Histology
Histology is the study of cells and the extracellular matix of tissues. Tissues are defined as cells with their ground substance acting together in the performance of a particular function. The primary tissue categories are epithelial, connective, muscle, and nervous. We include a discussion of blood here too as a special tissue type. The human body is composed of cells organized together with extracellular matrix into tissues having specific characteristics relating to specific functions. Each tissue is composed of several cell types and supporting extracellular matrix. About 200 cell types are found within the human body. Cells within a tissue must communicate with one another. This may be effected by a receptor mediated signalling mechanism or by direct cellular contact. In some tissues, cells are held tightly together by a series of cell junctions to form a barrier.

Cell Junctions
Cells are held together by the adhesive forces of cellular membrane molecules acting as a weak cellular glue. Several specialized cell junctions provide for tight cellular adhesion and communication. These junctions also serve as seals to prevent substances from flowing through the intracellular space.

Tight junctions
These junctions, also called zona occludens, are seen as a band that encircles the cell and fuses adjacent cell membranes closing off the intracellular space. The principle function of the tight junction is to form a more or less tight seal that prevents the flow of materials between cells. The membranes may have many fusion sites within the junction and therefore be more impermeable or have only a few and be relatively "leaky". The zona occludens are the first line of defense against unwanted molecules entering the body through the side door.

Zona adherens
This type of junction encircles the cell and provides for the adhesion of one cell to another. These junctions are rich in actin, myosin, tropomyosin and vinculin microfilaments.

Gap Junction 
Gap junctions are communicating junctions found in almost all mammalian tissues. These structures are characterized by very close apposition of adjacent membranes (2 nanometer electron dense space between membranes) and circular protein aggregates that form pores between adjacent cells. The aggregates, called connexons, are made of protein hexamers with a hydrophillic central pore. Connexons of adjacent cells are aligned to form a hydrophilic channel between the cells. Gap junctions permit the exchange between cells of molecules with molecular mass less than 1500 Da, which would include ions, some hormones, cAMP, cGMP causing cells in many tissues to act in a coordinated manner. Heart muscle and synctial smooth muscle are great examples of this principle.

Macula adherens
These junctions, also known as desmosomes, are complex disc shaped structures at the surface on one cell that is matched with an identical structure on the surface of the adjacent cell. Within each cell, in close approximation to the junction is a circular plaque made of some 12 proteins called the attachment plaque. The plaques have numerous intermediate filaments (keratin) entering and leaving them. These junctions seem to function as points of especially firm adhesion between adjacent cells. This is the only type of junction seen in the stratified squamous epithelium of the skin. Hemidesmosomes, or half desmosomes, can be found in the interaction of a cell membrane with the basement membrane where they serve to bind the cell to the membrane.

Extracellular Matrix (ECM)
A major portion of tissues is extracellular space that is filled by a network of macromolecules secreted by the cellular components within the tissue. The matrix is composed of a variety of polysaccharides and proteins. ECM is present to some degree in all tissues, even blood where the plasma might be considered the matrix, but is a functional determinant in connective tissues. In addition to serving as a scaffold or support for cells, cellular functions are regulated to some degree by the ECM. The matrix is made of polysaccharides bound to proteins (proteoglycans) and fibrous structural and adhesive proteins. The proteoglycans retain water and form a sort of gel substance through which ions, hormones and nutrients can freely move. Indeed the ground substance can act as a molecular sieve to regulate the traffic of molecules and cells according to charge or size. The fibrous structural proteins give the ECM strength, the elastic proteins give it resilience and the adhesive proteins help cells attach to the ECM. A chief component of ground substance is hyaluronic acid which is a large polysaccharide made of glucuronic acid and glucosamine that attracts water and is increased in tissues under repair or growth. Cells can easily migrate through swelled hyaluronic acid ground substance. It also serves as a lubricant in joint fluid.


Tissues <summary> 
Epithelial                   
Connective    loose (areolar)/dense irregular/dense regular/adipose/Cartilage/Bone
Muscle 
Nervous 
Blood    
 
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