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.
- Simple squamous can be found lining the blood vessels, Bowman’s capsules in kidneys, alveoli in lung, and covering mesentery.
- Stratified squamous functions as a barrier, protecting the body and can be keratinized. It can be found in the epidermis, lips, and lining the oral cavity, esophagus, and vagina.
Cuboidal, where the width, depth and height are approximately the same.
- Simple cuboidal can be found in the walls of thyroid follicles,, kidney tubules (especially collecting ducts), surface of the ovary (in the germinal layer), and the small ducts of the exocrine glands.
- Stratified cuboidal is rare, being found in the excretory ducts of salivary and sweat glands.
Columnar, where height of cell greatly exceeds width.
- Simple columnar can be found lining the intestinal tract from stomach to rectum, uterus, cervix, and gallbladder.
- Stratified columnar is rare, being found in the conjunctiva lining of the eyelid.
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:
- Lamina densa, which is 50-10nm thick and contains networks of 3-4nm filaments.
- Lamina lucida (or lamina rara), which has clear space between the base of the cells and the lamina densa. It is believed to be an artifact caused by the shrinkage of the epithelial cells.
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:
- Motile (active movement)
- Primary (passive movement)
- Nodal (active movement)
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:
- Zona occludens (aka tight junction): a diffusion barrier, forming a ring or circumferential band (zonula) around the cell
- Zona adherens: continuous band-like adhesion, surrounding cells and joining them to their neighbours.
- Macula adherens (or Desmosome): localized spot adhesions, located at multiple sites on the lateral surfaces of adjoining cells. They are a strong attachment structure. In simple cuboidal or simple columnar epithelium, they are found in conjunction with occluding and adhering junctions. TEM shows a very dense material on the cytoplasmic side of membranes of adjoining cells. These are called attachment plaque and attach to intermediate filaments.
- Fascia adherens: sheet-like junction, called fascia because it has a broad face. Found in the intercalated disc of cardiac muscle.
- Gap junctions: provide ionic continuity between the cells. They are found in epithelia, smooth muscle, cardiac muscle, and nerves.
- Hemidesmosomes: half-desmosome located on the basal surface of the cell. They are found in epithelial tissues subjected to abrasion and mechanical shearing, including cornea, skin, and the mucosae of the oral cavity, esophagus, and vagina.