye, parts of the male reproductive/urinary tract, the pharynx, and the epiglottis.
Ciliated pseudostratified columnar epithelium is found in the trachea.
Pseudostratified epithelium with stereocilia is found in the epididymis. Stereocilia are similar to microvilli but are longer and are often branched.
Associated with urinary system
Referred to as urothelium
Functions of epithelial tissues
-Protection Especially stratified squamous epithelia