Morphological classification
Histologically and anatomically, the endocrine apparatus is divided into several main groups:
- Central organs: hypothalamus, pituitary gland, and pineal gland.
- Peripheral glands: thyroid and parathyroid glands, adrenal glands.
- Mixed and transitory structures: pancreatic islets, gonads, as well as temporarily existing structures (corpus luteum, ovarian follicle).
- Diffuse endocrine system (DES): single hormone-producing cells scattered throughout various non-endocrine organs (e.g., in the intestinal tract).
General structural plan and cellular organization
Large organ glands are usually covered by a connective tissue capsule. Thinning trabeculae with vessels and nerves extend inward from it. Secretory cells form cords that closely abut the vascular beds. Excretory ducts are completely absent.
Cell ultrastructure directly depends on the chemical nature of the synthesized hormone:
- Peptide hormones: cells have a highly developed rough endoplasmic reticulum (rER) and Golgi apparatus. Often, the hormone is synthesized as a large precursor molecule, which then undergoes intracellular enzymatic processing with the cleavage of the active part.
- Steroid hormones: smooth endoplasmic reticulum (sER) predominates, and mitochondria possess specific tubular-vesicular cristae.
All tissue types possess the capacity for endocrine secretion. Epithelium forms the thyroid gland and the adrenal cortex. Nervous tissue forms the neurohypophysis and the adrenal medulla. Connective tissue is represented by Leydig cells, while muscular tissue is represented by juxtaglomerular cells of the kidneys and secretory cardiomyocytes.
Endocrine function of the gonads
The gonads ensure reproductive function, the development of secondary sex characteristics, and systemic metabolic effects.
Male gonads (testes) have a lobular structure. In the connective tissue between the convoluted seminiferous tubules lie Leydig cells, which synthesize testosterone. Within the tubules themselves, sustentocytes (Sertoli cells) function to secrete androgen-binding protein (ABP) to create the high local concentration of testosterone necessary for spermatogenesis. Testosterone controls the development of the prostate gland, exerts a powerful anabolic effect, stimulates erythropoiesis, and causes voice breaking.
Female gonads (ovaries) synthesize hormones in different compartments. Estrogens are produced by follicular cells (granulosa), progesterone by the luteal cells of the corpus luteum (corpus luteum), and androgens by the theca cells. Estrogens prevent the degeneration of the dominant follicle, regulate the proliferative phase of the endometrial cycle, initiate the closure of bone epiphyseal growth plates, and influence the lipid profile and hemostatic system.
Pathology: Type 1 diabetes mellitus
Impaired function of endocrine structures leads to severe systemic disorders. A classic example is type 1 diabetes mellitus, which features absolute insulin deficiency due to the destruction of $\beta$-cells in the pancreatic islets of Langerhans.
The pathogenesis is based on an autoimmune process — insulitis. T-lymphocytes (CD8+ and CD4+) recognize HLA system antigens on the surface of $\beta$-cells and trigger their destruction. Destruction occurs both via direct contact and indirectly through free radicals, cytokines (IL-1$\beta$, TNF-$\alpha$, $\gamma$-IF), and nitric oxide (NO). Nitric oxide has a pronounced cytotoxic effect and damages cellular DNA. As a result, the islets atrophy and are replaced by connective tissue.
To remember cell ultrastructure: peptides are proteins, and their assembly requires "ribosome workers," which is why the rough ER is well-developed in the cell. Steroids are lipids, requiring smooth (agranular) ER and mitochondria with unusual tubular-vesicular cristae.
Class and exam questions
Where do endocrine glands secrete their products?
Strictly into the blood, as they completely lack excretory ducts. Hormones penetrate through the walls of fenestrated or sinusoidal capillaries.
Is the "one cell — one hormone" rule true?
No, this principle has been rejected. It has been proven that many endocrine cells are capable of synthesizing several different hormones simultaneously.
Where do hormones accumulate prior to release into the blood?
Most often intracellularly in the form of secretory granules. Neurohormones accumulate in axons, forming neurosecretory bodies. The only exception with extracellular storage is the follicles of the thyroid gland.
Which cells of the testis produce testosterone?
Testosterone is synthesized by interstitial endocrinocytes (Leydig cells) located in the loose connective tissue between the convoluted seminiferous tubules.
What else to study on this topic
- Diffuse endocrine system (DES) and its elements
- Intracellular processing of polypeptide hormones
- Mechanisms of autoimmune destruction of pancreatic $\beta$-cells
- Endocrine functions of non-specialized tissues (myocardium, kidneys)
- Histophysiology of folliculogenesis and corpus luteum formation
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