It contains cells adapted to a secretory function for the benefit of the body, the epithelial cells.
Glandular epithelia are tissues made up of cells that produce substances for the benefit of the body.
These cells do not use this secretory product but make it available to other body elements through excretion.
Epithelial cells are generally organized into secretory units that, in association with connective tissue and blood vessels, and nerves, form glands.
Some secretory units are associated with myoepithelial cells that, by their contractions, favor the expulsion of the secretory product from the secretory team.
The product is excreted either:
- On the body’s surface (epidermis), either on the surface of a body cavity that communicates with the outside (mucous membrane) through an excretory canal or exocrine gland.
- Into the bloodstream through the endocrine gland.
- Tissue structure of the glandular epithelium
- The cells of the glandular epithelial tissue are generally cylindrical or cuboidal. Glandular epithelial tissue can be classified according to specific criteria:
The place of discharge
Secretion is described as exocrine when glandular cells discharge their secretory product outside the body or into a cavity in communication with the outside of the body.
The epithelial cells of an exocrine gland are organized into secretory units that have a closed-end located opposite an open end at the level of the lining epithelium.
Through the open end, the secretory product is discharged out of the body either directly (simple exocrine gland), through an unbranched excretory duct (simple exocrine gland), or a branched excretory duct system (compound exocrine gland). ).
Examples of exocrine glands: salivary, sebaceous, sweat, breasts, and tears.
In this type, the substances are very varied, distinguishing respectively in enzymes (salivary amylase, ptyalin); oily lipid-based secretions; those that produce sweat, containing water and salts; milk secretion during lactation; and the tear fluid, made up proportionally of water, salts, proteins, and fats.
A secretion is called endocrine when the epithelial cells discharge their secretory product within the body, more precisely in the bloodstream, without the intermediation of channels, being dispersed through the blood for the entire organism.
In the case of the endocrine glands: pituitary, thyroid, parathyroid, adrenal, secreting mainly hormones (stimulating follicle, luteinizing, somatropin, prolactin, thyroxine, triiodothyronine, parathormone, adrenaline, androgens, and others).
In the endocrine glands, except the thyroid, the epithelial cells are arranged in cords that anastomose and create spaces between them occupied by blood capillaries.
The glands that release secretions in and out of the body are called amphibians.
The liver and pancreas are amphiphilic glands.
In the liver, endocrine (production of various molecules released into the blood) and exocrine (production of bile) function is carried out by the same cell, the hepatocytes.
Hepatocytes are organized into lobes where cords of hepatocytes separated by blood capillaries converge in a central vein.
Blood capillaries (hepatic sinusoids) are found between the cords of hepatocytes. It is in these sinusoids that the endocrine secretion is discharged.
Bile is discharged into the bile canaliculi, formed by a depression of the plasma membranes between two neighboring hepatocytes.
The bile ducts are the confluence of the bile canaliculi to the excretory ducts of the liver.
These channels are seen in the connective tissue located on the periphery of the lobes. The media have different gauges and wall thicknesses.
Different epithelial cells provide the endocrine and exocrine functions in the pancreas.
Endocrine cells are grouped into rounded islets, better known as the islets of Langerhans.
The cells are arranged in anastomosed cords within each island and provide spaces for the blood capillaries between them.
Exocrine cells organize into acini and secrete enzyme precursors.
These glands perform endocrine and exocrine functions, releasing their secretions into the blood or cavities.
An example of mixed glands: the testicle and the ovary. These sex glands produce the hormones testosterone and progesterone, respectively, and sperm and eggs. Another example is the liver, creating and secretes proteins and bile.
The mode of secretion
Glandular epithelial cells use four secretion mechanisms: merocrine secretion, holocrine secretion, apocrine and eccrine secretion.
Merocrine secretion involves the mechanism of exocytosis. After the endoplasmic reticulum, the secretion product passes through the Golgi apparatus and is found in secretory vesicles.
Secretory vesicles fuse with the plasma membrane and release their contents outside the cell.
This mechanism is used by the serous and mucous epithelial cells of the salivary glands and goblet cells.
In the holocrine secretion used by the sebaceous gland, the cell cytoplasm is wholly filled with the lipid secretion product.
The nuclei enter the pyknosis, and the cells as a whole are excreted.
Apocrine secretion is used in particular by the mammary gland cells for the lipid fraction of their secretion.
These cells synthesize lipids that accumulate at the apical pole and fuse into a large lipid globule.
The apical pole is separated from the rest of the cell: the secretion is constituted by the lipid globule surrounded by a thin layer of cytoplasm and by the plasma membrane.
In the secretory units of the falsely apocrine sweat glands, the coexistence of cylindrical cells with a domed apical pole and cubic cells suggests that this gland secretes apocrine.
But this is not the case; sweat is secreted by eccrine, and, presumably, proteins are secreted by merocrine.
Eccrine secretion does not alter any cellular structure; the secretory product leaves the cell by diffusion or transport through the plasma membrane.
This mechanism is used in particular by the eccrine sweat gland.
The nature of the discharge
Glandular cells adapt to their secretory function by developing the organelles involved in the synthesis and excretion of the product they are developing.
The nature of the secretion and the development of the organelles involved in this secretion condition the morphological aspects of these cells.
