The endocrine system is made up of the endocrine glands that secrete hormones.

Some glands, for example, pancreas, ovaries and testes, also have non-endocrine regions that have functions other than hormone secretion. Primacy of thymus in mediating immune response regulation is now unquestionable.

                    

Some organs, for example, stomach, small intestines, heart, and placenta, produce hormones, but their primary function is not hormone secretion.

 

 

The pituitary gland or hypophysis is a small gland of about 1 cm dia. It is surrounded by bone as it rests in the sella turcica, a depression in the sphenoid bone. The gland is connected to the hypothalamus of the brain by a slender stalk called the infundibulum.

 

There are two distinct regions in the gland: the anterior lobe (adenohypophysis) and the posterior lobe (neurohypophysis). The activity of the adenohypophysis is controlled by releasing hormones, TRH (or TRF), CRH(or CRF), GnRH(or LHRH), GHRH(or GHRF, or GRF), GHIH(or SRIF or SS) and PIF(or PIH or DA) from the hypothalamus. Pituitary may be the master gland but hypothalamus is the power behind the throne. The hypothalamus acts as a store for release of ADH and oxytocin to the posterior lobe (neurohypophysis). The neurohypophysis is controlled by nerve stimulation.

 

 

Growth hormone or somatotropic hormone (GH or STH) is a protein that stimulates the growth of bones, muscles, and other organs by promoting protein synthesis. This hormone drastically affects the appearance of an individual because it influences height. Too little growth hormone in a child results in a pituitary dwarf of normal proportions but small stature. An excess of the hormone results in an exaggerated bone growth, and the individual becomes exceptionally tall or a giant.

 

Thyroid-stimulating hormone, or thyrotropin, (TSH) causes the glandular cells of the thyroid to secrete thyroid hormone. When there is a hypersecretion of thyroid-stimulating hormone (TSH), the thyroid gland enlarges and secretes too much thyroid hormone.

 

Gonadotropic hormones, that is, follicle-stimulating hormone (FSH) and luteinizing hormone or Interstitial Cell Stimulating Hormone (LH or ICSH ) react with receptor sites in the gonads (ovaries and testes) to regulate the development and function of these organs. They are controlled by hypothalamus and estrogen or androgen.

 

Prolactin (PRL) promotes the development of glandular tissue in the female breast during pregnancy and stimulates milk production after the birth of the infant.

 

Luteotrophic Hormone (LTH). A hormone released by the anterior of the pituitary gland that stimulates production of LH and maternal behaviour.

 

Luteinizing hormone or Interstitial Cell Stimulating Hormone (LH or ICSH). A small glycoprotein hormone secreted by the anterior pituitary. LH plays an important role in controlling ovulation and in controlling secretion of hormones by the ovaries and testes.

 

Adrenocorticotropic hormone (ACTH) reacts with receptor sites in the cortex of the adrenal gland to stimulate the secretion of cortical hormones, particularly cortisol.

 

 

Beta-lipotropin or β- lipotropic hormone (β-LPH). A pituitary hormone mobilizing fat from adipose tissue. β-lipotropin is a 90 amino acid polypeptide that contains the sequences of endorphins and metenkephalin, and may be a precursor of beta-melanotropin and beta-endorphin. Gamma-lipotropin is shorter and is identical in sequence to the first 58 residues of beta-lipotropin. Both contain sequences common to ACTH and beta-melanotropin. It stimulates melanocytes to produce melanin, and can also be cleaved into smaller peptides. β-lipotropin also performs lipid-mobilizing functions such as lipolysis and steroidogenesis.

 

Gamma-lipotropin or γ-lipotropin (γ-LPH). γ-lipotropin is the amino-terminal peptide fragment of β-lipotropin. In humans, it has 56 amino acids.

 

 

Met-enkephalin is a five-amino-acid-residue vasoactive neurotransmitter peptide with stimulative actions in both the nervous and immune response systems. Met-enkephalin precursors and receptors are widely distributed in the nervous, vascular and immune response system.

