What differentiates endocrine, exocrine and paracrine glands?

Glands are structures made up of secretory epithelial cellsin the glands substances are synthesized that are released into the bloodstream (endocrine glands) or into body cavities or their surfaces (exocrine glands).

All glands in the body are formed by an invaginative growth of an epithelial surface. The invagination usually begins as a tubular structure or as a cell column that later becomes tubular.

As the growth of the gland continues, the cell column can divide and form branches, giving rise to what are known as compound glandswith a characteristic arrangement of the ramifications according to the gland.

These tubular or branched glands are multicellular glandsbut there are also unicellular glands which are formed by a single secretory cell dispersed among nonsecretory cells, for example goblet cells.

Substances secreted by the glands are often long-distance chemical messengers, for example hormonesbut they can also exert an action or an effect directly on the environment in which they are secreted, which gives rise, in addition to endocrine and exocrine glands, to different types of communication or effect, such as autocrine effect or the paracrine effect.

endocrine and exocrine glands

The glands are usually classified into three groups: endocrine, exocrine and mixeddepending on where the secretion occurs.

endocrine glands

Endocrine glands secrete their products through the basal lamina of epithelial tissue into the bloodstream. In these glands, the basal lamina is often seen as a layer around the gland with millions of tiny blood capillaries.

The endocrine glands are usually hormone producingso they play a very important role in maintaining the homeostasis.

Examples of endocrine glands:

  • Pineal gland
  • thymus
  • Pituitary
  • Thyroid
  • adrenal glands

Mixed or amphricrine glands

The mixed glands, also called amphicrines, They have two types of secretion: endocrine and exocrine.. They produce substances that are secreted into the bloodstream and substances discharged into cavities and surfaces of the body.

As examples of amphricrine glands we can cite the pancreas or the sexual glands.

exocrine glands

The exocrine glands with those that secrete their products to a body surfaceeither from an internal cavity or from outside the body, for example, glands that secrete into the digestive tract.

The secretion is usually carried out through a duct directly to the apical surface of the epithelium where the gland is located.

The exocrine glands are further subdivided into three types or groups:

  • Apocrine glands: they are exocrine glands that lose part of their cytoplasm and their membrane to form extracellular vesicles that transport the secretion. For example, the mammary glands.
  • holocrine glands: the secretory cell completely disintegrates to secrete its products. For example, Meibomian sebaceous glands (in the eyelids).
  • merocrine glands: carry out secretion by exocytosis. They are also called eccrine glands. For example, sweat glands and salivary glands.

Furthermore, exocrine glands can be classified according to the type of secretion they produce:

  • serous glands: produce a watery secretion rich in proteins.
  • mucous glands: produce mucus, a viscous substance with a high content of glycoproteins.
  • Sebaceous glands: the secretion is greasy, rich in lipids.

Autocrine, Paracrine, and Other Types of Communication

As mentioned, the endocrine glands mainly secrete hormones, substances that act as chemical messengers in other parts of the body.

In general, the products of the endocrine glands are secreted into the bloodstream and exert their action at a distance, far from the place of synthesis and secretion, but they can also have an effect on the producing cells themselves (autocrine effect) or on nearby cells of the same tissue (paracrine effect).

The types of communication or types of action of hormones are classified into:

  • endocrine: Hormones can quickly reach any part of the body through the bloodstream. Although they usually have a target organ or cells, the action can potentially be on any cell in the body.
  • juxtacrine: there is direct cell-cell communication.
  • autocrine: occurs when the secretory cell itself responds to the secreted substances.
  • paracrine: secreted hormones or substances are dispersed in the extracellular matrix and exert their action at a short distance in nearby cells.
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