Gland

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Gland
Gray1026.png
Human submaxillary gland. At the right is a group of mucous alveoli, at the left a group of serous alveoli.
Latin glandula
Code TH H2.00.02.0.02002

A gland is an organ in an animal's body that synthesizes a substance such as hormones for release into the bloodstream (endocrine gland) or into cavities inside the body or its outer surface (exocrine gland).

Structure

Glands are organs in that synthesizes a substance such as hormones for release into the bloodstream (endocrine gland) or into cavities inside the body or its outer surface (exocrine gland). Numerous glands exist throughout the human body.

Development

This image shows some of the various possible glandular arrangements. These are the simple tubular, simple branched tubular, simple coiled tubular, simple acinar, and simple branched acinar glands.
This image shows some of the various possible glandular arrangements. These are the compound tubular, compound acinar, and compound tubulo-acinar glands.

Every gland is formed by an ingrowth from an epithelial surface. This ingrowth may in the beginning possess a tubular structure, but in other instances glands may start as a solid column of cells which subsequently becomes tubulated.

As growth proceeds, the column of cells may divide or give off offshoots, in which case a compound gland is formed. In many glands the number of branches is limited, in others (salivary, pancreas) a very large structure is finally formed by repeated growth and sub-division. As a rule, the branches do not unite with one another, but in one instance, the liver, this does occur when a reticulated compound gland is produced. In compound glands the more typical or secretory epithelium is found forming the terminal portion of each branch, and the uniting portions form ducts and are lined with a less modified type of epithelial cell.

Glands are classified according to their shape.

  • If the gland retains its shape as a tube throughout it is termed a tubular gland.
  • In the second main variety of gland the secretory portion is enlarged and the lumen variously increased in size. These are termed alveolar or saccular glands.

Function

Glands are divided based on their function into two groups:

Endocrine glands

Endocrine glands secrete substances that circulate through the blood stream. These glands that secrete their products through the basal lamina into the blood stream and lack a duct system. These glands often secrete hormones, and play an important role in maintaining homeostasis. The pineal gland, thymus gland, pituitary gland, thyroid gland, and the two adrenal glands are all endocrine glands.

Exocrine glands

Exocrine glands secrete their products through a duct onto an outer surface of the body, such as the skin or the human gastrointestinal tract. Secretion is directly onto the apical surface. The glands in this group can be divided into three groups:

    • Apocrine glands a portion of the secreting cell's body is lost during secretion. Apocrine gland is often used to refer to the apocrine sweat glands, however it is thought that apocrine sweat glands may not be true apocrine glands as they may not use the apocrine method of secretion.
    • Holocrine glands the entire cell disintegrates to secrete its substances (e.g., sebaceous glands)
    • Merocrine glands cells secrete their substances by exocytosis (e.g., mucous and serous glands). Also called "eccrine".

The type of secretory product of exocrine glands may also be one of three categories:

  • Serous glands secrete a watery, often protein-rich product.
  • Mucous glands secrete a viscous product, rich in carbohydrates (e.g., glycoproteins).
  • Sebaceous glands secrete a lipid product.These glands are also known as oil glands.

Clinical significance

Other animals

Additional images

References

Public Domain This article incorporates text from a publication now in the public domainChisholm, Hugh, ed. (1911). Encyclopædia Britannica (11th ed.). Cambridge University Press. 

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