BIOL 302 – Histology - Epithelial Tissues
Definition: Membranous tissue composed of one or more layers of cells separated by very little
intercellular substance and forming the covering of most internal and external surfaces of the body and
its organs. Epithelia are sheets of cells that are linked tightly together by intercellular junctions. The
major function of epithelia is to form barriers between two compartments.
Lies on basement membrane
Apical surface faces outside
Cells readily divide to make more cells; helps for recovery after an abrasion
No blood vessels; epithelial is often damaged when things move against it, it would not make
sense for blood vessels to be present since you would bleed every time you scraped your skin or
swallowed something rough
Tight junctions are present; to act as a barrier so fluids don’t leak in or out
Protection: abrasion resistance through dead epithelial cells at surface that can be sloughed off
(external) and mucous secreting cells (internal).
Absorption: epithelial facing the digestive tract must be very good at absorbing or else we would
not get nutrients we need. Folds in small intestine increase surface area for increased
Secretion: our bodies sometimes need to release materials, like hormones into the blood, and
epithelial cells must be able to accommodate this. Pancreatic cells secrete insulin and glucagon.
Transport: striation ducts in salivary glands allow for transport of IgA antibody to neutralize any
pathogens in substances being passed into the body.
Barrier: tight junctions act as a barrier between the inside and outside world. They force trans-
Types of Epithelial Layers:
Simple: single layer of cells; each cell is attached to basal lamina and has an apical surface.
Stratified: has two or more layers of cells; the cells on the apical surface and basal lamina are
separated by layers of cells.
Pseudostratified: all cells rest on basal lamina, but not all at apical surface. The bottom nuclei
look crowded; the more apical nuclei are messy, uneven, and are elongated.
Shapes of Epithelial Cells:
Squamous: flat in cross sections, thin, nucleus bulging
Cuboidal: round nucleus in center of cell
Columnar: taller than they are wide, elongated nuclei located at the bottom of cells
Types of Epithelial Tissue:
Simple Squamous: one layer, very thin, offers little protection, functions to allow materials to
Ex: endothelium (line blood vessels) in ear, esophagus, stomach, Bowman’s capsule o Keratinized Simple Squamous: superficial layer of dead cells, no nuclei; resists abrasion
and prevents moisture loss. Ex: Skin.
Simple Cuboidal: offers some protection, more commonly found where there is a lot of
secretion and absorption.
Ex: distal and collecting duct of kidneys, thyroid gland
Simple Columnar: offers some protection and where secretion or absorption is necessary.
Ex: digestive tract, goblet cells, oviducts, kidneys
**It is the top layer that determines classification of stratified tissues!**
Stratified Squamous: uppermost cellular layer, usually made up of many layers, offers most
protection to underlying tissue. Keratinized cells have lost their nucleus, ex outermost layer of
Ex: tongue, skin, esophagus.
Stratified Cuboidal: found where secretion is important.
Ex: sweat ducts, salivary glands
Stratified Columnar: transitional tissue.
Ex: lining of large salivary ducts
Transitional tissue: specialized stratified in bladder to accommodate stretch. When unstretched,
they appear cuboidal, pale, called pillow cells. When stretched, they become compressed and
appear flat. Have prominent nucleus: round with nucleoli visible as a dot in the middle of the
Small cylindrical projections, 0.5-1 um
Core contains actin bundles so they cannot actively move – non motile
Present on luminal surface
Increase surface area for increased absorption – commonly found in digestive tract, kidney
Brush border/”striated” border under light microscope – cannot see individual microvilli, length
of border <50% of nucleus.
Modified microvilli; specialized
Long, 75% of cell length
Sparse and ragged looking
Found in inner ear
Exact function unknown, possible absorptive role