[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).

[Encyclios]

Endocrine glandular epithelia

The endocrine glands are devoid of excretory ducts and pour their secretions (hormones) directly into the bloodstream to specific organs (called targets) in order to regulate their activity, thanks to the presence of a rich network of capillaries in the connective tissue support that surrounds the glands themselves. Hormones can be of amino acid or steroid origin.

The endocrine glands are generally formed by epithelial cells arranged to form cords or islands; in the case of the thyroid gland they form follicles instead. The endocrine glands constitute separate organs (e.g. pituitary, thyroid, parathyroid, adrenal glands) or alternatively are contained within other organs (e.g. islets of Langerhans in the pancreas).

There are pure endocrine glands, i.e. formed exclusively by endocrine glandular tissue, and amphiphrine glands, i.e. composed of both endocrine and exocrine glandular tissue (typical example is the pancreas, whose parenchyma, mostly exocrine secretion, has clusters of cells with endocrine activity, the pancreatic islets or Langerhans islets).

The endocrine glands are classified into:

• endocrine glands with solid epithelial cords: the secreting cells form cellular cords differently arranged in space. Most endocrine glands include: pituitary, parathyroids, adrenals, epiphysis, placenta, corpus luteum;
• endocrine glands with closed follicles: the secreting part is formed by follicles that contain within them the precursor of thyroid hormones (thyroglobulin), the only example is the thyroid;
• islet endocrine gland: the islets of Langerhans of the pancreas;
• interstitial endocrine gland: found in the interstitial spaces between the seminiferous tubules of the testis, in groups of six to eight or more cells placed to surround a capillary, into which they input the secretion (testosterone) and into the ovary.