Human Anatomy - Kotsan I. Ya. 2009

Endocrine glands

In the simplest unicellular organisms lacking a Nervous system, The regulation of all body Functions and their interaction with the external environment are carried out solely through chemical substances contained within their fluids. This is known as chemical or humoral regulation (from the Greek humer — moisture, fluid).

With the evolution of The Nervous System, neurohumoral regulation gradually emerges, establishing a close functional interaction between chemically active substances and neural elements.

Active chemical substances, produced during METABOLISM under the Influence of the nervous system, simultaneously act as its stimulants—mediators, meaning Transmitters of nerve impulses (e.g., histamine, etc.). In the Cytology/cytology/16.html">Early stages of phylogenesis, the transmission of mediators occurs directly from Cell to Cell and proceeds slowly (local activators). At later phylogenetic stages, distant activators appear; they act over long distances from their site of production and spread rapidly through the circulatory and lymphatic systems. These distant activators are produced in specialized Organs known as ductless glands, or Endocrine glands (glandulae endocrinae).

Endocrine glands (from endo — inward, crino — to secrete) are defined as glands that lack excretory ducts (ductless glands) and release their secretions directly into the Blood or Lymph, in

contrast to exocrine glands, whose secretions or excretions are discharged onto the Skin surface (sweat and Sebaceous Glands) or mucous membranes (Salivary Glands, Liver, etc.).

Despite differences in shape, size, and Location, individual endocrine glands share several common Anatomical and physiological properties. First and foremost, they lack excretory ducts. Because their secretions are released directly into the Circulatory system, endocrine glands possess an extensively developed network of Blood Vessels. These vessels permeate the gland in various directions, performing a function analogous to the ducts of exocrine glands. Glandular Cells are arranged around these vessels, releasing their secretions directly into the blood.

In addition to a rich vascular supply, endocrine glands share structural features in their capillary networks. The capillary bed of these glands may consist of significantly widened capillaries, known as sinusoids, whose endothelial walls lie directly against the glandular cells without intervening Connective Tissue. Furthermore, in some areas, the walls of the sinusoids are discontinuous, allowing epithelial cells to bulge directly into the vascular lumen. Blood flow is slowed within these relatively wide sinusoids, ensuring a more prolonged and intimate contact between the glandular cells and the blood flowing through them. The secretory products of endocrine glands are collectively termed increts, or Hormones.

Hormones (from hormao — to excite) are BIOLOGICALLY ACTIVE SUBSTANCES. Even in minute quantities, they exert a profound effect on The Human Body. Each hormone performs a specific function. For instance, THYROID HORMONES have a direct impact on metabolism, and their deficiency leads to nutritional and Metabolic Disorders. Other Hormones secreted by endocrine glands influence the body's GROWTH AND DEVELOPMENT. Despite entering the bloodstream in negligible amounts, hormones exhibit exceptionally high physiological activity.

All endocrine glands are functionally interconnected, forming a unified system in which the Pituitary Gland plays a leading role. Its tropic hormones regulate the secretion of dependent glands via feedback loops. Furthermore, There is a close, bidirectional relationship between the endocrine glands and the nervous system. On the one hand, endocrine glands are controlled by the nervous system, which coordinates their activity; on the other hand, glandular secretions act via the bloodstream on the nervous system, thus realizing neurohumoral regulation.

Unfortunately, the coordinated function of endocrine glands can be disrupted, leading to The Development of severe pathologies. Endocrine disorders arise from either the excessive or deficient release of hormones into the blood. Overproduction of hormones is termed hyperfunction, while insufficient production is termed hypofunction of the gland.

During evolution, endocrine glands emerged at different times, in various locations, and from diverse embryonic sources. Consequently, the placement, size, shape, Structure, and functions of these organs are highly diverse. Embryologically, endocrine glands also have different origins. In this respect, even distinct PARTS OF THE same gland may differ, such as the cortex and medulla of the Adrenal Glands. All three germ layers—endoderm, mesoderm, and ectoderm—participate in the development of endocrine organs.

According to the modern Classification, depending on their developmental origin, endocrine glands are divided into five groups:

1. Endodermal glands derived from the epithelial lining of the pharyngeal gut (embryonic branchial pouches) — the branchiogenic group (anterior lobe of the pituitary gland, thyroid, parathyroid, and Thymus glands).

2. Endodermal glands derived from the epithelium of the intestinal tube — the enteric group (the Endocrine portion of the Pancreas — pancreatic islets).

3. Mesodermal glands developed from the coelomic epithelium (the secondary body cavity) — the interrenal system (the adrenal cortex and interstitial Cells of the Gonads).

4. Ectodermal glands derived from the anterior region of the neural tube — the neurogenic group (the posterior lobe of the pituitary gland and the Pineal Gland).

5. Ectodermal glands originating from the Sympathetic division of the nervous system, formed through the migration of certain cell groups from the neural tube — the adrenal system group (The adrenal medulla and Paraganglia — aortic, carotid, etc.). Due to their selective staining with chromium salts (into a dark brown color), they are referred to as chromaffin organs.