HISTOLOGY OF THE EPITHELIAL TISSUES
By Farayola Rachael Oluwatoyosi, Faculty of Pharmacy, University of Ibadan
▶ What are Epithelia and Epithelial Tissues?
This article is about the histology of epithelial tissues.
Epithelia simply mean coverings and Epithelial tissues are tissues that cover or line all body surfaces, cavities, and tubes and form interface between different biological compartments. Epithelial tissues are involved in a wide range of activities such as absorption, secretion, and movement. Their thickness and surface specializations are determined by the function they perform.
▶ Characteristics of Epithelial Tissues
1. All epithelia are supported by a basement membrane with variable thickness
2. Cells of epithelial tissues are closely packed with little cementing materials between them and are arranged as a lining
3. They are held together and supported by basement membrane which separates them from the underlined connective reissue
4. They lack blood or lymphatic vessels. They drive nutrients from connective tissues. They also lack nerve fibres.
5. Epithelial cells that are involved in secretion are called Glands.
▶ Classification of Epithelial Tissue
Epithelial tissues are classified based on 3 characteristics. CELL LAYER, CELL SHAPE and SURFACE SPECIALIZATIONS
Classifications based on the cell layer
1. Simple epithelium consists a single layer of epithelial cells
2. Stratified epithelium consists of several layers of epithelial cells.
Classification based on Cell Shape
1. squamous- width is more than the length
2. Cuboidal- height is approximately same as the width
3. Columnar height is more than the width
Classification based on Surface Specializations
The presence of surface specializations such ad cilia, keratin, microvilli, and stereocilia.
1. Simple Epithelia are surface epithelial consisting of a single layer of cells. They provide little protection against mechanical abrasion. The nucleus is located near the centre of the cell. They may also exhibit surface specialization such as cilia. Simple epithelial includes simple squamous, simple cuboidal, simple columnar and pseudostratified epithelium
a. Simple Squamous Epithelium is composed of flattened irregularly shaped cells. The simple squamous epithelium is found lining surfaces involved in passive transport (as in the lungs) or fluids (walls of blood capillaries). It also forms the delicate lining of the pleura, pericardial, and peritoneal cavities.
b. Simple Cuboidal Epithelium has an appearance like a cube. The nucleus is usually round and centrally located in the cell. The simple cubical epithelium is found in the collecting tubules of the kidney, thyroid gland, excretory duct of the pancreas and salivary gland.
c. Simple Columnar Epithelium is similar to simple cuboidal epithelium except that the cells are taller and appear columnar in sections at right angles to the basement membrane. Simple columnar ciliates are found mainly in the female reproductive tract while simple columnar nonciliated is found in the small intestine and gallbladder.
d. Pseudostratified Epithelium- this type of epithelium appears to be stratified but is simple. Pseudostratified columnar epithelium can be distinguished from stratified epithelium by two main characteristics. First, stratified epithelia are never ciliated. Secondly, pseudostratified epithelia exhibit polarity with nuclei being mainly confined to the basal two-thirds of the epithelium. Pseudostratified epithelium is found in the respiratory tract.
Pseudostratified epithelium is found in the respiratory tract.
2. Stratified Epithelia- consists of two or more cell layers found where extra protection is needed, e.g. skin, oesophagus, vagina, anal canal. They are poorly suited for absorption and secretion. They are classified into stratified squamous, stratified cuboidal, stratified columnar and transitional epithelia.
a. Stratified Squamous Epithelium- consists of a variable number of cell layers that exhibit maturation from a cuboidal basal layer adherent to the underlying basement membrane to a flattened surface layer. In oesophagus and vagina, the epithelia are non keratinized stratified squamous. While in the skin, it is keratinized stratified squamous epithelium.
b. Stratified Cuboidal Epithelium- is a thin stratified epithelium that usually consists of only two or three layers of cuboidal cells. This type of epithelium is usually confined to the lining of larger excretory ducts of exocrine glands such as salivary gland and sweat glands.
c. Stratified Columnar Epithelium- found in the conjunctiva
d. Transitional Epithelium (Urothelium) – is a form of stratified epithelium found only in the urinary tract (bladder, ureter etc) where it is highly specialized to accommodate a greater degree of stretch and to withstand the toxicity of urine. It is intermediate (transitional) between stratified squamous and stratified cuboidal.