 

Antidiuretic hormone or vasopressin (ADH) promotes the reabsorption of water by the kidney tubules, with the result that less water is lost as urine. This mechanism conserves water for the body. Insufficient amounts of antidiuretic hormone cause excessive water loss in the urine.

 

Oxytocin causes contraction of the smooth muscle in the wall of the uterus. It also stimulates the ejection of milk from the lactating breast.

 

 

In human fetal life, this area produces melanocyte stimulating hormone (MSH) which causes the release of melanin pigment in skin melanocytes (pigment cells). However, the pars intermedia is normally either very small or entirely absent in adulthood.

 

 

The pineal gland, also called pineal body or epiphysis cerebri, is a small cone-shaped structure that extends posteriorly from the third ventricle of the brain. The pineal gland consists of portions of neurons, neuroglial cells, and specialized secretory cells called pinealocytes. Pinealocytes include 5-HT N-acetyl transferase and 5-hydroxyindole-O-methyltransferase which are used to convert methoxyindoles like serotonin to melatonin, 5-methoxy-N-acetyltryptamine, a methoxyindole which is secreted directly into the cerebrospinal fluid, which takes it into blood and plays a role in the regulation of the circadian rhythm of several biological functions. Many biological effects of melatonin are produced through activation of melatonin receptors, while others are due to its role as a pervasive and extremely powerful antioxidant with a particular role in the protection of nuclear and mitochondrial DNA.

 

 

The thyroid gland is a very vascular organ that is located in the neck. It consists of two lobes, one on each side of the trachea, just below the larynx or voice box. The two lobes are connected by a narrow band of tissue called the isthmus. Internally, the gland consists of follicles, which produce a prohormone, thyroxine, 3,5,3',5'-tetra­iodothyronine (T4),  which is converted by deiodinases to actriiodothyronine hormone(T3).

 

About 95 percent of the active thyroid hormone is thyroxine, and most of the remaining 5 percent is triiodothyronine. Both of these require iodine for their synthesis. Thyroid hormone secretion is regulated by a negative feedback mechanism that involves the amount of circulating hormone, hypothalamus, and adenohypophysis.

If there is an iodine deficiency, the thyroid cannot make sufficient hormone. This stimulates the anterior pituitary to secrete thyroid-stimulating hormone (TSH), which causes the thyroid gland to increase in size in a vain attempt to produce more hormones. But it cannot produce more hormones because it does not have the necessary raw material, iodine. This type of thyroid enlargement is called simple goiter or iodine deficiency goiter.

 

Calcitonin is secreted by the parafollicular cells (C cells) of the thyroid gland. This hormone opposes the action of the parathyroid glands by reducing the calcium level in the blood. If blood calcium becomes too high, calcitonin is secreted until calcium ion levels decrease to normal.

 

Four small masses of epithelial tissue are embedded in the connective tissue capsule on the posterior surface of the thyroid glands. These are parathyroid glands, and they secrete parathyroid hormone or parathormone (PTH). Parathyroid hormone (PTH) is the most important regulator of blood calcium levels. The hormone is secreted in response to low blood calcium levels, and its effect is to increase those levels.

 

Hypoparathyroidism, or insufficient secretion of parathyroid hormone, leads to increased nerve excitability. The low blood calcium levels trigger spontaneous and continuous nerve impulses, which then stimulate muscle contraction.

 

The adrenal, or suprarenal, gland is paired with one gland located near the upper portion of each kidney. Each gland is divided into an outer cortex and an inner medulla. The cortex and medulla of the adrenal gland, like the anterior and posterior lobes of the pituitary, develop from different embryonic tissues and secrete different hormones. The adrenal cortex is essential to life, but the medulla may be removed with no life-threatening effects.

 

The hypothalamus of the brain influences both portions of the adrenal gland but by different mechanisms. The adrenal cortex is regulated by negative feedback involving the hypothalamus and adrenocorticotropic hormone (ACTH); the medulla is regulated by nerve impulses from the hypothalamus.

