Morphological Features
The key anatomical feature of glands of internal secretion is the absence of excretory ducts. The tissue elements of endocrine glands (glandulae endocrinae) are abundantly penetrated and densely intertwined with a network of blood and lymphatic vessels.
The secretion produced by the glands—hormones—bypasses any ducts and enters venous or lymphatic capillaries directly. Such intensive vascularization ensures the fastest possible entry of hormones into the body's systemic circulation.
List of endocrine glands
The endocrine system includes the following glands:
- Thyroid gland — glandula thyroidea
- Parathyroid glands — glandulae parathyroideae
- Thymus gland (thymus) — thymus
- Adrenal glands — glandulae suprarenales
- Paraganglia (chromaffin bodies) — paraganglia
- Gonads: testes (testes) and ovaries (ovaria)
- Pituitary gland — hypophysis (or glandula pituitaria)
- Pineal gland — corpus pineale
- Pancreas (endocrine part) — pancreas
Classification by embryonic origin
Despite their close functional interrelationship, endocrine organs have completely different embryonic origins. Based on genetic features, they are divided into several groups.
1. Endodermal derivatives
- Pharyngeal pouches: thyroid gland, parathyroid glands, and thymus gland.
- Intestinal origin: endocrine part of the pancreas.
2. Mesodermal derivatives Develop from the coelomic epithelium. This group includes the adrenal cortex and the endocrine part of the gonads (testes and ovaries).
3. Ectodermal derivatives
- Sympathetic origin: adrenal medulla and paraganglia.
- Neurogenic glands: pituitary gland and pineal gland. Their development is directly related to the formation of the diencephalon.
Briefly: answers to topical questions
What hormones does the thyroid gland synthesize?
The thyroid gland synthesizes and secretes T3, T4, and calcitonin hormones. The classification of thyroid hormones includes thyroxine, triiodothyronine, and calcitonin. In a narrow sense, the term "thyroid hormones" refers to thyroxine and triiodothyronine; they are iodinated derivatives of the amino acid tyrosine. The thyroid gland regulates metabolism and body growth processes, while calcitonin is involved in regulating blood Ca2+ concentrations.
What hormones does the adrenal cortex secrete?
The adrenal cortex is the outer part of the adrenal gland and serves as the site of steroid synthesis. The general name for the products of adrenal cortex synthesis is corticosteroids; more than 50 types of them have been described. Functionally, corticosteroids are divided into precursors with low biological activity and true hormones that are vital for life. Adrenal cortex hormones include corticosteroids: glucocorticoids, mineralocorticoids, and androgenic hormones.
Where is the pineal gland anatomically located?
The pineal gland (epiphysis, corpus pineale) anatomically belongs to the epithalamus as part of the diencephalon and is located on the superior colliculus plate, which forms the roof of the midbrain. Topographically, this structure lies adjacent to the posterior wall of the third ventricle and serves as the posterior boundary of the diencephalon.
From which embryonic primordia is the pituitary gland formed?
The pituitary gland has a dual embryonic origin and is formed from two different primordia. The first primordium is ectodermal and develops from below as an invagination of the oral cavity ectoderm, forming Rathke's pouch, which gives rise to adenohypophysis structures. The second primordium is neural and develops from above as an outpocketing from the floor of the third ventricle of the diencephalon, from which the neurohypophysis is formed. The connective tissue of the pituitary gland develops from mesenchyme.
Which arteries supply blood to the thyroid gland?
Blood supply to the thyroid gland is provided by branches of the major trunk vessels of the neck. These include the superior thyroid artery, which branches off near the origin of the external carotid artery and gives off glandular branches to the lobes of the organ. Blood supply is also provided by the inferior thyroid artery from the thyrocervical trunk, which approaches the posterior surface of the gland and anastomoses with the superior thyroid artery.
Note the adrenal glands (glandulae suprarenales): they have a dual origin. The cortex develops from the mesoderm (like the gonads), while the medulla develops from the ectoderm (sympathetic origin).
Class and exam questions
What is the main anatomical difference between endocrine and exocrine glands?
Endocrine glands completely lack excretory ducts. They secrete their products directly into blood (venous) or lymphatic capillaries.
From which part of the brain do neurogenic endocrine glands develop?
The pituitary gland and the pineal gland are genetically linked to the development of the diencephalon.
What ensures the rapid entry of hormones into the blood?
This is achieved because the tissue elements of the glands are penetrated and intertwined with a dense network of blood and lymphatic vessels.
What else to study on this topic
- Anatomy and topography of the thyroid gland (glandula thyroidea)
- Connection of the pituitary gland (hypophysis) with the diencephalon
- Structure of the adrenal glands: cortex and medulla
- Endocrine part of the pancreas (pancreas)
- Endocrine function of the gonads (testes, ovaria)
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