Glands: Types, Function, Inflammation And Diseases

Illustration of the thyroid gland

What are the glands?

The glands are cells or tissues (cell groups) epithelial tissue which are responsible for the synthesis and release of substances into the bloodstream or into a body cavity internal.

In humans, most of the glands originate from epithelial cells that leave the surface on which they develop and “enter” into the underlying connective tissue, developing around them a basal lamina, which is not more than a thin layer of extracellular matrix that separates them from the rest of the epithelium.

The glands are made up of different secretory units, which are responsible for the synthesis and transport of the substances that these structures make. These units are the ducts, the parenchyma, and the stroma (elements of the connective tissue that invade and support the parenchyma).

The substances produced by the different glands (hormones, mucinogen, waxy substance, milk, waste solutions, etc.) are manufactured intracellularly and are subsequently stored in “secretory granules” until the moment of their release.

The different types of glands are of fundamental importance for the proper functioning of all body systems, which is evident from a digestive, respiratory and reproductive point of view, to name a few examples.

Types of glands

According to how they distribute their secretory products, the glands in humans can be classified into two large groups: the exocrine glands and the endocrine glands, which secrete their products towards the external or internal epithelial surface or towards the blood and lymphatic vessels, respectively. .

Exocrine glands

Sweat glands, a type of exocrine gland (Source: Possible2006, via Wikimedia Commons)

This group of glands is characterized by secreting their products through ducts to the surface (external or internal) of the epithelium where they originate. The skin and digestive tract, for example, receive sweat, bile, and digestive enzymes from the sweat glands, liver, and pancreas, correspondingly.

The classification of this group of glands depends both on the chemical nature of the products they secrete and on the number of cells that make them up, and is more or less as follows:

According to what they secrete

Mucous glands : those that secrete mucinogens (mucus substance), which are nothing more than large glycosylated proteins that, when hydrated, form a lubricating and viscous substance, similar in appearance to a gel (mucin), which is the main component of mucus. Examples of these glands are the minor salivary glands of the tongue and palate and the goblet cells.

–  Serous glands : secrete liquids rich in enzymes, a good example of these glands is the pancreas.

–  Mixed glands : as their name indicates, these glands have the ability to produce both mucinogenic-type substances and serous secretions. The sublingual and submandibular glands are mixed.

Salivary glands. 1. Parotid 2. Submaxillary 3. Sublingual.

According to the way they secrete it

In addition to this classification, some authors consider that the exocrine glands can be subclassified according to the mechanism they use to secrete their products, which can be merocrine, apocrine and holocrine.

Merocrine glands : these release their products through exocytosis (such as the parotid gland), so neither the cytosol nor the plasma membrane participate in this process.

Merocrine secretion. Source: CFCF, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons

–  Apocrine glands : are those glands whose cells release part of their cytosolic content with each secretion (example are the mammary glands).

Secretion of the apocrine glands. Source: CFCF, CC BY-SA 4.0, via Wikimedia Commons

Holocrine glands : the cells belonging to these glands, when they die, are the main part of the secretion product.

Example of a holocrine gland. 1. Hair, 2. Skin. 3. Tallow. 4. Hair follicle. 5. Sebaceous gland

Likewise, there are cells in many epithelia that have individual cells capable of secreting substances, that is, of behaving like glands, and whose product does not reach the bloodstream, but affects the functions of the cells around them. This secretory activity is known as paracrine.

According to the number of cells

Depending on the number of cells that make them up, the exocrine glands can also be classified as unicellular glands and multicellular glands.

–  Unicellular exocrine glands : these are the simplest glands, as they consist of a single cell that is responsible for the secretion of products. They are secretory cells isolated in an epithelium and the most outstanding example is a goblet cell, which can exist in different places in the body (the digestive tract, the respiratory tract, etc.).

Goblet cell of the epithelium of the trachea. Source: O’Boyle N., Sutherland E., Berry CC, Davies RL, CC BY 4.0 <https://creativecommons.org/licenses/by/4.0>, via Wikimedia Commons

–  Multicellular exocrine glands : their structure is made up of “clusters” of secretory cells that can be “accommodated” or organized in different ways. Since there are many cells in constant contact and communication, they behave like a secretory organ.

Mammary glands and lymph nodes

Multicellular glands can be simple or compound, whether their ducts are branched or not; or they can be tubular, acinar, alveolar or tubuloalveolar, depending on the appearance of their secretory units.

Endocrine glands

The endocrine glands do not have ducts through which to secrete the products they synthesize within, so their secretory products are discharged directly into the bloodstream or into the lymphatic vessels, as the case may be.

