الأربعاء , سبتمبر 23 2020
الرئيسية / التعليمي / Classification of epithelial tissues

Classification of epithelial tissues

We discussed the classification of epithelial tissues as:
1- Covering epithelia, we described before.
2- Glandular epithelia, glands are derived from covering epithelia by means of cell proliferation and penetration into the underlying connective tissue. According to type of cells, there are two types of glands: unicellular glands and multicellular glands.
According to type of gland channel, there are two types of glands:
a- Exocrine glands, their secretion pass to the surfaces to a duct. And according to the morphology of the glands, it is divided into:
1- Simple glands, which have a single un-branched duct.
2-Compound glands, which have a branched duct system.
Moreover, according to the way the secretory products leave the cell, it is divided into:
a- Merocrine glands, the secretory granules are shed by a process of exocytosis (e.g. exocrine pancreas).
b- Apocrine glands, the secretory product is shed together with parts of the apical cytoplasm (e.g. sweat glands).
c- Holocrine glands, the product of secretion is shed.
The secretory portion of the simple gland may be:
1- Tubular, such as gastric glands of the stomach.
2- Coiled tubular, such as sweat glands.
3- Branched tubular, such as glands of the pyloric stomach.
4- Acinar (alveolar), such as mucous glands in frog’s skin.
5- Branched acinar, such as sebaceous glands.
There are three types of compound glands:
1- Compound tubular glands, such as Brunner’s gland in the duodenum.
2- Compound acinar glands, such as exocrine glands of the pancreas.
3- Compound tubulo-acinar glands, such as the sub-mandibular salivary gland.
b- Endocrine glands, which lose their contact with the surfaces and lack a duct.
Endocrine glands have no duct and hence there secretory products (hormones) are passed directly into the blood. The hormones are carried by the bloodstream to specific organs called (target organs) which respond to the hormonal stimulus. The hormones and the nervous system control and coordinate the functions of all the physiologic systems of the body.
Endocrine secretory cells can form discrete organs or glands (e.g. pituitary gland and adrenal gland). They can occur as isolated groups of cells within a certain organ (e.g. islets of langerhans in the pancreas) or as scattered cells among the parenchyma cells of complex organs (e.g. endocrine cells in the digestive tract of kidney and lung).
In general, endocrine glands are composed of secretory cells of epithelial origin supported by connective tissue rich in blood and lymphatic capillaries.
According to cell grouping, there are two types of endocrine glands:
a- Glands consisting of clumps of secretory cells separated by a rich network of capillaries such as pituitary gland, adrenal gland and most endocrine glands. The endocrine cells release the hormones that diffuse rapidly into the surrounding blood vessels.
b- Follicular (vesicular) endocrine glands, in which the hormone is stored within spheroidal cavities enclosed by the secretory cells. The only example of this type is thyroid gland.