In this article we will discuss about the classification of simple epithelium.
1. Simple Squamous Epithelium (L., squame = scale):
Structure:
The simple squamous epithelium consists of thin, extremely flattened cells which are closely fitted like the tiles of a floor. Hence it is also called pavement epithelium. Although the cells are of irregular shape, in the surface view they appear roughly polygonal. The cells are fairly large in size with centrally placed nuclei. The nucleus of each cell is thicker than the cytoplasm and it gives bulged appearance on the surface.
Location:
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The simple squamous epithelium lines the Bowman’s capsule of the kidney tubules, the terminal bronchioles and alveoli of lungs, the blood vessels, lymph vessels, heart and coelomic cavities. Functions: The thin, delicate nature of squamous epithelium permits rapid movement of molecules (such as diffusion of gases). The squamous epithelium also helps in protection, excretion and secretion of coelomic fluid.
The squamous epithelium when occurs in the blood vessels and lymph vessels, it is called endothelium, in the chambers of the heart, endocardium and in the walls of thoracic and abdominal cavities mesothelium. The cell outlines of endothelium and mesothelium are wavy and termed tessellated epithelia.
2. Simple Cuboidal Epithelium:
Structure:
The cells of the cuboidal epithelium are almost cube-like, i.e., they are as tall as wide. The nucleus is spherical and lies in the centre of the cell. The cells appear as polygonal in surface view and cuboidal in vertical section.
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Location:
The cuboidal epithelium occurs in the kidney tubules, thyroid gland, gonads, and also in sweat, salivary glands and pancreatic ducts.
In the salivary and pancreatic ducts it is non-secretory. In the sweat, salivary and thyroid glands it is secretory. In the proximal convoluted tubule of the kidney, the cuboidal cells exhibit numerous microvilli at their free ends. Since the latter give brush like appearance, the epithelium is called brush-bordered cuboidal epithelium. The cuboidal epithelium which lines the ovaries and the seminiferous tubules is known as germinal epithelium and it produces new cells for the gamete formation.
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Functions:
Protection, secretion, absorption and gamete formation.
3. Simple Columnar Epithelium:
Structure:
The cells of the simple columnar epithelium are tall, pillar like and are placed side by side. The free surface of a columnar epithelial cell is smooth. The oval nucleus lies close to the base of the cell. The cytoplasm is either clear or with droplets of secretory material. The cells look polygonal in surface view.
Location:
Simple columnar epithelium forms a lining of the stomach, small intestine, colon, gall bladder, gastric glands, intestinal glands and pancreatic lobules. Since here it performs the secretory function, it is called glandular epithelium. Certain cells of the columnar epithelium of stomach and intestine produce mucus which forms a fine mucus coating over the epithelium. These mucus producing cells are called goblet cells.
The epithelium having mucus-secreting cells along with the underlying connective- tissue is called mucous membrane or mucosa. The other columnar cells lining the intestine are specialized for the absorption of water and digested food. In order to increase the surface area for absorption they bear microvilli at the free ends. The columnar epithelium of this type is called brush-bordered columnar epithelium.
Functions:
Protection, secretion and absorption.
4. Simple Ciliated Epithelium:
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Structure:
The simple ciliated epithelium consists of cells bearing fine, vibratile cilia on their free surface. Mixed with the ciliated cells, mucus secreting goblet cells are found at intervals. The mucus produced by the goblet cells spreads over the epithelium as a thin layer. The epithelial cells move the mucus and other substances over the epithelium by the lashing action of cilia.
The simple ciliated epithelium is of two types:
(i) Cuboidal Ciliated Epithelium:
It consists of cuboidal cells bearing cilia on their free surfaces. This type of epithelium is found in the nephrons. It helps in the flow of urine.
(ii) Columnar Ciliated Epithelium:
It consists of columnar cells that bear cilia on the free surface. It forms the lining of the oviducts, ventricles of the brain, central canal of the spinal cord, nasal passages and terminal bronchioles.
Functions:
Protection and movement of urine, mucus, eggs and cerebrospinal fluid in a particular direction.
5. Glandular Epithelium:
A cell, tissue or organ which is specialized to secrete mucus, enzymes, hormones or any other useful material is known as a gland. The gland is generally lined by the columnar or cuboidal epithelium and is called glandular epithelium.
Based on the presence or absence of ducts, kind and mode of secretion, number of cells in the gland, shape and complexity of the glands, etc.
The glands are classified into following types:
(A) Based on the Presence or Absence of Duct and Kind of Secretion:
Based on the presence of duct and kind of secretion, the glands are of three types:
a. Exocrine Glands:
A gland with a duct is called an exocrine gland (exo = outside, krinein = to secrete). The exocrine glands either pour their secretions on to the free epithelial surface directly or send their secretions through ducts to the sites of action. The salivary glands, gastric glands, intestinal glands, sweat glands, oil glands, tear glands, mammary glands, etc., are some examples for the exocrine glands. The exocrine glands associated with the alimentary canal produce enzymes. They may be unicellular or multicellular. They are highly vascular and are under nervous or hormonal control.
b. Endocrine Glands:
Glands without ducts are called endocrine (endo = inside, krinein = to secrete) or ductless glands. They pour their secretions directly into the blood stream. The endocrine glands secrete regulatory substances called hormones which reach the respective site of action called target organs, through the blood stream. Pituitary, thyroid, parathyroid, thymus, adrenal glands, etc., are some examples for endocrine glands.
c. Heterocrine Glands:
The gland which has both exocrine and endocrine component in it is called heterocrine gland. The exocrine part sends its secretion through a duct and the endocrine part releases the hormone directly into the blood. Pancreas and gonads are the examples for the heterocrine glands.