Histology

Histology is the study of tissues, derived from the medical terms "hist-"(tissue) and "ology"(study of).

Types of tissues

There are four types of tissues. These include epithelial tissue, connective tissue, muscle tissue, and nervous tissue.

Germ Layers

In the human body, the primary germ layers are the embryonic tissues from which all tissues and organs of the body develop. These germ layers begin to devlop as the zygote undergoes a special type of mitosis called clevage, forming a multicellular blastula. As this cell differentiates, three germ layers develop. These layers are called the ectoderm, mesoderm, and the endoderm.

The ectoderm is the outer germ layer. As the organism develops, this layer contributes to the formation of epithelial tissue and nervous tissue.

The mesoderm is the middle germ layer. As the organism develops, this layer develops into epithelial tissue, connective tissue and muscle tissue.

The endoderm is the inner germ layer. As the organism develops, this layer contributes to the formation of epithelial tissue and the absorptive regions of the digestive tract.

Cell Junctions

The points of attachment between adjacent cell membranes are called cell junctions. These junctions can be found with epithelial, nervous, and muscle tissue. There are five types of junctions:

• Tight junctions form fluid tight seals between cells. They are composed of a web like strip of protein and are associated mostly with epithelial tissue lining a cavity faces outside and with surfaces that cover organs.

• Adherens junctions are composed of plaque and form an adhesion belt that encirles each cell. A dense layer of proteins inside the plasma membrane attaches to microfilaments. The plague of one cell is attached to the plaque of another cell by a transmembrane glycoprotein. They are found mostly in epithelia and function to resist seperation between cells when tissue is stretched.

• Desmosomes are the most common and the strongest of cell junctions. They are composed of plaque and transmembrane proteins which link adjacent cells. These cell junctions contain intermediate filaments that travel through the cytoplasm from desmosomes of the same cell. This acts to stabalize the cell.

• Hemidesmosomes are similar to desmosomes but instead act to anchor the cell to extracellular material or to a different kind of tissue.