Glands of the human endocrine system: 1) Pineal gland; 2) Pituitary; 3) Thyroid gland; 4) Thymus; 5) Adrenal gland; 6) Pancreas; 7) Ovary and 8) Testicles (Source: Intermedichbo, via Wikimedia Commons)

These glands are mainly responsible for the release of hormones, which are distributed to the different “target” organs. The most important endocrine glands in the body are the adrenal or adrenal glands, the pituitary, the thyroid, the parathyroid and pineal gland, the ovaries, the placenta, and the testes.

– The adrenal glands are located in the upper pole of the kidneys and produce steroid hormones and catecholamines.

Adrenal or adrenal glands (Source: EEOC [Public domain], via Wikimedia Commons)

– The pituitary is responsible for producing and secreting different hormones that participate in the regulation of growth, reproduction and metabolism.

Pituitary in the human brain. Source: User Jomegat

– The thyroid is found in the neck and participates in the secretion of the hormones thyroxine, triiodothyronine and calcitonin, which also influence metabolism.

Thyroid and parathyroid glands (Source: CFCF, via Wikimedia Commons)

– The pineal gland is controlled by periods of light and dark and various lines of evidence suggest that they influence the activity and function of the gonads.

Pineal gland. Nephron [CC BY-SA (https://creativecommons.org/licenses/by-sa/3.0)]

– The ovaries, the placenta and the testicles are responsible for the secretion of “sex” hormones that influence the development of the sexual organs and therefore, on the definition of sex and reproduction.

Among the hormones secreted by the endocrine glands are peptides and proteins, modified amino acids, steroids, and glycoproteins.

The main characteristic of the endocrine glands is that their cells are organized in the form of “cords” or follicles.

Glands with internal cord-like organization are the most common and are distinguished by the fact that their cells “stack” around blood capillaries, towards which they release their products when the appropriate stimulus reaches them.

In the endocrine glands with a follicle-like organization, the cells responsible for secretion form a kind of cavity into which they discharge the hormones they produce. The hormones remain in this cavity until the appropriate stimulus arrives and are subsequently reabsorbed to pass into the blood vessels and capillaries.

It is important to note that some glands of the human body are mixed, which in other words means that they have endocrine portions and exocrine portions, examples of which are the pancreas, testicles and ovaries.

Gland function

The functions of the glands are very diverse and depend mainly on the type of gland in question.

Thus, these glands participate directly in the processing of food during its transit through the digestive tract, starting with the oral cavity (with the salivary glands) and continuing in the stomach and small intestine (intestinal glands and accessory glands of the digestive system).

The sweat glands, another example of exocrine glands, participate in the control of body temperature by eliminating evaporating liquid from the body, releasing calories.

The products secreted by the endocrine glands also exert “global” functions in the body, controlling virtually all physiological processes, especially those related to reproduction, metabolism, growth, control of plasma electrolyte levels, milk secretion, control of body water volume, etc.

Inflammation

Inflammation of the body glands can be due to various kinds of processes: infectious, tumor or traumatic.

Inflammation of the lymphatic glands, also known as “lymph nodes” is usually a symptom of the presence of an infection, since these are glands that are rich in cells or white blood cells, the same that circulate in the blood and participate in the reactions immunological.

The salivary glands can also become inflamed or regrowth, which prevents them from exercising their functions correctly and usually leads to the development of some disease. Mumps is a good example of viral infection inflammation of the parotid gland in the oral cavity.

Diseases

Different diseases are related to the body glands in humans, among them different types of cancers and tumors are common.

Diseases related to the endocrine glands

The most representative pathologies correspond to those that affect the endocrine glands, among which diabetes, osteoporosis, thyroid cancer, hypo- and hyperthyroidism, obesity, growth hormone deficiency, hyper- and hypoglycemia, low testosterone levels, menopause, etc.

There are also Addison’s disease (lesions in the adrenal glands), Cushing’s syndrome (due to excessive production of cortisol in the adrenal glands) and Grave’s disease (related to an increase in the activity of the thyroid gland).

Diseases related to the exocrine glands

Regarding the exocrine glands, some common conditions can be pointed out to the liver or pancreas, which are the well-known “accessory glands of the digestive system”. Such diseases may or may not be of viral origin or may be related to the lifestyle of the people who suffer from them.

Hepatitis is a viral disease that involves inflammation of the liver and affects thousands of people around the world. Alcoholic people also suffer from liver diseases such as cirrhosis, caused mainly by excessive alcohol consumption.

The exocrine portion of the pancreas, which is part of the digestive function, can also suffer from different disorders that affect the normal functioning of this organ or gland.

If the pancreas is unable to secrete the digestive enzymes it secretes (trypsinogen, chymotrypsinogen, procarboxypeptidase, lipase, amylase, elastase, ribonuclease, deoxyribonuclease), this can seriously affect gastrointestinal function.

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