Morphofunctional characteristics
The histological basis for the formation of most glands is glandular epithelium. In the body, it forms two types of glands: exocrine (having excretory ducts) and endocrine.
Endocrine glands are characterized by the following morphological features:
- Complete absence of terminal secretory sections and excretory ducts.
- Abundant vascularization (tissue is penetrated by a dense network of blood capillaries).
- Secretion is released directly into the internal environment of the body: predominantly into the blood, somewhat less frequently into the lymphatic bed.
Interestingly, in terms of its level of organization, the endocrine apparatus is represented not only by large multicellular organs, but also by separate isolated cells distributed across different tissues.
Nature of hormones and their mechanism of action
The product of the endocrine system structures is hormones. From the perspective of physiology and histology, these are specialized regulatory compounds.
- Delivery mechanism: hormones are secreted into the bloodstream and transported with the blood flow to organs distant from the site of synthesis.
- Mechanism of action: hormones do not affect all tissues indiscriminately. They interact exclusively with strictly specific target cells. The result of such interaction is specific changes in the metabolism of these cells.
Classification of endocrine structures
Depending on the level of tissue organization and functions performed, all hormone-producing formations are divided into 4 main groups:
- Central endocrine organs: hypothalamus, pituitary gland (hypophysis), and pineal gland (epiphysis).
- Peripheral endocrine glands: thyroid gland, parathyroid glands, and adrenal glands.
- Organs with mixed function: combine endocrine and non-endocrine tasks. These include the pancreas, gonads (testes and ovaries), and placenta. Note: the kidneys and thymus are also usually included in this group as they are capable of producing hormone-like substances.
- Solitary hormone-producing cells: form the diffuse endocrine system (DES). They do not form a single organ, but are localized in the structures of the nervous, digestive, respiratory, and other body systems.
Briefly: answers to topical questions
What types of blood capillaries are characteristic of the structure of endocrine glands?
The following types of capillaries are directly indicated for endocrine structures in sources:
- Fenestrated (visceral) capillaries — localized in endocrine glands; allow the passage of a large amount of water with dissolved substances and macromolecules. In pancreatic islets, between insulocytes, there are blood capillaries of the fenestrated type with a pericapillary space where hormones initially enter.
- Sinusoidal capillaries — described in the anterior lobe of the pituitary gland as a secondary capillary network of the portal system. They have a wide lumen and fenestrated endothelium; through their wall, releasing and inhibiting hormones (liberins and statins) reach adenocytes, and anterior lobe hormones are secreted back into the blood.
A common feature of endocrine glands is abundant vascularization, a dense network of capillaries.
What is the embryonic origin of central and peripheral endocrine glands?
Endocrine cells have diverse origins spanning all three germ layers and mesenchyme.
- Central endocrine organs:
- Hypothalamus — secretory neurons of the hypothalamus are derivatives of the neuroectoderm.
- Pineal gland (epiphysis) — classified as a derivative of the neuroectoderm.
- Pituitary gland (hypophysis): adenohypophysis is indicated as a derivative of the foregut epithelium with mixed ecto-entodermal origin; in embryogenesis, it forms from the oral bay (stomodeum) via the hypophyseal pouch, from the anterior wall of which the anterior lobe forms, and from the posterior — the intermediate lobe. Neurohypophysis develops as an outgrowth of the diencephalon: the proximal part forms the hypophyseal stalk, and the distal part forms the posterior lobe of the pituitary gland.
- Peripheral endocrine glands:
- Thyroid gland: follicular cells are indicated as derivatives of the foregut epithelium; calcitoninocytes (C-cells) as derivatives of the neuroectoderm.
- Parathyroid glands — indicated as derivatives of the foregut epithelium; in the entodermal derivative scheme, they are listed among the prechordal plate derivatives.
- Adrenal glands: cortical endocrinocytes originate from the coelomic epithelium (mesoderm), medullary endocrinocytes from the neuroectoderm.
Which specific cells are part of the APUD system?
The APUD series is described as a group of neuroendocrine cells united by biochemical features: they are capable of simultaneously producing specific peptide hormones and biogenic amines.
The sources directly mention the following cells and groups:
- Chromaffin cells — belong to the APUD series; in addition to catecholamines, they are capable of synthesizing regulatory peptides, including enkephalins and endorphins, in small amounts.
- Endocrinocytes (K-cells) of the tracheobronchial tree epithelium — belong to the diffuse endocrine system (APUD system); secrete basally bioamines (norepinephrine, serotonin) and hormone-like peptides (bombesin, calcitonin).
- Neuroendocrine cells of the nervous system, cells of certain glands, and solitary endocrinocytes — indicated as part of the APUD series.
- Solitary hormone-producing cells of the diffuse endocrine system are localized in the nervous, digestive, respiratory, and other systems.
To memorize the central organs of the endocrine system, use the rule of three "H-H-P": Hypothalamus, Hypophysis (Pituitary), Pineal gland.
Class and exam questions
How do endocrine glands differ histologically from exocrine glands?
Endocrine glands are completely devoid of excretory ducts and terminal sections. In addition, they are characterized by a very dense network of blood capillaries into which the secretion is directly released.
What belongs to endocrine organs with mixed function?
These include structures that perform both endocrine and other (e.g., exocrine) functions: the pancreas, gonads (ovaries, testes), placenta, as well as the kidneys and thymus.
Where are solitary hormone-producing cells located?
They form the diffuse endocrine system (DES) and are widely localized in the organs of the digestive, respiratory, nervous, and other systems.
What else to study on this topic
- Histological structure and cellular composition of the pituitary gland
- Morphology of peripheral glands: thyroid gland and adrenal glands
- Structural features of the diffuse endocrine system (DES)
- Types of capillaries in endocrine organs
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