Epithelial Tissues

Epithelial Tissues
The human body is composed of only four basic types of tissue: epithelial, connective, muscular, and nervous. These tissues, which are formed by cells and intracellular matrix forming different organs and systems of the body. The main characteristics of these basic types of tissue are shown in Table 1.

Table 1. Main characteristics of the four basic types of tissues.


Tissue Cells Inracellular Matrix Main Functions
Nervous Intertwining elongated processes None Transmission of nervous impulses
Epithelial Aggregated polyhedral cells Small amount Lining of surface or body cavities, glandular secretion
Muscle Elongated contractile cells Moderate amount Movement
Connective Several types of fixed and wandering cells Abundant amount Support and protection


Epithelial tissues are composed of closely aggregated polyhedral cells with very little extracellular substance. These cells have strong adhesion and form cellular sheets that cover the surface of the body and line its cavities.
The principal functions of epithelial tissues are:
1- Covering, lining, and protecting surfaces (like the skin)
2- Absorption (like the intestine)
3- Secretion (like the epithelial cells of glands)
4- Contractility (like the myo epithelial cells).
5-Specific cells of certain epithelia are also highly specialized sensory cells, such as those of taste buds or the olfactory epithelium. Because epithelial cells line all external and internal surfaces the body, everything that enters or leaves the body must cross an epithelial sheet.
Characteristic Features of Epithelial Cells
Epithelial are derived from all 3 embryonic germ layers, have little amount of intracellular matrix, the forms and dimensions of epithelial cells range from high columnar to cuboidal to low squamous cells. The nuclear form often corresponds roughly to the cell shape; thus, cuboidal cells have spherical nuclei, and squamous cells have flattened nuclei. The long axis of the nucleus is always parallel to the main axis of the cell.
Most epithelia rest on connective tissue. In the case of epithelia lining the cavity of internal organs (especially in the digestive, respiratory, and urinary systems) this layer of connective tissue is often called the lamina propria. The lamina propria not only serves to support the epithelium but also provides nutrition and binds it to underlying structures. The area of contact between epithelium and lamina propria is increased by irregularities in the connective tissue surface in the form of small evaginations called papillae. Papillae occur most frequently in epithelial tissues subject to friction, such as the covering of the skin or tongue.
The region of the cell that faces the connective tissue is called the basal pole, whereas the opposite pole, usually facing a space, is the apical pole and the intervening sides apposed in neighboring cells are the lateral surfaces.

Basal Laminae & Basement Membranes
All epithelial cells in contact with connective tissue have at their basal surfaces sheet of extracellular structure called the basal lamina, consisting of a network of fine fibrils, the dense layer or lamina densa .The basal laminae may have electron-lucent layers on one or both sides of the dense layer, called laminae lucida,the basment memberane is often thicker due to the fusion of the basal lamina from each epithelial layer. The most macromolecular components of basal laminae are:
1-Laminin: These are large glycoprotein molecules that self-assemble to form a lace-like sheet immediately below the cells basal poles where they are held in place by the trans membrane integrins.
2- Type IV collagen: Monomers of type IV collagen contain three polypeptide chains and self-assemble further to form a felt-like sheet associated with the laminin layer.
3- Entactin (nidogen), a glycoprotein, and perlecan, a proteoglycan with heparan sulfate side chains: these glycosylated proteins and others serve to link together the laminin and type IV collagen sheets.
All these components are secreted at the basal poles of the epithelial cells. Basal laminae are attached to reticular fibers made of type III collagen in the underlying connective tissues , These proteins are produced by cells of the connective tissue and form a layer below the basal lamina called the reticular lamina.
Basal laminae are found not only in epithelial tissues but also where other cell types come into contact with connective tissue. Muscle cells, adipocytes, and Schwann cells secrete laminine, type IV collagen, and other components that provide a barrier limiting or regulating exchanges of macromolecules between these cells and connective tissue.
Basal laminae have many functions. In addition to simple structural and filtering functions, influence cell polarity; regulate cell proliferation and differentiation by binding and concentrating growth factors; influence cell metabolism and survival; organize the proteins in the adjacent plasma membrane and serve as pathways for cell migration. The basal lamina seems to contain the information necessary for many cell-to-cell interactions, such as the enervation of enervated muscle cells. The presence of the basal lamina around a muscle cell is necessary for the establishment of new neuromuscular junctions. The basement membrane is formed by the combination of a basal lamina and a reticular lamina and is therefore thicker

Intercellular Adhesion & Other Junctions
Several membrane-associated structures contribute to adhesion and communication between cells. They are present in most tissues but are particularly numerous and prominent in epithelia and will be described here. Epithelial cells are extremely cohesive and relatively strong mechanical forces are necessary to separate them. Intercellular adhesion is especially marked in epithelial tissues that are subjected to traction and pressure (ex, in the skin).
The lateral membranes of epithelial cells exhibit several specialized intercellular junctions. Various junctions serve to function as:
1- Seals to prevent the flow of materials between the cells (occluding junctions).
2- Sites of adhesion (adhesive or anchoring junctions).
3-Channels for communication between adjacent cells (gap junctions).
In several epithelia such junctions are present in a definite order from the apical to the basal ends of the cells.
Tight junctions, or zonulae occludens, are the most apical of the junctions. "Zonula" indicates that the junctions form bands completely encircling each cell, and "occludens" refers to the membrane fusions that close off the space between the cells. Thus, the principal function of the tight junction is to form a seal that prevents the flow of materials between epithelial cells .

Besides forming a seal between compartments on either side of an epithelium, the zonulae occludens of epithelial cells help prevent the integral membrane proteins of the apical surface from being transferred to the basolateral surface. This allows the two sides of the epithelium to maintain different receptors and function differently.
The next type of junction is the adherent junction or zonula adherens . This junction also encircles the cell, usually immediately below the zonula occludens, and provides for the firm adhesion of one cell to its neighbors.
Another junction specialized for adhesion is the desmosome or macula adherens. The desmosome is a disk-shaped structure at the surface of one cell that is matched with an identical structure at the surface of an adjacent cell.