 

 

The adrenal cortex consists of three different regions, with each region producing a different group or type of hormones. Chemically, all the cortical hormones are steroid.

Mineralocorticoids are secreted by the outermost region of the adrenal cortex. The principal mineralocorticoid is aldosterone, which acts to conserve sodium ions and water in the body. Glucocorticoids are secreted by the middle region of the adrenal cortex. The principal glucocorticoid is cortisol, which increases blood glucose levels.

The third group of steroids secreted by the adrenal cortex is the gonadocorticoids, or sex hormones. These are secreted by the innermost region. Male hormones, androgens, and female hormones, estrogens, are secreted in minimal amounts in both sexes by the adrenal cortex, but their effect is usually masked by the hormones from the testes and ovaries. In females, the masculinization effect of androgen secretion may become evident after menopause, when estrogen levels from the ovaries decrease.

 

 

The adrenal medulla develops from neural tissue and secretes two hormones, epinephrine (adrenalin) and norepinephrine. These two hormones are secreted in response to stimulation by sympathetic nerve, particularly during stressful situations. A lack of hormones from the adrenal medulla produces no significant effects. Hypersecretion, usually from a tumor, causes prolonged or continual sympathetic responses.

 

The pancreas is a long, soft organ that lies transversely along the posterior abdominal wall, posterior to the stomach, and extends from the region of the duodenum to the spleen. This gland has an exocrine portion that secretes digestive enzymes that are carried through a duct to the duodenum. The endocrine portion consists of the pancreatic islets of Langerhans, wherefrom glucagon is secreted by α -cells in response to a low concentration of glucose in the blood and insulin is secreted by β-cells in response to a high concentration of glucose in the blood.

 

 

The gonads, the primary reproductive organs, are the testes in the male and the ovaries in the female. These organs are responsible for producing the sperm and ova, but they also secrete hormones and are considered to be endocrine glands.

 

Male sex hormones, as a group, are called androgens. The principal androgen is testosterone, which is secreted by the testes. A small amount is also produced by the adrenal cortex. Production of testosterone begins during fetal development, continues for a short time after birth, nearly ceases during childhood, and then resumes at puberty. This steroid hormone is responsible for:

•                   The growth and development of the male reproductive structures

•                   Increased skeletal and muscular growth

•                   Enlargement of the larynx accompanied by voice changes

•                   Growth and distribution of body hair

•                   Increased male sexual drive

Testosterone secretion is regulated by a negative feedback system that involves releasing hormones from the hypothalamus and gonadotropins from the anterior pituitary.

 

Two groups of female sex hormones are produced in the ovaries, the estrogen and progesterone. These steroid hormones contribute to the development and function of the female reproductive organs and sex characteristics. At the onset of puberty, estrogens promote:

•                   The development of the breasts

•                   Distribution of fat evidenced in the hips, legs, and breast

•                   Maturation of reproductive organs such as the uterus and vagina

Progesterone causes the uterine lining to thicken in preparation for pregnancy. Together, progesterone and estrogens are responsible for the changes that occur in the uterus during the female menstrual cycle.

 

 

 

A thymic hormone influencing lymphoid cell immunological competence. One of several polypeptide hormones secreted by the thymus that control the maturation of T cells. They are derived from a polypeptide called prothymosin-alpha (PTMA) or alpha thymosin.

 

THF selectively activates thymus-derived (T) cell population.THF confers immunological reactivity upon noncompetent lymphoid cell populations. Induction of adenyl cyclase and a rise in cellular levels of cAMP are necessary events for acquisition of competence by the noncompetent lymphoid cell populations. Calf derived THF has been found particularly helpful in rebuilding immune response destroyed by radiation and chemotherapy.

 

This hormone dependent on zinc for its biological activity, is believed to be involved in T-cell differentiation and enhancement of T and NK cell actions. Thymulin seems to have neuroendocrine effects as well. There exist bidirectional interactions between thymic epithelium and the hypothalamus-pituitary axis (for example, thymulin follows a circadian rhythm and physiologically elevated ACTH levels correlate positively with thymulin plasma levels and vice versa.

