I. Covering and lining epithelium
II. Glandular epithelium
I. Covering and lining epithelium:
According to arrangement of layers covering epithelium is divided into:
Single layered, every cell rests on basement membrane. According to cell type it is divided into many types:
(a) Squamous epithelium:
It consists of flat disc like cells with a centrally located and oval or spherical nucleus (polygonal in surface view). It looks like tiles of floor, hence called pavement epithelium. The cells of endothelium (that lines the lumen of cardiovascular system) and mesothelium (that lines the serous cavity) are wavy in outline hence they are called tesselated epithelia.
It forms the inner lining of lungs alveoli, heart (endocardium), blood vessels (endothelium), outer and inner layer of Bowman’s capsule, coelomic epithelium (parietal and visceral peritoneum), lining of eye lens.
(b) Cuboidal epithelium:
It consists of cuboidal cells which are as tall as wide with centrally placed nucleus (polygonal in outline). It lines the respiratory bronchioles, sweat glands, salivary glands and thyroid follicles (secretory in nature).
The cells of cuboidal epithelium often forms microvilli on their free surfaces which gives a brush like appearance, hence called brush bordered cuboidal epithelium, (e.g. proximal part of uriniferous tubules).
It also lines pancreatic duct (non-secretory), testis (seminiferous tubules) and ovary. In ovaries and testis it is called germinal epithelium because these cuboidal cells are capable of producing gametes (ova and sperms respectively). Cuboidal cells also have cilia which occur in certain parts of nephron of the kidney. Hence called ciliated cuboidal epithelium.
(c) Columnar epithelium:
Cells are taller with basally located nucleus. It is secretary in nature (glandular) in gastric glands of stomach, intestinal glands, pancreatic lobules and gall bladder. It is mucus secreting (goblet cells) in lining of stomach and intestine.
It is specialized for absorption because it bears microvilli as it has more absorptive surface area, hence microvilli (brush bordered columnar epithelium) is present in intestine.
It is ciliated (called ciliated columnar epithelium) in bronchioles and fallopian tube. It bears non-motile (because the sperm can move on its own) stereocilia in the lining of epididymis.
It is also known as stratified epithelium. It has more than one layer of cells and only deepest cells rest on the basement membrane. Different types of compound epithelium are:
(a) Stratified squamous epithelium:
Superficial layers are of squamous cell and deeper layers of cuboidal cells. It covers buccal cavity, pharynx, and oesophagus, and vagina, outermost part of urethra, cornea and conjunctiva of eye. This epithelium is of two types – keratinized and non-keratinized
(b) Stratified cuboidal epithelium:
Superficial cells are also cuboidal. It is found on the inner surfaces of large ducts of sweat and mammary glands.
(c) Stratified columnar epithelium:
This type is also uncommon in the body; usually the basal layers or layers consist of shortened, irregularly polyhedral cells. Cells of the upper layer of this epithelium are columnar. It lines some parts of pharynx and epiglottis.
(d) Transitional epithelium:
This is a stratified epithelium which contains cuboidal or columnar shaped cells, which are thin and stretchable. No basement membranes are present as it would impede stretch ability.
It lines the inner surface of renal calyces, urinary bladder, and ureter. Because of its distribution, it is also called urothelium.
A single layer of cells is present but some cells are shorter than other cells. Due to difference in size of cells it appears as many layered. In trachea, bronchi and bronchioles, cilia are present on longer cells as they perform the function of upward movement of tenacious / sticky sputum towards the mouth. In epididymis stereocilia are present. Some portion of urethra is lined with this layer.
II. Glandular epithelium
These glands secrete hormones which directly reach the blood. E.g., Pituitary, thyroid etc.
Their secretions are poured by ducts at the site of action. E.g. Salivary, gastric etc. Number of Cells
Goblet cells found in lining of respiratory tract and alimentary canal secrete mucous.
Usually situated in deeper layers and can be divided into three types according to the shape of secretory units. These are tubular, alveolar and tubuloalveolar glands.
Secretory portion is straight and tubular.
E.g. – Sweat glands (Coiled tubular), intestinal glands, crypts of leiberkuhn.
Simple branched tubular:
A single duct with branched tubes.
E.g. – Gastric glands, Brunner’s glands, Bartholin’s glands.
Duct branched aggregation of tubules.
E.g. – bulbourethral gland and liver.
Simple (acinar) alveolar:
A single duct (flask shaped) with sac.
Examples – Mucous and poison glands in amphibian skin.
Simple branched alveolar:
Duct branched but many sacs (flask shaped).
E.g. – Sebaceous glands, Meibomian glands.
Compound alveolar glands:
The secretory portion is flask shaped.
E.g. – Salivary glands (sublingual and submandibular).
The tube ends in a sac like dilation.
E.g. – Mammary glands, pancreas, lacrimal gland.
Apical portion (where secretory products accumulates) of cell goes with secretion. Example – Mammary glands
Secretory cell comes out with the secretion. Example – Sebaceous glands. Sometimes holocrine glands are described as those endocrine glands which secrete only hormones. Example – Thyroid, PTH, adrenals, etc.
Only secretion is secreted without any loss of cells or their parts.
Example- Most of the glands such as salivary glands sweats glands etc.
Mucous glands – Secretion is thick and sticky. E.g. – Goblet cells.
Serous glands – Secretion is watery. E.g – Sweat glands, intestinal glands.
Mixed glands – Secretion is mixed. E.g. – Pancreas, gastric glands.