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General Structure of the Endocrine System

Systema endocrinum

For medical students2 min readUpdated 2026-10-10

The endocrine system is a collection of specialized glands and solitary cells that secrete regulatory substances into the internal environment. Unlike exocrine structures, they lack excretory ducts and are richly vascularized, allowing their secretions to enter the bloodstream instantly.

SecretionHormones are released directly into the internal environment—into the blood or lymph.

MorphologyEndocrine glands lack secretory end-pieces and excretory ducts.

Target CellsHormones act selectively on strictly specific cells.

ClassificationAll hormone-producing formations are divided into 4 major groups.

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 secretory end-pieces and excretory ducts.
Abundant vascularization (the tissue is permeated by a dense network of blood capillaries).
Secretion is released directly into the internal environment of the body: predominantly into the blood, and somewhat less frequently into the lymphatic vessels.

Interestingly, in terms of its level of organization, the endocrine apparatus is represented not only by large multicellular organs, but also by individual isolated cells distributed across various tissues.

Nature of Hormones and Their Mechanism of Action

The product of the endocrine system's activity is hormones. From a physiological and histological standpoint, these are specialized regulatory compounds.

Delivery mechanism: hormones are secreted into the bloodstream and transported via 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 this 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 commonly included in this group as they are capable of producing hormone-like substances.
Solitary hormone-producing cells: form the diffuse neuroendocrine system (DNES). They do not form a single organ, but are localized within the structures of the nervous, digestive, respiratory, and other body systems.

Mnemonic

To remember the central endocrine organs, use the "H-P-P" rule: Hypothalamus, Pituitary (hypophysis), Pineal gland (epiphysis).

Frequently asked questions

What types of blood capillaries are characteristic of the structure of endocrine glands?

The sources explicitly indicate the following types of capillaries for endocrine structures:

Fenestrated (visceral) capillaries — localized in endocrine glands; they pass large amounts of water with dissolved substances and macromolecules. In pancreatic islets, fenestrated blood capillaries with a pericapillary space are located between insulocytes, where hormones are initially received.
Sinusoidal capillaries — described in the anterior lobe of the pituitary gland as the secondary capillary network of the portal system. They have a wide lumen and fenestrated endothelium; through their wall, releasing and inhibiting hormones (releasings and statins) reach adenocytes, and anterior lobe hormones are secreted back into the blood.

A general feature of endocrine glands is abundant vascularization and a dense capillary network.

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 neuroectoderm.
Pineal gland (epiphysis) — derived from neuroectoderm.
Pituitary gland (hypophysis): the adenohypophysis is indicated as a derivative of the foregut epithelium with mixed ecto-entodermal origin; in embryogenesis, it forms from Rathke's pouch (oral bay), the anterior wall of which forms the anterior lobe, and the posterior wall forms the intermediate lobe. The neurohypophysis develops as an outgrowth of the diencephalon: the proximal part forms the pituitary 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; calcitonin-secreting cells (C-cells) are derived from neuroectoderm.
Parathyroid glands — indicated as derivatives of the foregut epithelium; listed among entodermal derivatives originating from the prechordal plate.
Adrenal glands: adrenocortical endocrinocytes originate from coelomic epithelium (mesoderm), while medullary endocrinocytes originate from 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 explicitly 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); basally secrete biogenic amines (norepinephrine, serotonin) and hormone-like peptides (bombesin, calcitonin).
Neuroendocrine cells of the nervous system, cells of certain glands, and solitary endocrinocytes — indicated as components of the APUD series.
Solitary hormone-producing cells of the diffuse endocrine system are localized in the nervous, digestive, respiratory, and other systems.

How do endocrine glands differ histologically from exocrine glands?

Endocrine glands completely lack excretory ducts and secretory end-pieces. In addition, they feature a very dense network of blood capillaries into which secretions are directly released.

What structures are considered 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 make up the diffuse neuroendocrine system (DNES) and are widely localized in the organs of the digestive, respiratory, nervous, and other systems.

Go deeper

Histological structure and cellular composition of the pituitary gland
Morphology of peripheral glands: thyroid and adrenal glands
Structural features of the diffuse neuroendocrine system (DNES)
Types of capillaries in endocrine organs