Generality
The exocrine glands are the glands of the human body that release their own secretion on the epithelial surface of the skin or of some hollow internal organs, such as the mouth, trachea or stomach.
The exocrine glands can be multicellular structures (ie consisting of multiple cells) or unicellular (ie formed by a single cell).
The exocrine glands include: the sweat glands, the sebaceous glands, the mammary glands, the glands that produce gastric digestive enzymes, the muciparous glands, the salivary glands, the lacrimal glands, the pancreatic exocrine glands, the hepatic exocrine glands and the prostate.
Short review of what a gland is
In medicine, the word " gland " identifies an organ, a grouping of cells (such, however, that it cannot be defined as an organ) or a simple single cell, having the capacity to produce a particular substance, which in technical jargon is called secreted .
What are the Exocrine Glands?
The exocrine glands are the characteristic glands of the human body that release their secretion, usually by means of a special expulsion channel, on the epithelial surface of the skin (thus outside the human body) or of some hollow internal organs, such as the mouth, the trachea or the stomach (in this situation, the release of the secretion takes place on the inner surface of these hollow organs).
Examples of exocrine glands
The main exocrine glands of the human body are:
- Salivary glands . Located in the mouth, they are the exocrine glands that produce saliva;
- The sweat glands . Located on the surface of the skin, more diffusely in some areas of the body rather than in others (eg, armpits), are the sweat glands responsible for the production of sweat;
- Mammary glands . They are the exocrine glands that, following certain hormonal stimuli, produce milk;
- The sebaceous glands . Based on the skin, often in association with the hairs, are the exocrine glands responsible for the production of sebum;
- The mucipar glands . Located on the inner surface of the respiratory tract and digestive organs, are the exocrine glands with the task of secreting the mucus;
- Lacrimal glands . Based in the eye, it is the exocrine glands that have the task of secreting tears;
- Peptic glands . Located in the stomach, due to the precision of the so-called fundus, are the exocrine glands that secrete essential enzymes for the digestive process;
- Pancreatic exocrine glands . Belonging to the pancreas, the exocrine glands are responsible for the production of pancreatic juices, essential substances in the digestive process;
- The hepatic exocrine glands . Belonging to the liver, they are the exocrine glands delegated to the production of bile, another important substance involved in the food digestion process;
- The prostate . A fundamental component of the male reproductive system (therefore present in men), it is the glandular organ that secretes the prostatic fluid, which serves to make up the sperm.
Exocrine Glands and Endocrine Glands: the differences
The exocrine glands are opposed, in a sense, to the endocrine glands .
The endocrine glands are the characteristic glands of the human body that release their secretion - usually a protein-based secretion with hormonal function - in the circulatory stream, ie in the blood .
Did you know that ...
Pancreas and liver act not only from exocrine glands, but also from endocrine glands. In fact, it is up to the pancreas and the liver to secrete and release into the blood some fundamental hormones of life ( insulin and glucagon, as far as the pancreas is concerned, and angiotensinogen and somatomedin, as far as the liver is concerned).
Structure
Except in special cases, the exocrine glands have a structure comprising two elements: the so-called glandular portion (or adenomer ) and the so-called excretory duct .
The glandular portion is the part responsible for the production of the secretion; in fact, therefore, it represents the actual gland.
More cells ( multicellular glands ) participate in the formation of the glandular portion, which can be arranged so as to give the aforesaid glandular portion a rounded shape (exocrine acinar glands) or an elongated shape (tubular exocrine glands).
The type of cell or cells of the gland portion determines the nature of the secretion (ex: the cells of the mammary glands have such characteristics that they produce a different secretion from the cells of the sweat glands.
Corresponding to the expulsion channel mentioned in the initial definition, the excretory duct is the tubular-shaped structural element used to transport the product of the glandular portion towards the epithelial surface.
Culminating with a characteristic opening (a sort of "vent"), the excretory duct can be simple (simple exocrine gland) or branched (branched exocrine gland); when it is simple, it can be linear or similar to a spiral.
Did you know that ...
In the human body, there are exocrine glands that are at the same time acinar and tubular, as they have both the cells that give the classic round shape, and the cells that give the classic elongated shape.
These acinar and tubular exocrine glands are called tubulo-acinar exocrine glands .
Special cases: Exocrine Glands without Ductus Escretor
The particular cases to which the article referred previously are the exocrine glands lacking the excretory duct, therefore constituted by the single glandular portion.
Although they are devoid of the duct necessary for the expulsion of the secretion, these particular exocrine glands are in any case capable of expelling their secretion adequately, as they have a chalice-like shape which guarantees communication with the epithelial surface.
The exocrine glands lacking an excretory duct are unicellular glands - that is, constituted by a single cell - and, in the human body, they are always used for the production of mucus (muciparous glands).
Did you know that ...
Due to their typical calyx shape, the cells that represent the exocrine glands lacking an excretory duct are known as goblet cells .
Classification
There are three different ways of classifying exocrine glands: a first way is based on structure; a second way is based on the secretion mechanism; a third way is based on the type of secreted product.
Classification based on the structure
The classification based on the structure distinguishes the exocrine glands in two types: the unicellular exocrine glands and the multicellular exocrine glands.
Of the unicellular and multicellular exocrine glands it has already been widely spoken of previously, consequently it is superfluous to add more in this site, except that the parameter of distinction is the number of cells constituting the glandular portion .
- Examples of unicellular exocrine glands are the muciparous glands (ie that produce mucus), present on the epithelial surface of the trachea, bronchi or stomach.
- Examples of multicellular exocrine glands are the mammary glands, the salivary glands and the sebaceous glands.
Classification based on the secretion mechanism
The classification based on the secretion mechanism distinguishes the exocrine glands in three types: the merocrine glands, the apocrine glands and the olocrine glands .
- Merocrine glands: they are the exocrine glands whose constituent cells expel their own secretion through small channels or pores located on the cell membrane; the use of this method of expulsion of the secretion is called exocytosis.
The exocytosis mechanism adopted by the merocrine glands does not undermine its integrity and does not alter its shape.
Examples of merocrine glands are the salivary glands and the acinar glands of the pancreas.
- Apocrine glands: these are the exocrine glands whose constituent cells release their own secretions expelling the part of the cytoplasm that includes the aforementioned secretion and sacrificing a portion of the plasma membrane.
The apocrine glands, therefore, resort to a secretion mechanism that modifies the structure of their cells, not only from the point of view of the cytoplasm but also of the plasma membrane.
Examples of apocrine glands are the mammary glands.
- Olocrine glands: these are the exocrine glands whose constituent cells release their own secretions by self-destructing within the excretory duct.
To implement such a secretion mechanism, a high cellular renewal capacity is needed; the olocrine glands perfectly meet this requirement.
Examples of olocrine glands are the sebaceous glands.
Classification based on the type of secreted product
The classification based on the type of secretion produced distinguishes the exocrine glands in 4 types: serous glands, muciparous glands, serum-mucous glands (or mixed glands), and sebaceous glands.
- Serous glands: these are the exocrine glands whose cells produce a protein-rich, watery and light-colored secretion.
Examples of serous glands are the peptic glands of the stomach.
- Mucipar glands: as stated on more than one occasion, they are the exocrine glands whose cells secrete mucus.
- Serum-mucosal glands: these are the exocrine glands equipped with both mucus-producing cells and cells producing the same protein secretion deriving from the serous glands.
In other words, they are, at the same time, serous glands and muciparous glands.
Examples of serum-mucous glands are the salivary glands.
- Sebaceous glands: already known to readers just like previous mucipar glands, they are the exocrine glands whose cells produce sebum.
