Histology - The study of tissues.

Tissue - a group of similar cells specialized in a particular way for the performance of a common function. All tissues arise from the three embryonic germ layers, the ectoderm, the mesoderm and the endoderm.

Four Categories of Tissues:

1. Epithelium - a sheet-like covering of cells with one surface free (apical) and the other attached (basal). Epithelia arise from all three germ layers.

2. Connective tissues - arising from the mesoderm, these tissues connect, bind and support other tissues and organs.

3. Muscle tissues - muscle tissues are derived from the mesoderm. They respond to nervous or hormonal stimulation by contracting.

4. Nerve tissue - Nerve cells are of ectodermal origin. They are highly specialized in the transmission of electro-chemical impulses.

I. Epithelium

A. An epithelium is a layer(s) of contiguous cells that covers the external and internal free surfaces of the body, e.g., surface of skin or inner surface of a small blood vessel.

B. The epithelium rests on a supporting extracellular layer called a basement membrane.

C. The free or apical surface of the epithelial cell often shows specializations in structure which relate to the function of the tissue.

1. Nonmotile processes are cytoplasmic projections from the free surface. They serve
to increase the surface area of the cell and, therefore, facilitate transport of materials into and out of the cells.

a. Microvilli are closely bunched, tiny projections from the free surface of cells lining the intestinal cavity (striate border) and kidney tubules (brush border). In these tissues their chief function is absorption. In the stomach and uterine epithelium they function primarily for secretion.

b. Sensory hairs are receptor processes found in certain sensory epithelia concerned with taste (taste buds), smell (olfactory epithelium), hearing (Organ of Corti) and equilibrium (vestibular apparatus). Some show the same internal structure as motile cilia but cannot move.

c. Stereocilia are nonmotile, cilia-like extensions from the free surfaces of epithelial cells lining the male sex duct. They do not show the 9:2 arrangement of microtubules internally. They have a secretory function.

2. Motile Processes:

a. Cilia are short extentions of the cytoplasm and cell membrane at the free surface of some epithelial cells. As revealed by electron microscopy, a cross section of a cilium shows a central pair of microtubules surrounded by nine peripheral pairs of microtubules. Cilia can be seen on the free surfaces of epithelia lining the respiratory tract and oviducts tubes in mammals.

b. A flagellum has the same internal structure as a cilium but is much longer and usually found one to a cell.

D. A variety of intercellular junctions can be seen between the cells of an epithelium.

1. Desmosomes are dense regions of attachment between epithelial cells. Under the electron microscope there is no continuity between the adjacent cells across the desmosome. The very narrow extracellular space is filled with a dark staining "glue" of mucopolysaccharides, sialic acid and protein. These structures are very common in the stratified squamous epithelium of the epidermis and greatly contribute to the strong bond between the cells in the skin.

2. Zona adherens is a region resembling the desmosome but lacks the intense staining capacity of the latter.

3. Zona occuldens is usually found closest to the free surface of the epithelial cell. It consists of an apparent fusion of the cell membranes of adjacent cells. In the epithelium of the small intestine, this so-called "tight junction" seems to prevent materials from the intestinal lumen from leaking into the intercellular spaces of the epithelium.
     These three junctions are commonly found together between epithelial cells lining the digestive lumen and constitute the Junctional Complex.

4. A gap junction represents a very small (20A) continuity between the cytoplasms of adjacent cells. They appear to represent sites of cell to cell communication.

E. Lateral cell modifications:

1. Interfacial canals - The adjacent cells of the stratified squamous of the skin are held together with desmosomes. Between the desmosomes, large, intercellular spaces can be found. They would permit the circulation of nutrients by diffusion, passage of leukocytes especially during inflammation and the innervation of the epidermis with the processes of sensory neurons.

2. Interdigitations of lateral cell processes increase the contact between adjacent cells for a variety of purposes, i.e., transport (renal tubule cells) or secretion (salivary gland cells, sweat gland cells or the epithelium of the gall bladder.

F. Specializations of the basal surface:

1. Basement membrane - This structure is misnamed in that it is not a cell membrane or membranous layer. It consists of two components:

a. Basal lamina - a dense (500 to 1000 A) filamentous sheet which appears to be attached to the basal surface of the epithelial cell by hemi-desmosomes (half desmosomes). It is a product of the epithelial cell, itself.

b. Reticular lamina - located under the basal lamina of most basement membranes, this layer consists of a condensed ground substance mixed with collagenous fibers.
  
The basement membrane supports the epithelium and functions as a semipermeable
filter separating the epithelium from the underlying tissues.

2. Basal cell processes extend from the basal membrane of epithelial cells into the underlying connective tissue. These serve to anchor the epithelium to the underlying tissues.

G.  Epithelia are avascular, i.e., epithelia lack a direct blood supply. The cells of an epithelium must receive nutrients and oxygen from and eliminate wastes to capillaries across the basement membrane. Transport of these materials is achieved by diffusion.

H.  Epithelia are capable of considerable regeneration. This is an especially important characteristic since epithelia are exposed to hostile environmental conditions and must constantly replace surface cells lost due to abrasion, microbial activity, toxic substances or extremes of temperature.