Mixed glands

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Sherman Hoover
Mixed glands

What are mixed glands?

The mixed glands They are a type of exocrine glands, present in different tissues of the human body, that produce combined secretions of serous and mucous substances. Mixed glands are also considered those that function as exocrine and endocrine glands at the same time..

Glands are defined as cells or organized tissues (aggregates of cells) that specialize in the secretion or excretion of substances that they receive from the body's blood and that they process in some way, so that they can be used by other tissues or discarded as waste.

Generally they are structures that are composed of cuboidal epithelium -with cells of equal width and height- or columnar epithelium -with cells of greater height than width-, which rests on a basement membrane and is closely related to a network of blood vessels that irrigate.

Types of glands

There are two main types of glands: endocrine and exocrine. The former secrete their products towards the blood and lymphatic vessels (pancreas, adrenal glands, ovaries, testicles, etc.), while the latter do so towards the external or internal epithelial surfaces of the body..

Some authors define a third type of glands -the mixed glands- that behaves as both an endocrine gland and an exocrine gland.

Exocrine glands and endocrine glands

According to the chemical nature of the substances they secrete, the exocrine glands can be:

  • Mucous: secrete glycosylated proteins, mucinogens.
  • Serous- secrete enzyme-rich fluids.
  • Mixtas- secrete a combination of mucinogens and enzyme fluids.

These glands have special ducts to “discharge” their products towards the surfaces with which they are related..

On the other hand, the endocrine glands generally produce substances of a hormonal nature and are ductless glands, which secrete their products directly into the bloodstream, from which they use for their distribution and transport..

Examples of mixed exocrine glands are the sublingual and submandibular salivary glands, which are found in the region corresponding to the oral cavity and are related to the digestive system..

The main example of mixed exocrine / endocrine glands is the pancreas, as it is responsible for both the production and release of insulin and glucagon (two hormones) and the digestive enzymes necessary for the processing of food at the intestinal level..

Characteristics of mixed glands

Mixed glands have several characteristics:

- They are glands of the exocrine (mucous glands) or exocrine / endocrine type, so their products are released through ducts to the internal or external surface of the body or directly into the bloodstream..

- Mixed exocrine glands produce substances that are a combination of mucinogens (glycosylated proteins that form a type of mucus when hydrated) and fluids with high enzyme content..

- Mixed exocrine / endocrine glands produce both hormonal and enzymatic substances.

- Like all glands, these originate from invaginations of epithelial cells.

Features

The mixed glands, whether we define them as a type of exocrine gland (seromucosal) or as a type of gland that behaves both as an exocrine and an endocrine gland, give epithelial tissues secretory functions of great importance for the production of:

  • Precursors of mucous substances, which function in the protection and lubrication of some tissues.
  • Precursors of enzymes and other molecules, for the hydrolysis of different types of molecules.
  • Hormone precursors, which function in controlling the functioning of different organs and processes of the human body.

Examples of mixed glands

Mixed exocrine glands

The major salivary glands: 1) parotid glands, 2) submandibular glands, and 3) sublingual glands (Source: Goran tek-en, via Wikimedia Commons)

Two good examples of mixed exocrine glands correspond to the submandibular and sublingual glands..

  • The sublingual glands

They belong to the group of major salivary glands, which are responsible for more than 90% of the total daily saliva production. Like the parotid and submandibular glands, the sublingual glands release their products into the oral cavity through long ducts..

These glands are the smallest of the major glands and are found on the "floor" of the mouth, below the tongue, where they release their excretion products, which consist of a mixture of serous and mucous substances in a ratio of 1: 3.

Humans have two sublingual glands, one on each side of the tongue..

  • The glands submandibular

Also known as the submaxillary glands, they are the second largest in the human body and are found, as the name implies, in the region under the jaw. They pour their products into the oral cavity through ducts that open in the membrane under the tongue.

The products of these glands form a thick saliva, rich in glycoproteins and growth factors essential for the lubrication and protection of the mucosa that lines the mouth on the inside. Humans have two submandibular glands, one on each side of the jaw..

Mixed exocrine / endocrine glands

This other type of mixed gland is characterized by having the ability to release some of its secretion products through ducts and some directly into the bloodstream..

  • The pancreas

If the pancreas is considered as a glandular organ, then it is said that this is a type of mixed gland, since it has cells specialized in the production of exocrine secretions and cells specialized in the production of endocrine secretions (some authors consider it an organ formed by two glands).

Among its main functions as an exocrine gland, the pancreas is responsible for producing the enzymes necessary for the processing or digestion of proteins, lipids, carbohydrates and nucleic acids present in the food we eat daily..

Accordingly, the pancreas participates as an accessory organ of the digestive system, since its exocrine products are released into the region corresponding to the duodenum of the small intestine through a special glandular duct, the pancreatic duct..

On the other hand, the endocrine functions of the pancreas are related to the production of two hormones with antagonistic functions: the insulin and the glucagon.

Both hormones participate in the maintenance of glucose homeostasis in the blood, allowing it to be released into the bloodstream (glucagon) or introduced into the cells (insulin), decreasing its blood levels..

  • The kidneys

3d illustration of kidneys

The kidneys are the organs responsible for the production of urine (exocrine function), one of the liquid waste products of animal metabolism.

However, they are also recognized for their dual endocrine function, since they are responsible for the production of some important hormones such as renin, angiotensin, erythropoietin and prostaglandins, among others, which function in the control of blood pressure , in the excretion of sodium and water ions, in the production of red cells, in the homeostasis of calcium, etc..

  • The testicles and ovaries

These two organs, belonging to the male and female reproductive systems, respectively, behave as mixed endocrine and exocrine glands..

As endocrine glands, the ovaries produce, among other things, estrogen, a female sex hormone, and the testes produce testosterone (and others), a male sex hormone.

From the exocrine point of view, on the other hand, the ovaries are in charge of the production of female sex cells and the testes produce male sex cells..

References

  1. Dudek, R. W. (2000). High-yield histology. Williams & Wilkins.
  2. Gartner, L. P., & Hiatt, J. L. (2012). Color atlas and text of histology. Lippincott Williams & Wilkins.
  3. Johnson, K. E. (1991). Histology and cell biology.
  4. Lower, W. E. (1934). The Exocrine and Endocrine Functions of the Testes. The Journal of Urology, 31 (3), 391-396.
  5. Renner, I.G., Savage, W. T., Stace, N. H., Pantoja, J. L., Schultheis, W. M., & Peters, R. L. (1984). Pancreatitis associated with alcoholic liver disease. Digestive diseases and sciences, 29(7), 593-599.
  6. Ross, M., & Pawlina, W. (2017). Histology, A text and atlas, with correlated cell and molecular biology. Lippincott Williams & Wilkins.

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