We are talking about organs that are responsible for developing and secreting substances necessary for the body to function or that it needs to eliminate.
Some glands have dual functions, for example, the liver, pancreas, ovary, and testes produce both a secretion that is emitted through a duct and a hormone that is taken up by the blood.
Such structures are called mixed glands.
What is a gland?
A gland is an organ that produces and releases substances that perform a specific function in the body. There are two types of glands. The endocrine glands are ductless glands and release the substances they produce (hormones) directly into the bloodstream.
These glands are part of the endocrine system . There is another type of gland called the exocrine gland (for example, sweat glands, lymph nodes). These are not considered part of the endocrine system, since they do not produce hormones and release their product through a duct.
Endocrine glands, such as the pancreas and thyroid gland, use the bloodstream to control the internal environment of the body and communicate with each other through substances called hormones, which are released into the bloodstream.
The adrenal glands are small structures attached to the top of each kidney. The human body has two adrenal glands that release chemicals called hormones into the bloodstream. These hormones affect many parts of the human body.
A gland can range from a single cell to a complex system of tubes that join and open to a surface through a duct. The endocrine glands, for example the thyroid, adrenal glands, and pituitary glands, produce hormones that are secreted directly into the bloodstream.
The exocrine glands secrete their substances on an external or internal surface of the body. Most exocrine glands, for example the salivary and lacrimal glands, release their secretions through the ducts.
However, some open directly on a body surface, as in the sebaceous glands of the skin and the digestive glands of the intestinal mucosa. A simple exocrine gland may consist only of a tube lined with secretory cells.
In more complex types, groups of cells produce the secretion and a duct or duct system discharges the secreted material. Among the substances produced by the exocrine glands in humans are sweat, lubricants such as mucus and tears, and digestive juices.
There are specialized exocrine glands in the animal world that produce substances such as the shells of bird eggs, cobwebs, and the cocoons of silkworm larvae.
Simple glands are also common in the plant kingdom. The sweet nectar of the flowers and the resin-hued pine trees are substances produced by the plant glands.
Classification of the glands
The glands are classified according to:
- The nature of the product.
- The structure.
- The manner by which the secretion is delivered to the area of use.
- The manner of cellular activity in the formation of secretion.
- Unicellular (mucous goblet cells).
- Glandular cell sheets (choroid plexus)
- Restricted nests of gland cells (urethral glands)
- Invaginations of varying degrees of complexity.
- Simple or branched tubular (intestinal and gastric glands): no duct interposed between the surface and the glandular portion
- Simple coiled (sweat gland) -duct interposed between glandular portion and surface
- Simple, branched, acinous (sebaceous gland): spherical or ovoid glandular portion, connected to the surface by Compound duct, tubular glands (gastric cardias, renal tubules): branching ducts between the surface and the glandular portion
- Compound tubular-acinous glands (pancreas, parotid gland): branched ducts, ending in a secretory portion that can be tubular or acinar.
Mode of secretion
- Exocrine : the secretion is passed directly or through ducts to the outer surface (sweat glands) or to another surface that is continuous with the outer surface (intestinal glands, liver, pancreas, submaxillary gland).
- Endocrine : the secretion is passed into adjacent tissue or area and then into the bloodstream directly or through the lymphatic vessels; These organs are generally circumscribed, highly vascularized and generally have no connection to an external surface (adrenal, thyroid, parathyroid, islets of Langerhans, parts of the ovary and testes, anterior lobe of the pituitary, intermediate lobe of the pituitary, clusters of nerve cells of the hypothalamus, and the neural portion of the pituitary).
- Mixed exocrine and endocrine glands: (liver, testes, pancreas).
- Cytocrine : passage of a secretion from one cell directly to another (melanin granules from melanocytes in the connective tissue of the skin to the epithelial cells of the skin).
Nature of the secretion
- Cytogen (testis, perhaps spleen, lymph node, and bone marrow) -gland “secrete” cells.
- Acellular (intestinal glands, pancreas, parotid gland): the gland secretes a non-cellular product.
Cytological changes of the glandular portion during secretion
- Merocrine (sweat glands, choroid plexus): no loss of cytoplasm.
- Holocrine (sebaceous glands) -gland cells undergo dissolution and are entirely extruded, along with the secretory product.
- Apocrine (mammary gland, axillary sweat gland) – only part of the cytoplasm is extruded with the secretory product.
Chemical nature of the product
- Goblet mucous cells (submaxillary glands, urethral glands) : secretion contains mucin.
- Serosa (parotid gland, pancreas) : the secretion does not contain mucin.