Role of Hormones in Female Reproduction
Hormones play an important role in regulating the female reproductive system, ensuring proper development, ovulation, and fertility.
Endocrinology
A science concerned with chemical integration of the body. Integration is a key word related to the function of the nervous system.
Endocrine glands
Endocrine glands are those ductless glands of the body whose secretion goes directly in to the blood stream.
Exocrine glands
Exocrine glands are those glands of the body whose secretion is carried away by a duct.
Hormones
The word hormone originated from a Greek word meaning “I Stir up or stimulate”. A hormone is a chemical substance produced in one part of the body (restricted area) that diffuses or is transported to another area where it influences activity and tends to integrate component parts of the organism.
Hormones regulate (decrease or increase) the rates of specific processes but do not contribute energy to the process or initiate metabolic reactions. Instead, hormones influence an existing reaction which is usually one involving enzymes.
Proteins
Proteins or polypeptide hormones have molecular weight of 300 to 70,000 daltons and are easily broken down by enzymes. They must be administered by injections.
Steroids
Steroids have a molecular weight of 300 to 400 daltons. Natural steroids are not effective by oral administration. Synthetic and plant steroids can be administered orally and by injection.
Fatty acids
Fatty acids have a molecular weight of approximately 400 daltons and can be administered by injection.
Glandular Origin
Reproductive hormones originate from the hypothalamus, pituitary, gonads, uterus and placenta.
Neurohormones
Neurohormones are synthesised by neurons and released directly into the blood and cause response in a target tissue. Eg. Oxytocin. A neurohormone can act on any number of tissues provided that the tissue has cellular receptors for the neurohormone.
Releasing Hormones
Releasing hormones are synthesised by neurons in hypothalamus and cause release of hormones from pituitary. Eg. GnRH.
Gonadotropins
Gonadotropins released by the gonadotroph cells of the anterior pituitary and stimulate gonads. The suffix tropin means having an affinity for eg. FSH and LH.
Sexual Promoters (Steroids)
Are produced by the gonads of both male and female to stimulate the reproductive tract, to regulate function of hypothalamus and anterior pituitary and to regulate reproductive behaviour.
- Pregnancy maintenance hormones are responsible for maintenance of pregnancy (eg. Progesterone) and in some cases, assist the female in her lactation ability).
- General metabolic hormones: promote metabolic well being eg. Thyroxin, adrenal corticoids and somatrophin.
- Luteolytic hormones: cause destruction of the corpus luteum. The suffix ‘lytic’ is a derivative of word lysis. Eg. PGF2 alpha.
Local Hormone or Para Hormones: In strict sense are not hormones, but are chemical messengers or regulators. Eg. Prostaglandins, erythropoietin, and histamine.
Hypothalamus
The hypothalamus, the key brain center that controlls all our reproductive activities lies at the base of the brain, bordered anteriorly by the optic chiasma, posteriorly by the mammillary bodies, dorsally by the thalamus and ventrally by the sphenoid bone.
- Its size is 1/300 of the entire brain.
- Within the hypothalamus are numerous neurons. Clusters or groups of these neurons are called hypothalamic nuclei, each of which have a specific name.
- The hypothalamic nuclei of importance are:
- The preoptic nucleii(PON), the anterior hypothalamic area (AHA), suprachiasmatic nucleii(SCN) that make up the surge center
- The ventromedian nucleii(VMN), the arcuate nucleii(ARC) and the median eminence(ME) that make up the tonic center, and
- paraventricular nucleus(PVN).
- The medial portion of the hypothalamus known as third ventricle of the brain separates most of the paired nuclei.
- Neurons in the hypothalamus communicate with the anterior lobe of the pituitary using a special circulatory modification known as the hypothalamo-hypophyseal portal system.
Pituitary Gland
The pituitary gland lies below the hypothalamus in a bony depression in the sphenoid bone called the Sella turcica.
It consists of anterior and posterior lobes.
Hypothalamo-Hypophysesal Portal System
The hypothalamus, the key brain center that controlls all our reproductive activities lies at the base of the brain, bordered anteriorly by the optic chiasma, posteriorly by the mammillary bodies, dorsally by the thalamus and ventrally by the sphenoid bone.
- Its size is 1/300 of the entire brain.
- Within the hypothalamus are numerous neurons. Clusters or groups of these neurons are called hypothalamic nuclei, each of which have a specific name.
- The hypothalamic nuclei of importance are:
- The preoptic nucleii(PON), the anterior hypothalamic area (AHA), suprachiasmatic nucleii(SCN) that make up the surge center
- The ventromedian nucleii(VMN), the arcuate nucleii(ARC) and the median eminence(ME) that make up the tonic center, and
- paraventricular nucleus(PVN).
- The medial portion of the hypothalamus known as third ventricle of the brain separates most of the paired nuclei.
- Neurons in the hypothalamus communicate with the anterior lobe of the pituitary using a special circulatory modification known as the hypothalamo-hypophyseal portal system.
Pituitary Gland
The pituitary gland lies below the hypothalamus in a bony depression in the sphenoid bone called the Sella turcica. It consists of anterior and posterior lobes.
The portal system consists of:
- The Superior hypophyseal artery(SHA).
- Primary portal plexus(PPP) (where the neurons of the surge center and tonic center terminate,
- The medial hypophyseal artery (MHA)that supplies part of the anterior lobe of the pituitary,
- The portal vessels(PV) that transport the blood containing releasing hormones and,
- The secondary portal plexus(SPP) that delivers blood and releasing hormones to the cells of the anterior lobe.
The terminal portion of the hypothalamic neurons release neuropeptides that enter specialized capillary system at the stalk of the pituitary. Blood enters the capillary system from the superior hypophyseal artery that divides into small arterial capillary forming plexus (primary portal plexus). The releasing hormones are transferred to the secondary portal plexus in the anterior lobe of the pituitary where the releasing hormones cause pituitary cells to release other hormones.
The hypothalomo-hypophyseal portal system is important as it allows for minute quantities of releasing hormones to act directly on the cells of the anterior lobe of the pituitary before GnRH gets diluted by the circulation.
The posterior lobe of the pituitary does not have a portal system. Neurons from certain hypothalamic nuclei (PVN) extend directly into the postetrior lobe of the pituitary where the neurohormone is released into a simple arteriovenous capillary plexus. For eg. Cell bodies in the Para Ventricular Nucleus (PVN) synthesize oxytocin that is transported down the axon to the terminals in the posterior lobe. If the neuron is stimulated, oxytocin is released into the blood.
