The Endocrine System

The lymphatic system | Main Anatomy Index | Female reproductive system

Last updated 30 March 2006

This page was contributed by David Boshell.

The Endocrine System

• The endocrine system parallels the nervous system; their primary function is intracellular communication.
• It produces a slower, more prolonged response than the nervous system.

• Hormones are typically released into fenestrated capillaries, and reach receptors of target cells via the circulatory system.

• The endocrine system includes pituitary gland, adrenal glands, thyroid and parathyroid glands, the pineal gland, and other endocrine centres.

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The Pituitary Gland (Hypophysis)

• This is a pea sized, compound endocrine gland lying within the sella turcica of the sphenoid bone at the base of the skull.
• It is joined to the hypothalamus by the stalk-like infundibulum.

• There are two functional components of the pituitary:

1. The anterior pituitary (adenohypophysis): glandular epithelial tissue derived from the ectoderm of the oropharynx (Rathke’s pouch).
2. The posterior pituitary (neurohypophysis): neural secretory tissue derived from the neuroectoderm of the diencephalon.

The Anterior Pituitary (Adenohypophysis)

• This is the master gland of the endocrine system, regulating other endocrine glands.
• It has the typical arrangement of endocrine tissue, consisting of cords of cells separated by large, fenestrated sinusoidal capillaries.

• Upon signalling from the hypothalamus, these cells synthesise and secrete tropic hormones that regulate the activity of other endocrine tissues in the body.
• These regulatory hormones are either releasing or inhibiting.

• The anterior pituitary has 3 parts, the pars distalis, the pars intermedia and the pars tuberalis.

The Pars Distalis

• The pars distalis comprises most of the anterior pituitary.
• Its has 3 types of cells based on their staining:

1. Basophils
2. Acidophils
3. Chromophobes

• Many hormones are secreted from this region include:

1. Adrenocorticotropin (ACTH), that stimulates the adrenal cortex to produce cortisol and aldosterone.
2. Lipotropic hormone (LPH), which may be cleaved to form melanocyte stimulating hormone and endorphins which have analgesic effects.
3. Luteinizing hormone (LH), which stimulates ovulation in the ovaries and the production of testosterone from the Leydig cells in the testis.
4. Follicle-stimulating hormone (FSH), that stimulates follicle development in the ovaries and the Sertoli cells in the testis.
5. Thyroid-stimulating hormone (TSH), causing release of T4 and T3 from the thyroid gland.
6. Somatotropin, or Growth hormone (GH), that causes release of insulin-like growth factors (IGFs) in the liver, and inhibits the effects of insulin on carbohydrate and fat metabolism.

The Pars Intermedia

• This is a thin zone of tissue between the pars distalis and the posterior pituitary.
• It includes:

1. Basophils
2. Chromophobes
3. Colloid-filled cysts, representing the residual lumen of Rathke’s pouch.

• Hormones produced in this region include:

• Melanotropin (MSH), stimulating the production of the pigment melanin by melanocytes in the skin.

The Pars Tuberalis

• This is a highly vascular region surrounding the infundibulum and forming a collar around the pars intermedia.
• It contains:

1. Squamous cells;
2. Small follicles lined with cuboidal cells;
3. Veins of the hypophyseal portal system.

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The Posterior Pituitary (Neurohypophysis)

• This is a storage site for neurosecretions from the neurons of the supraoptic and paraventricular nuclei (SON & PVN) of the hypothalamus.
• It is not an endocrine gland.

• The posterior pituitary has 2 parts, the pars nervosa and the infundibulum.

The Pars Nervosa

• This is the neural lobe of the pituitary, containing non-myelinated axons and nerve endings of many neurosecretory neurons.
• These cells’ cell bodies lie in the supraoptic and paraventricular nuclei.
• They convey neurosecretions to the pars nervosa, where they are stored and released.

• These neurons have 2 unique features:

1. They end close to fenestrated capillaries in the pars nervosa;
2. And they contain membrane bound secretory granules in all parts of the cells, not just the cell body.

• The membrane bound granules come in 3 sizes:

• 10-30 nm neurosecretory granules, accumulating in axon terminals and forming Herring bodies, which are dilations in the axon near the terminals.

• 30nm terminal vesicles containing acetylcholine.

• The larger 50-80nm terminal vesicles.

• Hormones contained in membrane bound Herring bodies are direct acting and include:

1. Antidiuretic hormone (ADH) or vasopressin, produced in the SON, which affects blood pressure and increases water resorption in the kidney tubules.
2. Oxytocin (OT), produced in the PVN, promotes contraction of the smooth muscle of the uterus and the myoepithelial cells of the breast (releasing milk).

