The endocrine system is one of the most important systems of the body, which along with the nervous system controls and coordinates all the systems of the body. An endocrine gland is a ductless gland that synthesizes one or more hormones and secretes them directly into the blood. Endocrine glands, present in different regions of the body, work in coordination as a combined system. They secrete a wide variety of chemical compounds known as hormones, also called informational molecules. These hormones enter the blood circulation and ultimately reach the target organ on which they act. The study of endocrine glands is called endocrinology.
Types of Gland
Glands are the organs of secretion. There are two types:
- Exocrine gland: glands whose secretions are carried by a duct to the site of action. Example: pancreas, salivary gland.
- Endocrine gland: also called ductless glands, as they pour their secretion directly into the blood. Example: pituitary gland.
Hormones
Hormones are non-nutrient chemicals that act as intercellular messengers and are produced in very small amounts. They are secretions of endocrine glands that do not act like enzymes, but influence the synthesis, activation, or inhibition of certain metabolic processes in the body.
The first hormone, secretin, was discovered in 1903 AD by William M. Bayliss and Ernest H. Starling. The term "hormone" was coined by Starling in 1905. Banting and Macleod isolated insulin, the first hormone to be isolated, in 1920 AD, for which they received the Nobel Prize in 1923 AD. Thomas Addison is regarded as the father of endocrinology.
Properties of hormones
- They have a low molecular weight.
- They are the chemical messengers of the body.
- Chemically, hormones are either steroids or peptides.
- They are effective even in minute quantities.
- Hormones are transported to the target organ through blood, not by ducts.
- Reactions controlled by hormones are not immediately reversible.
- Hormones are water soluble, and therefore can be transported in the blood.
- They can easily diffuse through the cell membrane.
- Hormones cannot be stored, as they are readily destroyed.
- Excess or deficiency of any hormone can lead to serious physiological dysfunction.
Types of hormones
Hormones are classified as follows:
- Amine hormones: derived from the amino acid tyrosine and have an amino group. Example: thyroxine, adrenaline.
- Steroid hormones: derived from cholesterol and have a sterol group. Example: cortisol, aldosterone.
- Proteinous/peptide hormones: have peptide bonds and are made up of 3 to 200 amino acids. This includes:
- Proteinous hormones: TSH, FSH, GH.
- Long peptides: insulin, glucagon.
- Small peptides: ADH, oxytocin.
Human Endocrine Glands
Hypothalamus
The hypothalamus develops from the ectoderm of the embryo. It is located at the base of the forebrain, in the post-thalamus region, just below the anterior and posterior lobes of the diencephalon, and above the pituitary gland.
Structure: the hypothalamus is composed of nervous tissue with certain nuclei (N1 and N2). It is connected with the anterior lobe of the pituitary gland by the hypophyseal portal vein, and with the posterior lobe of the pituitary gland by axons of its neurons. The portal vein and axons pass through the infundibulum to the pituitary lobes.
Hormones: the neurosecretory cells of the hypothalamus secrete hormones called neurohormones. These hormones are carried by the portal vessels to the anterior lobe of the pituitary gland and stimulate the pituitary to release hormones. They are of two kinds: releasing hormones (or factors) and inhibiting hormones.
- Corticotrophin releasing hormone (CRH): stimulates the anterior pituitary to release adrenocorticotropic hormone (ACTH), which stimulates the adrenal cortex to secrete cortisol and aldosterone.
- Thyrotropin releasing hormone (TRH): stimulates the anterior pituitary gland to release thyroid stimulating hormone (TSH), which causes the release of thyroid hormones from the thyroid gland.
- Gonadotropin releasing hormone (GnRH): stimulates the anterior pituitary gland to release follicle stimulating hormone (FSH) and luteinizing hormone (LH), which stimulate the sex glands to release testosterone, oestrogen, and progesterone.
- Growth hormone releasing hormone and somatostatin: growth hormone releasing hormone stimulates release of growth hormone, while somatostatin inhibits the anterior pituitary from releasing growth hormone.
- Prolactin releasing hormone (PRH) and prolactin inhibiting hormone (PIH): PRH stimulates release of prolactin from the anterior pituitary and causes secretion of milk; PIH inhibits the release of prolactin from the anterior pituitary and therefore inhibits milk secretion.
Pituitary Gland
The pituitary gland originates from the ectoderm of the embryo and is located just below the hypothalamus, connected to it by a short, thin stalk called the infundibulum. The pituitary gland works under the influence of the hypothalamus and is also called the hypophysis.
Structure: the pituitary is a very small, brownish, pea sized gland, about 1.2 cm in diameter and weighing about 0.5 gm. It is composed of three lobes:
- Anterior lobe, or adenohypophysis, or pars distalis.
- Posterior lobe, or neurohypophysis, or pars nervosa.
- Intermediate lobe, or pars intermedia.
Hormones of the anterior pituitary
- Follicle stimulating hormone (FSH): promotes oogenesis in females and spermatogenesis in males. FSH acts mainly on the gonads; it stimulates the testes in males to produce sperm, and the ovaries to produce ova, and also releases sex hormones.
- Luteinizing hormone (LH): stimulates the testes and ovaries. In males it is also called interstitial cell stimulating hormone (ICSH), stimulating the interstitial cells of Leydig to produce the male sex hormone testosterone. In females, LH stimulates ovulation, formation of the corpus luteum, and secretion of estrogen and progesterone.
- Prolactin: stimulates lactation (milk production) in females by stimulating the mammary gland during lactation.
- Adrenocorticotropic hormone (ACTH): stimulates the adrenal cortex to produce mineralocorticoids (example: aldosterone), glucocorticoids (example: cortisol), and androgens (example: testosterone).
- Thyroid stimulating hormone (TSH): stimulates the thyroid gland to produce thyroid hormone; it increases the rate of absorption of iodine from the blood to produce the normal amount of thyroid hormone.
- Somatotropin (growth hormone, GH): controls the growth and physical development of the body.
- Melanocyte stimulating hormone (MSH): secreted by the intermediate lobe of the pituitary; stimulates melanocytes to produce melanin, responsible for skin colour. It appears to be functional mainly in cold blooded animals such as fishes and amphibians, making the skin darker to protect them from predators.
Disorders of the anterior pituitary gland:
- Prolactin: hypersecretion, usually due to a prolactinoma, causes galactorrhea in men and amenorrhea in women.
- Growth hormone: hyposecretion in children causes dwarfism, with small body size and sexual immaturity. Hypersecretion in children causes gigantism (abnormal, excessive height), whereas hypersecretion in adults results in acromegaly, an abnormal growth of the feet, hands, and face, especially the lower jaw, giving a gorilla-like appearance.