What are glands and its types?
A cell, tissue or organ which secretes specific chemical secretion is known as a gland.
The glands are classified in different ways:
1. Site of Secretion
According to the site where secretion is released, the glands are of three types: exocrine, endocrine and heterocrine.
A. Exocrine Glands
The glands which pour their secretions on the epithelial surface, directly or through ducts
are called exocrine glands (= extemally secreting glands). These include salivary glands, gastric glands, intestinal glands, tear glands, sweat glands, oil glands, mammary glands, etc.
The exocrine glands may be unicellular or multicellular. When unicellular glands secrete mucus, they are called mucous cells (= goblet cells or mucocytes). Mucus is a proteinaceous viscous and slimy substance. The mucous cells are common in the columnar epithelium of the intestine. When unicellular glands secrete a clear watery fluid, they are called serous cells (= serocytes). Serous cells are present in parotid salivary glands, intestinal glands and sweat glands. When the glands secrete both mucous and serous secretions, they are called mixed glands. There include pancreas, most gastric glands, sub mandibular salivary glands, etc.
Multicellular glands consist of secretory portion and duct, both formed of epithelial cells. Multicellular glands are further of two types : tubular and saccular (= alveolar or acinous).
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