As a result, the type of substances they produce can be characterized as:
- Mucous membranes secrete fluids when they are highly viscous (mucus).
- Serous: when secretions are more fluid and rich in protein.
- Mixtures: when they eliminate associated mucous and serous secretions.
The shape of the secretory unit
Epithelial cells, exocrine or endocrine, can gather and organize themselves into structures in specialized ways: the secretory units.
Secretory units of the exocrine glands
The secretory units of the exocrine glands have a blind end located opposite an open end through which the secretory product of the epithelial cells escapes.
These exocrine secretory units are tubular (straight, circumvallate, twisted, curly, branched), acinar or alveolar.
Secretory units of the endocrine glands
Each endocrine gland has cordial secretory units except the thyroid gland.
To allow temporary storage of the secreted product, the secretory units of the thyroid gland are spherical and form thyroid vesicles or follicles lined by epithelial cells.
The structure of the excretory tree
To carry their secretion out of the body or into a cavity in communication with the outside of the body, the secretory units of the exocrine glands emerge directly outside the body or use excretory ducts that conduct the secretion out of the body.
The structure of the excretory ducts makes it possible to distinguish between simple exocrine glands and compound exocrine glands.
Simple exocrine glands
Simple exocrine glands may not have an excretory duct. For example, the secretory unit of the Lieberkühn gland opens directly into the intestinal cavity.
Simple exocrine glands can use an unbranched excretory duct.
For example, the secretory unit of the eccrine sweat gland discharges sweat into the sweat duct, the excretory channel of this same gland.
Thanks to this unbranched excretory duct, sweat can be carried to the body’s surface.
Each sebaceous gland discharges its secretion into a hair follicle through a short duct lined with an epithelium comparable to the epidermis.
Compound exocrine glands
The compound exocrine glands use a system of branched excretory ducts.
The coexistence of excretory canals of different sizes and wall thicknesses is an argument for the existence of a branched excretory tree.
For example, in this salivary (parotid) gland, two types of excretory ducts are visualized: the intermediate canal (narrow and consisting of a simple cuboidal epithelium that rests on a basal lamina) and the Pflüger canal that follows (more comprehensive and consists of a simple columnar epithelium that rests on a basal lamina and presents basal folds).
The possible presence of various types of epithelial cells
The secretory units can be pure or mixed. Secretory units are called “pure” when they consist of a single type of epithelial cells.
They are otherwise described as “mixed” when they have at least two types of epithelial cells.
By extension, glands containing a single epithelial cell type will be described as pure; Glands showing at least two types of epithelial cells will be mixed.
Pure secretory unit (gland)
The parotid salivary gland, all of whose acini are serous cells, can be described as pure.
An enlargement of a serous acinus shows the pyramidal acinar cells that constitute it, bordering a very narrow central cavity.
This cavity, where the secretion is poured, communicates with the excretory canal.
Mixed secretory unit (gland)
The submaxillary and sublingual salivary glands can be described as mixed because specific secretory units consist of serous cells and cells of the closed apical mucosa.
This enlargement shows the arrangement of gland cells within a mixed secretory unit.
Cylindrical mucosal cells (apical pole closed) show a flattened nucleus in the basal bar and mucinous grains of poor color that occupy the entire cytoplasm.
The serous cells are rejected in the remote part of the secretory units in the Gianuzzi crescent.
The intercellular canaliculi allow the secretory product to flow into the secretory unit’s lumen.
The phases of secretion function
The secretion function is carried out in 4 phases:
It consists of capturing the metabolites necessary for the synthesis of the product from the blood vessels.
Synthesis or product development phase
A secretory polarity is observed in each epithelial cell and depends on the position of the organelles in their cytoplasm.
Ergastoplasma is found at the basal pole of the cell.
The synthesis begins, which continues in the Golgi apparatus located most often supranuclear.
The Golgi apparatus marks the secretory pole of the cell. In this way, subsequently, the substances formed accumulate in the apical pole of the cell, better known as the pole of excretion.
Storage phase (inconstant)
The secretory product can be stored intracellularly in the form of secretory grains.
A nervous or hormonal effect can regulate the excretion phase during which the product is expelled from the cell.
Epithelial tissues are formed from the embryonic period by cells specialized in the synthesis of secretions.
As these cells multiply, they become embedded in the connective tissue.
However, in addition to the multicellular glands, for example, the pancreas and gonads, there are glands of the unicellular type, such as goblet cells, between the epithelium lining the trachea.
The glands are formed by budding and invagination of a lining epithelium in the mesenchyme.
If the connection with the shunt epithelium persists, we are talking about exocrine glands.
The product of the secretion of this type of gland is shed on the body’s surface (epidermis) or the surface of a body cavity in communication with the outside (mucous membrane).
If the connection with the bypass epithelium is lost, then it is called endocrine glands, which in most cases, shed their secretory product into the bloodstream.
There are glands with the two modes of secretion called ampicillin glands (pancreas).
Examples of glands
- Simple exocrine glands.
- The Sebaceous Glands.
- The glands of Lieberkhün.
- Eccrine sweetfish glands.
- The falsely apocrine sweat glands.
- The exocrine compound glands.
- The parotid glands.
- The submandibular glands.
- The sublingual glands.
- The endocrine glands.
- The adrenal cortex.
- The thyroid glands.