[Encyclios]

Exocrine glandular epithelia

Exocrine glands consist of a secreting part called the adenomere, and a part that drains the secretion to the outside, called the excretory duct. Depending on their topographical location, they may be intraepithelial or extraepithelial (which in turn are divided into parietal and extraparietal).

Exocrine glands can be unicellular or multicellular.

• Unicellular exocrine glands consist of a single secreting cell; a classic example are the mucipolar calycephalic cells, intercalated with enterocytes in the intestinal mucosa.
• Pluricellular exocrine glands consist of a secreting portion, the adenomere, and an excretory duct that allows the secretion to be poured out.

Pluricellular exocrine glands can be classified according to several criteria:

1. according to the shape of the adenomere: if the adenomere is elongated with a fairly obvious lumen we have tubular glands. A particular case of tubular glands are the tubulo-glomerular glands (the adenomere has the shape of a curled tubule) represented only by sweat glands. If the adenomere is roundish with a small lumen and therefore very little evident we have an acinous gland. If the adenomere is round, large and with a very evident lumen we have an alveolar gland. In the case of branched or compound glands we can have a combination of tubular adenomers with acinar adenomers (tubulo-acinar glands) or with alveolar adenomers (tubulo-alveolar glands);
2. according to the complexity of the organization: if a single adenomere is drained by a single excretory duct we have a simple gland. If, on the other hand, several adenomeres are drained by a single excretory duct we have a branched gland. If the draining excretory ducts are more than one and converging into each other and then into a common excretory duct we have a compound gland;
3. based on the mode of secretion:
   • merocrine secretion: the release of the secretion occurs through the cytoplasmic membrane leaving the cell perfectly intact (exocytosis), examples are the parotid, the exocrine pancreas, salivary glands.
   • apocrine secretion: with the secretion there is a loss of part of the cytoplasm, surrounded by plasma membrane, which thus becomes an integral part of the product of secretion. This group includes the mammary gland (limited to the mechanism of secretion of lipids), the sweat glands with large lumen.
   • holocrine secretion: it occurs when the secretion is expelled in the excretory duct through the disintegration of the cell itself, sebaceous glands are an example of this type of secretion.
4. according to the type of secretion (only for merocrine glands): serous (if the secretion is formed mainly by proteins and water), mucous (if the secretion has a mucopolysaccharide nature and is formed mainly by glycoproteins and water) and serum-mucous (mixed, they are made up of mucous cells and serous cells).