Glands are structures formed by secretory epithelial cells. In 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 invaginating growth of an epithelial surface. Invagination usually starts as a tubular structure or as a column of cells that later becomes tubular.
As gland growth continues, the cell column can divide and form branches, giving rise to what are known as compound glands with a characteristic arrangement of branches according to the gland.
These tubular or branched glands are multicellular glands, but there are also unicellular glands that are formed by a single secretory cell dispersed among non-secretory cells, for example, goblet cells.
Substances secreted by the glands are often long-distance chemical messengers, for example hormones, but they can also exert an action or effect directly on the environment in which they are secreted, which gives rise, in addition to the endocrine and exocrine glands, to different types of communication or effect. , such as the autocrine effect or the paracrine effect.
endocrine and exocrine glands
Glands are generally classified into three groups: endocrine, exocrine, and mixed, depending on where secretion occurs.
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.
Endocrine glands are generally hormone producing so they play a very important role in maintaining homeostasis.
Examples of endocrine glands:
Mixed glands or amphirines
Mixed glands, also called amphicrine, have two types of secretion: endocrine and exocrine. They produce substances that are secreted into the bloodstream and substances that are discharged into cavities and body surfaces.
As examples of amphicrine glands we can mention the pancreas or the sex glands.
Exocrine glands with those that secrete their products to the surface of the body either from an internal cavity or from outside the body, for example, glands that secrete into the digestive tract.
Secretion is usually carried out through a duct directly to the apical surface of the epithelium where the gland is located.
Exocrine glands are subdivided into three types or groups:Apocrine glands: are exocrine glands that lose part of their cytoplasm and membrane to form extracellular vesicles that transport 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: secrete by exocytosis. They are also called eccrine glands. For example, sweat glands and salivary glands.
In addition, exocrine glands can be classified according to the type of secretion they produce:Serous glands: produce a watery secretion rich in protein. 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, endocrine glands primarily 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 site of synthesis and secretion, but they can also have an effect on the producing cells themselves (autocrine effect) or on neighboring cells. cells from the same tissue (paracrine effect).
The types of communication or types of hormone action 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 secreted substances. Paracrine: hormones or secreted substances are dispersed in the extracellular matrix and exert their action at a short distance on nearby cells.