[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.