The Infundibulum

• This connects the pars nervosa to the hypothalamus.

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Pituicytes

• These cells are unique to the neurohypophysis, found amongst the fenestrated capillaries.

• They are irregular in shape, with many branches, and resemble glial cells.
• They have round or oval nuclei and pigmented granules in their cytoplasm.
• They often have processes terminating in the perivascular space, and may have a similar role to that of the astrocyte in the rest of the CNS.

Blood Supply of the Pituitary

• The blood supply of the pituitary arises from 2 sets of vessels:

1. The superior hypophyseal arteries, from the internal carotid arteries and circle of Willis. This supplies the pars tuberalis, median eminence, and infundibular stem.
1. The inferior hypophyseal arteries, from the internal carotid arteries alone. This supplies the pars nervosa.

• Most of the anterior lobe of the pituitary has no direct arterial supply.

The Hypophyseal Portal System

• Arteries supplying the pars tuberalis, median eminence and infundibular stem give rise to the primary capillary plexus of fenestrated capillaries.
• These capillaries drain into the hypophyseal portal veins running along the pars tuberalis and give rise to the secondary capillary plexus.

• This system carries neuroendocrine secretions from hypothalamic nerves to the cells in the pars distalis.

• Most blood then drains into the cavernous sinus via the hypophyseal veins, but some blood may flow into the pars nervosa, and then back to the hypothalamus.
• This provides a direct feedback mechanism to the brain that bypasses the greater circulation.

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The Adrenal Glands (Suprarenal glands)

• These are flattened triangular glands embedded in perineal fat on the superior poles of the kidneys.

Structure of the Adrenal Glands

• There is a thick connective tissue capsule from which trabeculae extend into the parenchyma, carrying blood vessels and nerves.

The Cortex of the Adrenal Glands

• The outer cortex of the parenchyma is beneath the capsule.
• This comprises 90% of the gland, and is the steroid secreting portion.
• Its cells are derived from mesodermal mesenchyme, and are controlled by the pituitary and function in regulating metabolism and maintaining electrolyte balance.

• It is divided into 3 zones, the zona glomerulosa, zona fasciculata and zona reticularis.

The Zona Glomerulosa

• This is a narrow outer zone, 15% of cortical volume.

• It has small, densely staining, columnar or pyramidal cells with spherical nuclei.
• They are closely packed into clusters and columns that are continuous with the cords of the next zone.

• It secretes mineralocorticoids, of which aldosterone functions to control blood pressure.
• The renin-angiotensin system provides feedback control of this zone.

The Zona Fasciculata

• This is the thick middle zone, 80% of cortical volume.

• It consists of pale staining cells are large and polyhedral with spherical nuclei and lipid droplets in an acidophilic cytoplasm.
• They are arranged in long, straight cords 1 or 2 cells thick that are separated by sinusoidal capillaries.

• It secretes glucocorticoids that regulate glucose and fatty acid metabolism.
• The most important of which is hydrocortisone (cortisol) that acts to mobilise glucose and fatty acids for energy.

• This zone is under feedback control of the hypothalamic-hypophyseal corticotropin-releasing factor-adrenocorticotropic hormone (CRF-ACTH) system.

The Zona Reticularis

• This is the inner zone, 5% of cortical volume, but is thicker than the glomerulosa due to its central location.

• It has small cells with darker nuclei, arranged in narrow, anastomosing cords between fenestrated capillaries.

• It secretes weak androgens (sex hormones), mostly dehydroepiandrosterone (DHEA), as well as a small amount of glucocorticoids, namely cortisone.

• This zone is also under the CRF-ACTH system.

The Medulla of the Adrenal Glands

• The inner medulla forms the centre of the gland.
• It is derived from neural crest origin, and its parenchyma contains chromaffin cells: modified neurons.

• These large, epithelioid cells are characterised by membrane-bound secretory granules of 100-300nm, and are organised as clusters and short interconnecting cords.

• They are innervated by preganglionic sympathetic neurons.
• Upon the release of acetylcholine they are stimulated to release the catecholamines epinephrine and norepinephrine.
• Sinusoidal capillaries are intimately related to these, originating from either cortical capillaries or branching from cortical arterioles.

• Ganglion cells are also present.

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Blood Supply of the Adrenal Glands

• The adrenal glands are supplied by the superior, middle and inferior adrenal arteries.
• These branch before and inside the capsule, producing 3 distribution systems:

1. Capsular capillaries, supplying the capsule;
2. Fenestrated cortical sinusoidal capillaries, supplying the cortex and draining "venous blood" into the medullary capillary sinusoids;
3. Medullary arterioles traversing the cortex within trabeculae and bringing arterial blood to the medullary capillary sinusoids.

