Producer cells and the organization of endocrine tissue
In the human body, hormones are synthesized not only in classic glandular structures. There are several different types of producer cells, each with its own origin and specific localization:
- Glandocytes — true, highly specialized glandular cells. Their main task is the production of hormones.
- Neurosecretory cells — unique cellular elements that combine the morphological features of nerve cells (neurons) and the functional properties of glandocytes. A major source of such cells is the hypothalamus. It synthesizes controlling releasing and inhibiting hormones (releasing factors and statins), as well as effector hormones (antidiuretic hormone and oxytocin).
- Chromaffin cells — specific secretory elements.
- Unspecialized cells — structures capable of secreting so-called tissue hormones in addition to their primary functions.
All endocrine tissue is divided in terms of architecture into two global pools. The first is endocrine glands (organs whose primary function is hormone production). The second is the diffuse endocrine system, in which single producer cells are distributed across various organs and tissues.
Classification of endocrine glands
True endocrine glands have distinct morphofunctional features. First and foremost is the complete absence of excretory ducts: all produced secretions enter the internal environment directly. In addition, they are characterized by extremely intense cellular metabolism and a very dense capillary network ensuring rapid hormone transport. Some organs function as mixed-secretion glands (e.g., the pancreas and gonads), combining both exocrine and endocrine components.
The physiological classification of glands is based on their localization and dependence on higher regulatory centers:
1. Central glands They are located within the brain. This group traditionally includes the pituitary gland and the pineal gland.
2. Peripheral glands These organs are located outside the central nervous system and are divided into two subgroups depending on whether they are controlled by the pituitary gland:
- Pituitary-dependent: the production of effector hormones in these glands is strictly controlled by trophic hormones of the adenohypophysis. This category includes the adrenal cortex, thyroid gland, and incretory cells of the gonads.
- Pituitary-independent: function autonomously from adenohypophyseal hormones. These include the adrenal medulla, parathyroid glands, and the endocrine part of the pancreas.
Diffuse endocrine system
In addition to structured glands, the diffuse endocrine system is of colossal importance in physiology. Its largest representative is the enteric system — the pool of endocrine cells in the gastrointestinal tract.
Here, crucial gastrointestinal hormones are produced: gastrin, cholecystokinin, secretin, VIP (vasoactive intestinal peptide), somatostatin, and enkephalin. They are synthesized in the digestive organs as well as in neurons of the central nervous system. Their tasks include regulating gastrointestinal motility, secretion, and absorption, controlling metabolism, releasing regulatory peptides, and shaping feeding behavior.
Secretory cells of the diffuse system are also present in many other organs:
- Heart: cardiomyocytes, predominantly located in the right atrium, produce atrial natriuretic peptide (ANP, or atriopeptin). It exerts a wide range of renal and vascular effects, lowering blood pressure and stimulating natriuresis.
- Kidneys: serve as the source of erythropoietin (a hormone stimulating hematopoiesis — erythropoiesis), calcitriol (responsible for calcium metabolism regulation), and renin (an enzyme that acts as the initiating link in the renin-angiotensin-aldosterone system).
- Liver: synthesizes somatomedins and angiotensinogen (a protein that is a precursor to angiotensin I).
- Other organs: hormonally active cells have also been found in the epithelial cells of the bronchi and lungs, as well as in fat cells (adipocytes).
Briefly: answers to topical questions
Where are chromaffin cells located and what hormones do they secrete?
Chromaffin cells are located in the adrenal medulla (layer), which represents a modified sympathetic ganglion. Upon excitation, these cells secrete catecholamines and associated substances into the blood:
- Adrenaline (Epinephrine) — accounts for about 90% of secretion upon excitation.
- Noradrenaline (Norepinephrine) — accounts for about 10% of secretion.
- ATP and enkephalins — are released into the blood together with catecholamines.
What hormones does the pineal gland secrete?
The pineal gland (epiphysis cerebri) secretes peptide hormones and biogenic amines, the main one being melatonin.
- Melatonin — the main hormone of the pineal gland, synthesized from serotonin; involved in the regulation of circadian and seasonal rhythms.
- Serotonin — a biogenic amine.
- Noradrenaline (Norepinephrine) — a biogenic amine.
- Histamine — a biogenic amine.
What hormones do adipocytes produce?
Adipocytes synthesize hormone-like substances with systemic effects — adipokines. Visceral adipocytes secrete:
- Leptin — regulates appetite, fat composition, insulin sensitivity, and inflammation.
- Adiponectin — regulates appetite, insulin sensitivity, and inflammation; secreted exclusively by adipocytes.
In obesity, a decrease in adiponectin secretion is observed.
To easily remember pituitary-independent glands, use the mnemonic: Adrenal medulla, Parathyroid glands, Endocrine pancreas. They don't need orders from the center!
Class and exam questions
What is the main feature of endocrine glands?
They completely lack excretory ducts. Produced hormones are released directly into the internal environment (into the rich capillary network), bypassing any external cavities.
What are mixed-secretion glands?
The pancreas and gonads. They have both an endocrine part (releasing hormones into the blood) and an exocrine part (releasing secretions through ducts).
What hormones do the kidneys produce?
The kidneys synthesize erythropoietin (stimulates red blood cell production), calcitriol (regulates calcium metabolism), and the enzyme renin (triggers the cascade that raises blood pressure).
What else to study on this topic
- Mechanisms of action of hypothalamic releasing hormones and statins
- Physiological effects of ADH and oxytocin
- Interaction of adenohypophyseal trophic hormones and pituitary-dependent glands
- Mechanisms of the renin-angiotensin-aldosterone system (RAAS)
- Influence of gastrointestinal hormones on feeding behavior
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