Epithelial Tissue

Histology
Humans are composed of billions of cells. A tissue is a collection of cells with similar structural characteristics. Organs are groups of tissues. Most organs are complex groupings of different tissue types. An organism is composed of organs that are grouped together and functionally integrated.
The four types of human tissue
1. Epithelium. Epithelial cells form sheets of cells that are tightly joined to one another. These cells cover surfaces, line cavities, and form the secretory portions of many glandular structures, which are complex imaginations of the body surface.
2. Connective tissues join epithelial structures to other parts of the body. They exist under all epithelial layers and have a cellular component and an extracellular matrix. Bone, cartilage, and blood are specialized connective tissues.
3. Muscle tissue
a- Skeletal muscle tissue is found in the gross muscles that cause skeletal movement.
b- Cardiac muscle tissue is found in the heart wall and proximal portions of the aorta.
c- Smooth muscle tissue is a prominent component of blood vessel walls and visceral organs such as those of the gastrointestinal, urinary, and reproductive systems.
4. Nerve tissue. The brain, spinal cord, autonomic ganglia, peripheral nerves, and portions of sensory organs are composed of nerve tissue.

Tissue derivation.
The three primary germ layers in embryology are ectoderm (the outer germ layer), mesoderm (the middle germ layer), and endoderm (the inner germ layer).
1. Epithelium is derived from each of these layers.
A. Ectoderm forms epidermis.
B. Mesoderm forms mesothelium.
C. Endoderm forms the lining of the gastrointestinal tract.
2. Most connective tissue and muscle tissue is derived from mesoderm.
3. Most nerve tissue is derived from ectoderm.

Epithelial Tissue
Epithelia are specialized layers that line the internal and cover the external sur¬faces of the body. An epithelium consists of a sheet of cells lying close together with little intercellular space. These cells have strong adhesion and form cellular sheets that cover the surface of the body and line its cavities.
General classifications
1. Epithelia are classified by the number of cell layers they contain.
a- Simple epithelia have one cell layer. All cells rest on the basement membrane and reach the apical surface.
b- Stratified epithelia have more than one cell layer, consequently not all cells rest on the basement membrane or reach the apical surface.
c- Pseudostratified epithelia are simple epithelia that appear to be stratified. In these epithelia, all cells rest on the basement membrane; however, not all cells reach the apical surface. The stratified appearance occurs because nuclei lie at different levels in the epithelia.
2. Epithelia are further classified by cell shape.
a-Squamous cells are flat.
b-Cuboidal cells have approximately equal height and width.
c-Columnar cells are taller than they are wide.