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An Exocrine gland is distinguished by the fact that it excretes its essential product by way of a duct to some environment external to itself, be it either inside the body or on a surface of the body.
An Endocrine gland is its counterpart. It secretes its essential product without the use of a duct directly into the bloodstream or else by diffusion into its surrounding tissue (paracrine signaling) where it often affects only target cells near the release site.
Examples of endocrine glands include the adrenal glands, located atop the kidneys and responsible for the secretion of certain hormones such as adrenaline, cortisol and others. The testes, in males and ovaries in females are not only gonads, organs which generate male and female germ cells respectively, but are also endocrine glands in that they produce various androgens and estrogens together known as steroidal sex hormones.
Exocrine glands contain a glandular portion and a duct portion, the structures of which can be used to classify the gland.
- The duct portion may be branched (called compound) or unbranched (called simple).
- The glandular portion may be tubular or acinar, or may be a mix of the two (called tubuloacinar). If the glandular portion branches, then the gland is called a branched gland.
By method of excretion
Exocrine glands are named apocrine glands, holocrine glands, or merocrine glands based on how their products are excreted.
- Merocrine glands or (eccrine glands) - cells excrete their substances by exocytosis; for example, pancreatic acinar cells.
- Apocrine glands - a portion of the plasma membrane buds off the cell, containing the excretion.
- Holocrine glands - the entire cell disintegrates to excrete its substance; for example, sebaceous glands of the skin and nose.
By product excreted
- Serous cells excrete proteins, often enzymes. Examples include chief cells and Paneth cells
- Mucous cells excrete mucus. Examples include Brunner's glands, esophageal glands, and pyloric glands
- Mixed glands excrete both protein and mucus. Examples include the salivary glands, although the parotid gland is predominantly serous, the sublingual gland is predominantly mucous, and the submandibular gland is both serous and mucous.
- Wikipedia:MeSH A10#MeSH A10.336 --- exocrine glands
- List of specialized glands within the human integumentary system