Epithelial tissue is one of the four basic tissue types found in the human body. There are two types of epithelial tissue. First of all, it may exist as a contiguous sheet of cells that lines the internal or external surface of the body. Invaginated epithelial cells may also give rise to the second type of epithelial tissue – glands.
General Properties of Epithelium
Epithelium is derived from all three embryonic primary germ layers, mainly ectoderm and endoderm. The various forms it can take on results in a number of functions such as protection of the tissues, secretion, and absorption and so on. Epithelium is avascular, therefore nourishment is supplied via adjacent connective tissue and oxygen is diffused through the basal lamina.
Classification of Epithelial Membranes
There are two factors in regards to the classification of epithelial membranes, namely cell arrangement and cell morphology. Simple epithelium is a membrane that is made up of a single cell layer, whereas a stratified epithelium is composed of more than a single layer of cells. Morphology of the cells can be squamous, cuboidal or columnar. In a stratified epithelium, the morphology on the surface layer of cells is observed for classification.
Besides the simple and stratified epithelium, there are two other types of epithelium that fall beyond the classification system. They are known as the pseudostratified columnar epithelium and the transitional epithelium.
Simple Squamous Epithelium
Simple squamous epithelium consists of a flat, single cell layer. Due to the flattened shape, its nuclei are often elliptical. This particular epithelium functions as a limiting membrane, offers lubrication which reduces friction, and also acts as a lining membrane. These simple squamous epithelial cells are tightly packed, further decreasing friction and commonly lines the pulmonary alveoli, loop of Henle and blood and lymphatic vessels.
Simple Cuboidal Epithelium
Simple cuboidal epithelium is a basic layer of cells that have a square side view and a central spherical nucleus. They are often found gland ducts, the germinal covering of the ovary and they also form absorptive kidney tubules. Simple cuboidal epithelium is also involved with secretion, absorption and protection.
Simple Columnar Epithelium
A tall layer of hexagonal shaped cells represents the simple columnar epithelium. They have an elongated, ovoid nucleus usually found near the basal area of the cell. Simple columnar epithelium may present microvilli or cilia from the apical surface. This epithelium commonly lines the digestive tract, gall bladder, uterus, oviducts, small bronchi and functions in transportation, secretion, absorption and protection.
Psuedostratified Columnar Epithelium
The pseudostratified columnar epithelium may look stratified but in fact is a single layer of cells. It consists of basal, fusiform and columnar cells. All of these cells are attached to the basal lamina, but only columnar cells extend to the free surface.
Due to the differing heights, nuclei are also located at different levels resulting in the impression of a stratified epithelium and hence, the name – psuedostratified columnar epithelium. The extended columnar cells are generally ciliated and this epithelium lines most trachea, nasal cavity, male urethra, auditory tube and lacrimal sac.
Stratified Squamous Epithelium
Constituting several layers of cells, only the bottom layer is connected to the basal lamina. The cell morphology are varying from basal to apical, as such, only surface cells are observed while classifying cell morphology for stratified epithelium.
If the superficial layer of cells is nucleated, they are nonkeratinized. Stratified squamous nonkeratinized epithelium cells generally lines the oral pharynx, esophagus and vocal folds. However, if the topmost layers of cells are made up of dead and non-nucleated cells, they are keratinized (whereby keratin has replaced nuclei and cytoplasm). Keratin waterproofs the cell. Lastly, it forms the epidermis of skin as it is tough and offers protection.
Stratified Cuboidal Epithelium
This epithelium is made up of two layers of cuboidal cells. It generally lines the ducts of sweat glands. It offers secretion and absorption properties.
Stratified Columnar Epithelium
The stratified columnar epithelium has more than one layer of cells, with columnar cells extending to the surface. Surface cells may be ciliated and this epithelium may be confused with pseudostratified columnar epithelium. It can be found in the conjunctiva of the eye and certain areas of the male urethra.
Transitional Epithelium
Transitional epithelium appears to be made of several layers of cells. Basal cells are either low columnar or cuboidal cells whereas apical cells consist of a few layers of polyhedral cells. This epithelium is only found in the urinary system and lines continuously from the renal calyces to the beginning of the urethra. The surface cell layer is sometimes bi-nucleated. The transitional epithelium is able to distend – dome shaped cells become flattened.
Sources:
Gartner, L.P. & Hiatt, J.L. 2001. Color Textbook of Histology (2nd ed.). Pennsylvania: Saunders.
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