EXOCRINE GLANDS: CHARACTERISTICS, FUNCTIONS, TYPES - BIOLOGY - 2024

The exocrine glands are a type of gland that secrete their products through ducts that open to the external surface of the body or to the epithelial surface, external or internal, of the epithelium that gives rise to them.

A gland is a functional unit of cells that work together to synthesize and release a product in a duct or directly into the bloodstream. In the human body there are two main types: the exocrine glands and the endocrine glands.

Types of exocrine glands (Source: OpenStax College via Wikimedia Commons)

The exocrine glands differ from the endocrine glands in that the latter have lost their ducts and, consequently, secrete their products directly into the blood or lymphatic vessels, through which they distribute and reach their target organs.

Said structures arise through a process of "budding" of the epithelium, which is the result of a complex interaction between mesenchymal and epithelial cells and is promoted by different growth factors.

The exocrine glands are very diverse, both in number and functions, which is why many organ systems use them to carry out their functions, examples of which are the skin, mouth, stomach, pancreas, duodenum and breasts..

characteristics

All types of glands have their origin in epithelial cells. These cells leave the surface where they developed and invade the underlying connective tissue, where they make a basal lamina around it.

The ducts and secretory units of the glands form what is known as "glandular parenchyma", while the connective tissue that invades and supports the parenchyma is known as "glandular stroma".

The secretions produced by the glands originate intracellularly in the cells that make them up, and are synthesized as macromolecules that are grouped or stored in special vesicles known as "secretory granules".

The products of the exocrine glands may or may not be modified as they pass through the glandular ducts, since substances can be removed or added to them.

This happens, for example, in the major salivary glands, where there are ion pumps that modify the composition of the substances made by the secretory cells.

Features

Because the exocrine glands are distributed in many different organs and tissues in the body, they perform a variety of functions.

In the skin there are sweat glands and sebaceous glands. The former are from the most abundant exocrine glands in the body, as they are scattered throughout the skin and are responsible for secreting hyaline fluids that help regulate body temperature.

The sebaceous glands are also very abundant and are responsible for the production of oily or fatty liquids that constantly lubricate the skin.

In the mouth, the salivary, parotid, submandibular and sublingual glands work together to secrete semi-serous products that act directly in the first stage of food digestion and lubricate mucosal surfaces.

Salivary glands (Source: BruceBlaus. When using this image in external sources it can be cited as: Blausen.com staff (2014). «Medical gallery of Blausen Medical 2014». WikiJournal of Medicine 1 (2). DOI: 10.15347 / wjm /2014.010. ISSN 2002-4436. Via Wikimedia Commons)

In the stomach, the pyloric glands, the cardiac glands, and the fundus glands participate in the release of digestive enzymes, regulate stomach pH, ​​and participate in the absorption of some vitamins and minerals.

The duodenal and pancreatic glands have digestive functions and also participate in the protection of the mucosa.

Another of the most relevant examples of exocrine glands are the breasts, since the mammary glands are housed in these, responsible for the production and secretion of milk and the transmission of passive immunity from the mother to the neonate.

Types

Exocrine glands are classified according to the nature of the products they secrete, their shape and the number of cells that compose them (uni- or multicellular).

According to the type of secretion, these glands are classified as mucous glands, serous glands, mixed glands, and waxy glands.

Mucous glands

They secrete mucinogenic substances, which are compounds rich in glycosylated proteins and that, when hydrated, swell and form a moisturizing substance known as mucin, which is the most important component of mucus.

Examples of these glands are the goblet cells in the intestine and the minor salivary glands in the tongue and palate.

Serous glands

These glands secrete a watery fluid rich in enzymes. Serous glands are those of the exocrine portion of the pancreas, for example, that secrete proteolytic digestive enzymes.

Mixed glands

Mixed glands contain secretory units, also known as acini, which can produce mucous secretions and serous secretions, hence their name "mixed."

The sublingual and submandibular glands are good examples of mixed glands in man.

Waxy glands

These are the ceruminous glands of the external auditory canal. These are responsible for the secretion of wax in this canal.

In turn, depending on the secretion mechanism of the cells belonging to the glands, the exocrine glands can be classified as merocrine, apocrine and holocrine.

Exocrine gland secretion mechanism (Source: Fulvio314 via Wikimedia Commons)

- If the release of secretion products occurs through exocytosis, the glands are merocrine (parotid gland).

- If the secretion involves the release of internal products, such as the apical part of the cytosol of the secretory cell, for example, the glands are apocrine (case of the mammary glands of lactating mammals).

- When the secretion corresponds to the mature glandular cells that have died, then the glands are holocrine glands and an example of these are the sebaceous glands in the skin.

If the classification is related to the number of cells, then there are unicellular and multicellular glands.

Unicellular exocrine glands

These are the simplest exocrine glands, as they are made up of a single cell that is distributed in an epithelium.

The goblet cells of the intestine and the respiratory tract are the most prominent examples of this type of gland. They are mucous glands that secrete mucus that protects the pathways where it is found and its name derives from its morphology (they are similar to a balloon).

Its basal portion is attached to the basal lamina of the epithelium where they are located, while its expanded apical portion, called "theca", is oriented towards the lumen of the digestive tract or respiratory system.

Large numbers of mucin-laden "droplets" are found in teak and their release is stimulated by parasympathetic innervation and by local chemical irritation.

Multicellular exocrine glands

These types of glands are made up of more than one cell and consist of organized "clusters" of different secretory units (secretory cells) that are organized in different ways, according to which they are classified, and that function as a secretory organ.

Thus, there are compound and simple multicellular glands, whether their excretory duct is branched or not, respectively. Depending on their morphology, they can be tubular, acinar (alveolar) or tubuloalveolar.

Large multicellular exocrine glands are surrounded by a kind of "capsule" and have internal divisions known as "lobes" or "lobules" that are produced by the segmentation of said capsule; the vessels, nerves and ducts, enter and leave these glands through the partitions or segmentations.

References

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