 

A polypeptide hormone that induces differentiation of lymphocytes to thymocytes.

 

Thymosin, produced by the thymus gland, plays an important role in the body's immune system.

 

In the two thymic lobes, lymphocyte precursors from the bone-marrow become thymocytes, and subsequently mature into T cells. Once mature, T cells emigrate from the thymus and constitute the peripheral T cell repertoire responsible for directing many facets of the adaptive immune system. Loss of the thymus through genetic mutation, radiation or surgical removal results in severe immunodeficiency and a high susceptibility to infection.

 

The ability of T cells to recognize foreign antigens is mediated by the T cell receptor. The T cell receptor undergoes genetic rearrangement during thymocyte maturation, resulting in each T cell bearing a unique T cell receptor, specific to a limited set of peptide:MHC combinations. The random nature of the genetic rearrangement results in a requirement of central tolerance mechanisms to remove or inactivate those T cells which bear a T cell receptor with the ability to recognise self-peptides.

 

The generation of T cells expressing distinct T cell receptors occurs within the thymus, and can be conceptually divided into three phases:

 

A rare population of hematopoietic progenitors enters the thymus from the blood, and expands by cell division to generate a large population of immature thymocytes.

 

Immature thymocytes each make distinct T cell receptors by a process of gene rearrangement. This process is error-prone, and some thymocytes fail to make functional T cell receptors, whereas other thymocytes make T cell receptors that are autoreactive. Growth factors include thymopoietin and thymosin.

 

Immature thymocytes undergo a process of selection, based on the specificity of their T cell receptors. This involves selection of T cells that are functional (positive selection), and elimination of T cells that are autoreactive (negative selection).

 

T cells that pass both levels of selection are released into the bloodstream to perform vital immune functions including interaction with B cells.

 

The lining of the stomach, the gastric mucosa, produces a hormone, called gastrin, in response to the presence of food in the stomach. This hormone stimulates the production of hydrochloric acid and the enzyme pepsin, which are used in the digestion of food.

 

The mucosa of the small intestine secretes the hormones secretin and cholecystokinin. Secretin stimulates the pancreas to produce a bicarbonate-rich fluid that neutralizes the stomach acid. Cholecystokinin stimulates contraction of the gallbladder, which releases bile. It also stimulates the pancreas to secrete digestive enzyme.

 

The heart also acts as an endocrine organ in addition to its major role of pumping blood. Special cells in the wall of the upper chambers of the heart, called atria, produce a hormone called atrial natriiuretic hormone, or atriopeptin.

 

The placenta develops in the pregnant female as a source of nourishment and gas exchange for the developing fetus. It also serves as a temporary endocrine gland. One of the hormones it secretes is human chorionic gonadotropin (CG), which signals the mother's ovaries to secrete hormones to maintain the uterine lining so that it does not degenerate and slough off in menstruation. Also releases chorionic growth hormone prolactin (CGP), uterine relaxing factor (URF), progesterone and estrogen.

 

 
Note on Circumventricular Organs

 These allow factors (hormones, toxins etc)  to 'circumvent' the blood-brain barrier & include the following:

1. Pineal gland - secretes melatonin and is associated with circadian rhythms

2. Subfornical organ- regulates body fluids

3. Organum vasculosum of the lamina terminalis- detects peptides

4. Choroid plexus

5. Area Postrema- the vomiting centre of the brain (the brain can detect noxious substances in the blood and stimulate vomiting in order to rid the body of the substances)

6. Median eminence- is part of the inferior boundary for the hypothalamus part of the human brain; regulates the anterior pituitary through the release of neurohormones; it is integral to the hypophyseal portal system, which connects the hypothalamus with the anterior lobe of the pituitary gland. It is in this structure that the secretions of the hypothalamus (releasing and inhibiting regulatory hormones) collect before entering the portal system.

7.Subcommissural organ

8. Posterior pituitary (neurohypophysis)- to detect levels of oxytocin and ADH in the blood