Cell Types and Modifications

Epithelia can be divided into one cell layer (simple) and two or more layers (stratified). This can be further divided into cell types:

Squamous, where the width of the cell is greater than the height. Cuboidal, where the width, depth and height are approximately the same. Columnar, where height of cell greatly exceeds width.

Pseudostratified columnar have the appearance of being stratified, however some cells do not reach the free surface. However, ALL cells rest on the basal lamina. It can be found in the upper respiratory tract (trachea and bronchi), epididymis, and ductus deferens.

Transitional is stratified and functional accommodates distension and serves as a barrier. It can be found in the renal calyces, ureters, urinary bladder, and urethra.

Basal lamina (aka the Basement Membrane) is the structural attachment site for overlying epithelial cells and underlying connective tissue. Its components are synthesized and secreted by epithelial cells, and assembly occurs extracellularly at the base of their base. The basal lamina can be demonstrated with hematoxylin & eosin, although it usually requires special stains to visualize (i.e. periodic acid-schiff stain (PAS) or silver salts). Transmission electron microscopy (TEM) shows discrete structure of electron-dense material. It consists of two layers:

External lamina is a peripheral extracellular electron-dense material visible in TEM. It also has positive staining with PAS and silver salts. It is found on the surface of non-epithelial cells, including muscle and nerve-supporting cells (i.e. Schwann cells).

Cell surface modifications and elements of the cytoskeleton include the following:

Microvilli: cytoplasmic extensions at the apical cell surface seen with light microscopy (LM). Seen in cells of the gut and kidneys involved in transporting fluids. They provide an enormous increase in free surface area and contain a conspicuous core of actin microfilaments.
Stereocilia: extremely long immotile processes extending from the apical cell surface (also composed of actin filaments). Found in the epididymis and in the sensory hair cells of the inner ear. They serve as a receptor device (rather than an absorptive structure).
Cilia: Hair-like extension of the apical plasma membrane present on nearly every cell in the body. Contain axoneme, which has a microtubule-based core and extends from the basal body (i.e. a microtubule organizing center (MTOC)). There are three classes of cilia:

Lateral and basal cell surface folds and processes: invaginations and evaginations of the cell surface. This creates interdigitating and interleaving tongue and groove margins for opposed cells. Prominent in cells that transport fluid rapidly, such as the intestinal epithelium (i.e. water in intestine enters the cells apically and leaves at the lateral surface). Water osmotically follows sodium ions that are actively transported across the lateral plasma membrane.

Cell-to-Cell Adhesion/Junction: Epithelial cells are tightly adherent to each other and to the underlying extracellular matrix through junctions.
Terminal bar: Located in the apical part of the cell. It has a bar-like configuration, as seen in LM. TEM shows it to be the site of specialized attachment of adjoined cells. It is the barrier site to the diffusion of molecules across the epithelium, containing the junctional complex. The junction complex consists of the following: