A hormone  is any member of a class of signaling molecules produced by glands that are transported by the circulatory system to target distant organs to regulate physiology and behavior. Hormones have diverse chemical structures, mainly of 3 classes: eicosanoids, steroids, and amino acid derivatives (amines, peptides, and proteins). The glands that secrete hormones comprise the endocrine signaling system. The term hormone is sometimes extended to include chemicals produced by cells that affect the same cell (autocrine or intracrine signalling) or nearby cells (paracrine signalling).

Hormones are used to communicate between organs and tissues for physiological regulation and behavioral activities, such as digestion, metabolism, respiration, tissue function, sensory perception, sleep, excretion, lactation, stress, growth and development, movement, reproduction, and mood.

Hormones affect distant cells by binding to specific receptor proteins in the target cell resulting in a change in cell function. When a hormone binds to the receptor, it results in the activation of a signal transduction pathway. This may lead to cell type-specific responses that include rapid non-genomic effects or slower genomic responses where the hormones acting through their receptors activate gene transcription resulting in increased expression of target proteins. Amino acid–based hormones (amines and peptide or protein hormones) are water-soluble and act on the surface of target cells via second messengers; steroid hormones, being lipid-soluble, move through the plasma membranes of target cells (both cytoplasmic and nuclear) to act within their nuclei.

Hormone secretion may occur in many tissues. Endocrine glands are the cardinal example, but specialized cells in various other organs also secrete hormones. Hormone secretion occurs in response to specific biochemical signals from a wide range of regulatory systems. For instance, serum calcium concentration affects parathyroid hormone synthesis; blood sugar (serum glucose concentration) affects insulin synthesis; and because the outputs of the stomach and exocrine pancreas (the amounts of gastric juice and pancreatic juice) become the input of the small intestine, the small intestine secretes hormones to stimulate or inhibit the stomach and pancreas based on how busy it is. Regulation of hormone synthesis of gonadal hormones, adrenocortical hormones, and thyroid hormones is often dependent on complex sets of direct influence and feedback interactions involving the hypothalamic-pituitary-adrenal (HPA), -gonadal (HPG), and -thyroid (HPT) axes.

Upon secretion, certain hormones, including protein hormones and catecholamines, are water-soluble and are thus readily transported through the circulatory system. Other hormones, including steroid and thyroid hormones, are lipid-soluble; to allow for their widespread distribution, these hormones must bond to carrier plasma glycoproteins (e.g., thyroxine-binding globulin (TBG)) to form ligand-protein complexes. Some hormones are completely active when released into the bloodstream (as is the case for insulin and growth hormones), while others are prohormones that must be activated in specific cells through a series of activation steps that are commonly highly regulated. The endocrine system secretes hormones directly into the bloodstream typically into fenestrated capillaries, whereas the exocrine system secretes its hormones indirectly using ducts. Hormones with paracrine function diffuse through the interstitial spaces to nearby target tissue.

Major Hormones involved in pregnancy

Human chorionic gonadotropin (hCG):This is a hormone produced by the embryo after implantation.The presence of hCG is detected in some pregnancy tests (HCG pregnancy strip tests).

The hormone stimulates the corpus luteum (the part of the egg follicle left behind in the ovary after ovulation) to produce estrogen and progesterone in the first 10 weeks after conception, until the placental cells can do so themselves.

Progesterone: This hormone is an endogenous steroid and progestogen sex hormone involved in the menstrual cycle, pregnancy, and embryogenesis. It belongs to a group of steroid hormones called the progestogens, and is the major progestogen in the body. Progesterone is also a crucial metabolic intermediate in the production of other endogenous steroids, including the sex hormones and the corticosteroids, and plays an important role in brain function as a neurosteroid.

Progesterone is sometimes called the “hormone of pregnancy”,and it has many roles relating to the development of the fetus:

Progesterone converts the endometrium to its secretory stage to prepare the uterus for implantation. At the same time progesterone affects the vaginal epithelium and cervical mucus, making it thick and impenetrable to sperm. Progesterone is anti-mitogenic in endometrial epithelial cells, and as such, mitigates the tropic effects of estrogen. If pregnancy does not occur, progesterone levels will decrease, leading, in the human, to menstruation. Normal menstrual bleeding is progesterone-withdrawal bleeding. If ovulation does not occur and the corpus luteum does not develop, levels of progesterone may be low, leading to anovulatory dysfunctional uterine bleeding.

During implantation and gestation, progesterone appears to decrease the maternal immune response to allow for the acceptance of the pregnancy.

Progesterone decreases contractility of the uterine smooth muscle.  In addition, progesterone inhibits lactation during pregnancy. The fall in progesterone levels following delivery is one of the triggers for milk production.

A drop in progesterone levels is possibly one step that facilitates the onset of labor.

The fetus metabolizes placental progesterone in the production of adrenal steroids.

Estrogen: This hormone is the primary female sex hormone. It is responsible for the development and regulation of the female reproductive system and secondary sex characteristics.

During pregnancy it stimulates the growth of the uterus and improves blood flow between the uterus and placenta by enhancing the effect of nitric oxide, a gas that widens blood vessels. It also prepares the breasts for milk production by enlarging the milk ducts. Estrogen secretion peaks right before birth and declines afterward.

Relaxin: This is a  polypeptide hormone secreted by the corpora lutea  during pregnancy. It facilitates the birth process by causing a softening and lengthening of the pubic symphysis and cervix; it also inhibits contraction of the uterus and may play a role in timing of parturition.

Oxytocin: This is a nonapeptide neurohypophysial hormone that causes the muscular wall of the uterus to contract at term and promotes milk release during lactation; used to induce or stimulate labor, to manage postpartum hemorrhage and atony, and to relieve painful breast engorgement.

Hormone. (2016, October 13). In Wikipedia, The Free Encyclopedia. Retrieved 00:52, October 13, 2016, from https://en.wikipedia.org/w/index.php?title=Hormone&oldid=744083064

 

 

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