Epithelium is one of the four primary tissues in the human body. Epithelia are composed of closely aggregated cells that cover most body surfaces, cavities, and tubes. These include the outer surface of the body (skin), tracts traversing the body (gastrointestinal tract), dead-end tracts that have openings at the body surface (respiratory, urinary, and genital tracts), and ducts that open into these tracts (exocrine glands). The functions of epithelia are numerous, and a single epithelium may have several functions. The most important of these include physical protection and selective transport (diffusion, absorption, secretion).
Epithelia comprise a continuous sheet of cells that can have one or multiple layers. Cells in an epithelium are linked by several different types of cell junctions that provide physical strength and means of communication. A basement membrane lies beneath the epithelium and separates it from underlying tissue; because blood vessels do not penetrate the basement membrane, nutrients like oxygen and metabolites reach the epithelium by diffusion. Epithelia are polarized, with an apical surface that faces the external environment and a basal surface that faces the basement membrane.
Epithelia are classified based on three criteria:
Simple squamous epithelium is characterized by a continuous surface of irregularly shaped, flat cells supported by an underlying basement membrane. Note the appearance of simple squamous epithelium in section: the irregular cell boundaries, bulges in the area around the nuclei, and generally flat appearance. The thinness of simple squamous epithelium make it ideal for diffusion of gases.
Depending on its embryonic origin and its position in the adult body, simple squamous epithelium is referred to by a variety of names. The lining of blood and lymphatic compartments is called endothelium. Endothelial cells are long and polygonal-shaped, and form barriers of variable permeability that partially restrict the diffusion of macromolecules. The basement membrane here is thin and stains similarly to the cytoplasm, making it difficult to recognize. Also visible are red blood cells in the lumen of the blood vessel.
Simple cuboidal epithelia occur widely in the body in many glands and glandular ducts, such as the salivary ducts, pancreatic duct, bile duct, and kidney tubules. These epithelia are composed of cells that are short prisms with a top, bottom, and six sides. The side of the cell that faces the free surface is called the apex, and the side that faces the underlying tissues is the base. The apical surface of the epithelia forms the lumen. Note the pink-stained basement membrane that lies beneath the basal surface of the epithelia.
These cells are similar to cuboidal epithelia but are taller than they are wide. The nuclei are elongated and are often located to one side of the cell base. Simple columnar cells are prominent in epithelia where there is active absorption of material (e.g. small intestine, gall bladder).
Many cells in a simple columnar epithelium contain microvilli on their apical surface. When the cells have a lot of microvilli, it forms a brush border that is often visible in H&E-stained samples. This image shows the brush border of an epithelium from the intestine. Note the size of the microvilli relative to the cells that they cover. Microvilli are outward folds of the cell membrane that are supported by parallel arrays of actin filaments. Cutting microvilli in cross-section reveal the individual actin filaments.
In some pseudo-stratified epithelia, the cells become so crowded that the nuclei become displaced and form several rows in the epithelium. This makes the epithelium appear to be stratified, even though all of the cells are attached to the basement membrane, a criterion for simple epithelium. Pseudo-stratified epithelia are most commonly found along the respiratory airways.
Stratified squamous epithelia have two or more layers of cells, with a superficial squamous layer and basal layers that are usually cuboidal or columnar. The cells in the basal slowly migrate upward toward the surface of the epithelium, and along the way, their nuclei condense and the cells flatten. Eventually, the most superficial layer of cells is sloughed off and replaced by the layer of cells beneath. This type of epithelium can withstand abrasion because the loss of cells from the surface does not compromise the underlying tissue.
In this image note the layers of material on the apical surface of the epithelium. This is keratin which is prominent on the the apical surface of the epithelia of skin. Keratin protects against desiccation and abrasion and is composed of a combination of intermediate filaments and lipid. Keratin will be discussed in more detail in the lab on skin.
Transitional epithelia is a specialized form that is found int eh bladder. The hallmark of transitional epithelia is that the shape of the cells and number of layers of the cells depends upon the state of the bladder. When the bladder is filled with urine and becomes distended, the epithelia stretches to accommodate the increase in volume of the bladder and the cells become flatter with fewer layers. In an empty bladder, the decrease in volume of the bladder allows the epithelia to relax and cells become rounder and pile up into more layers.
Besides microvilli (see above), cilia and stereocilia are two other surface specializations that are prominent in certain epithelia. Cilia are composed of microtubules and the motor protein dynein. Dynein slides the microtubules back and forth within the cilia, creating a wave-like motion of the cilia. The synchronized beating of cilia is capable of moving large objects along the surface of an epithelium.
Although stereocilia share part of their name with cilia, they are very different structures. Stereocilia contain actin filaments instead of microtubules and are not motile. Stereocilia facilitate absorption of material and are prominent in the male reproductive tract.
All epithelial cells are held together by a set of junctional complexes. These complexes can be seen in electron micrographs. The tight junction is closest to the apical surface, followed by the adhering junction and desmosome. Note the relative locations of each junction and the distance between the two adjacent cells at each point. Tight junctions determine the permeability of epithelia by controlling which ions and molecules diffuse between epithelial cells. Adhering junctions and desmosomes hold together adjacent epithelia cells.
Gap junctions allow for direct exchange of small molecules and ions between adjacent cells. In electron micrographs, gap junctions can be identified as locations where the outer leaflets of the cell membranes of adjacent cells appear fused.
Underlying most epithelia is a basement membrane (basal lamina). The basement membrane provides structural and metabolic support to epithelia. It is usually 30 - 60 nm thick and made up of a network of collagenous and non-collagenous glycoproteins and proteoglycans. Integrins on the basal surface of epithlial cells interact with proteins of the basement membrane. This layer can best be seen in samples stained with the periodic acid Schiff (PAS) procedure