• Thus the medulla has a dual arterial and "venous" blood supply.

• Venules from cortical and medullary sinusoids drain into medullary veins, the large medullary (adrenal) vein, and the IVC.

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The Thyroid Gland

• This is a bi-lobed endocrine gland located in the neck.
• Its lobes (about 25g each) are on either side of the larynx and trachea, and are connected by the isthmus.
• It is fully functional from 10 weeks, required for embryonic and neural development.

Hormones Produced by the Thyroid Gland

• It produces 3 hormones, each essential to normal metabolism and homeostasis, as well as foetal development.

1. Thyroxine (tetraiodothyronine - T4) and triiodothyronine (T3), which regulate cell and tissue metabolism, growth and differentiation.
2. Calcitonin, which lowers blood calcium levels, stimulates osteoblasts, inhibits osteoclasts and stimulates calcium excretion from the kidney.

• Thyroid hormone deficiency in foetal development leads to irreversible damage called cretinism.

Structure of the Thyroid Gland

• The structure of the thyroid gland includes:

Capsule of the Thyroid Gland

• There is a thin connective tissue capsule, sending trabeculae into the parenchyma, creating irregular lobes and lobules.

Secretory Follicles of the Thyroid Gland

• This constitutes the structural and functional units of the gland.

• These are spheroidal compartments with a wall of simple cuboidal follicular epithelium.
• The follicular lumen is filled with gel-like colloid containing the iodinated glycoprotein thyroglobulin.

• There are 2 basic cell types in the follicles:

1. Follicular (principal) cells that secrete T4 and T3, derived from the endoderm of the pharyngeal part of the foregut;
2. Parafollicular (C) cells that secrete calcitonin, of neural crest origin. These are solitary or a small cluster of pale staining cells, also found in the interfollicular space. They are characterised by membrane bound secretory granules (0.1-0.5 um diameter).

• The follicles are surrounded by many fenestrated capillaries, from the superior and inferior thyroid arteries.
• Lymphatic capillaries are also present in the interfollicular connective tissue, providing a secondary route for hormonal secretions from the gland.

Thyroid Hormone Synthesis

• Thyroglobulin is synthesised and glycosylated in the follicular epithelial cells and secreted into the colloid.
• Tyrosine residues on thyroglobulin are iodinated in the colloid, and then coupled to form T3 and T4 residues.
• Thyroglobulin is endocytosed into the follicular cells, where T4 and T3 are released by lysosomal enzymes.

• This uptake and release is subject to feedback control.
• Low levels of T3 and T4 cause the release of thyroid releasing hormone (TRH) from the hypothalamus, stimulating the pituitary to release thyroid stimulating hormone (TSH).
• This increases the uptake and release of T3 and T4.
• Conversely, high T3 and T4 levels inhibit the release of TSH, along with somatostatin from the hypothalamus.

The Parathyroid Glands

• These are small, superior and inferior laterally paired glands on the posterior aspect of the thyroid gland.
• They receive their blood supply from the thyroid arteries and contain fenestrated capillaries.

Structure of the Parathyroid Glands

• There is a thin connective tissue capsule, separating it from the thyroid, with small septa extending into the gland.
• This poorly defines the lobules and separating densely packed cords of cells.

• The parenchyma contains 2 types of cells, the chief cells and the oxyphil cells.

Chief (Principal)Cells

• These are small, polygonal cells with central nuclei and a pale staining cytoplasm that secretes parathyroid hormone (PTH).
• This has the reciprocal effect of calcitonin, and increases calcium resorption from bone and urine, and from the G.I. tract via the production of 1,25-dihydroxycholecalciferol.

Oxyphil Cells

• These are only a minor portion of the parenchymal cells.
• They are larger and more rounded than chief cells, with a very acidophilic cytoplasm.

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The Pineal Gland

• This is a small, flattened, cone-shaped structure weighing between 100-200mg.
• It is attached to the brain by a short stalk.
• Its function is not yet clearly defined.

• The pineal gland has 2 basic parenchymal cell types:

• Pinealocytes are the most common cell type, arranged in clumps or cords within lobules formed by septa extending from the gland’s surrounding pia mater.
• They have large, infolded nuclei with prominent nucleoli, and lipid droplets in their cytoplasm.

• Interstitial (glial) cells constitute about 5% of the gland.
• These cells closely resemble astrocytes.

• The human pineal gland also has calcified concretions called corpa arenacea or brain sand.

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Other endocrine centres

• Endocrine subsystems are also found in:

1. The kidney
2. The gastro-intestinal system
3. The liver and pancreas
4. The testes and ovaries

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