Role of Hormones In Regulating Reproduction In Farm Animals
Reproduction is regulated by marked interplay between nervous system and endocrine system. These two systems interact to initiate, coordinate and regulate (ICR) all reproductive functions. Nervous system generates two types of reflexes, i.e., simple neural reflex and neuroendocrine reflex. After receiving an external stimulus (stress, temperature, photoperiod, visual and tactile), message is conveyed to sensory neurons (recognition of stimulus) and is further transmitted (through spinal cord and inter-neurons) to efferent neurons. After this stage, treatment of massage is different between simple neural and neuroendocrine reflexes. In simple neural reflex, efferent neurons innervate target tissue and release neurotransmitter for action, e.g., muscles for sexual behavior and ejaculation. In neuroendocrine reflex, efferent neurons reach hypothalamus, thus stimulating release of neurohormone into blood. Neurohormones reach target tissue and response is shown by target tissue, e.g., suckling reflex and milk ejection. In suckling reflex, message goes to hypothalamus, oxytocin is released which reaches mammary glands (Target). Reproduction is a fundamental aspect of livestock farming, and understanding the intricate hormonal mechanisms that govern the reproductive processes in farm animals is essential for optimizing breeding outcomes. Hormones play a pivotal role in regulating the reproductive cycles, influencing fertility, and facilitating successful reproduction. This article explores the key hormones involved in the reproductive physiology of farm animals and their roles in various stages of the breeding process.
Endocrinology
Deals with biochemistry, physiology, pharmacology and molecular biology of hormones.
Hormones
chemical substances synthesized and released by endocrine ductless glands directly into blood stream and are transported to target tissue to stimulate or inhibit the functional activity of a target organ. Functional activity involves alterations in metabolism, synthetic activity or secretory activity. Target tissue contains specific receptors for specific hormones to produce new products or hormones. Blood concentrations of hormones are in nanograms (10-9) to pictograms (10-12) per ml. e.g. FSH, LH.
Hypothalamus
Hypothalamus is a neural control center for reproductive hormones. Approximately 0.003% part of entire brain and is composed of bilaterally paired nuclei. Hypothalamic nuclei are clusters of nerve cell bodies. A group of hypothalamic nuclei influencing reproduction are named surge and tonic center. Surge center is composed of preoptic nuclei (POA) and anterior hypothalamic area, whereas tonic center is composed of ventromedial nucleus (VMN) and arcuate nucleus (ARC). Neurons in these regions secrete GnRH. Various hypothalamic nuclei have different functions and are stimulated by different sets of conditions.
Hypothalamo-hypophyseal portal system (for anterior pituitary/adenohypophysis)
GnRH neuron axons from cell bodies located in surge and tonic center extend into pituitary stalk region (the median eminence) where neuron endings terminate on sophisticated and highly specialized capillary network called HHP system. Hypothalamic neuron terminals release neurohormones into HHP system. This portal system enables extremely small (pg) quantities of releasing hormones to be deposited in the capillary plexus of the pituitary stalk/the median eminence, thus enabling direct action of neurohormones on cells of anterior pituitary before being diluted by systemic circulation. Posterior pituitary (neurohypophysis) does not contain portal system. Neurohormones are deposited directly into capillaries in the post pituitary, e.g. oxytocin which is synthesized from the supraoptic nuclei and the paraventricular nuclei (PVN) located neuron cell bodies is transported down axons to post pituitary and is thus stored here before being released to systemic circulation upon receiving stimuli.
Role of Reproductive Hormones
Hypothalamic hormones
GnRH/LHRH (Gonadotropin releasing hormone/ luteinizing hormone releasing hormone)
It is a decapeptide with MW around 1183D. GnRH is synthesized in hypothalamus and is carried to anterior pituitary gland. Target cell of GnRH is gonadotroph cells in anterior pituitary. Functions: It stimulates synthesis and secretion of FSH and LH from anterior pituitary. Clinical applications of GnRH are based upon its ability to stimulate surge release of LH and FSH upon single GnRH injection. Deficiency leads to delayed puberty and infertility related to ovulatory problems. Commercial preparation available is Receptal containing GnRH analogue named Buserelin acetate.
Oxytocin
Hypothalamic hormone (based on site of synthesis) and posterior pitutary hormone (based upon site of release). This is also produced in small amounts from corpus luteum. Target tissue is myometrium and endometrium of uterus and myoepithelial cells of mammary gland.
Functions
Induces contractions of uterus and oviduct musculature, help transport of male and female gametes, involved in milk letdown and promotes uterine prostaglandin synthesis. Clinical applications involve milk letdown, difficult parturition related to uterine inertia, expulsion of fetal membranes and uterine involution. Commercial preparation is Pitocin etc.
Anterior pituitary hormones FSH (Follicle stimulating hormone)
It is released from gonadotroph cells and MW is 32,000D. Target tissue is ovary (granulose cells). Functions involve stimulating the growth of growing follicles. FSH in combination with estrogen causes formation of FSH and LH receptors in granulose cells, thus stimulating adequate development of granulose cells. Overall the functions of FSH are follicle development and estrogen synthesis. Clinical applications involve use for superovulation during embryo transfer. Preparations available are Gonadotropin FSH, Folltropin-V.
LH (luteinizing hormone)
It is released from gonadotroph cells and MW is 30,000D. Target tissue is ovary (theca interna and luteal cells). Functions involve final maturation of follicle; LH surge release is required for ovulation, transforms granulose cells to lutein cells thus leading to formation of corpus luteum and stimulates progesterone secretion from luteal cells in the ovary. Clinical applications involve use in ovulatory problems. Preparation available is Gonadotropin LH, Lutropin-V.
Prolactin
A peptide released from anterior pituitary and MW is 24,000D. Target tissue is mammary cells.
Functions
Prolactin is luteotropic in rat, mouse and sheep i.e., helps in maintenance of corpus luteum. In large animals, prolactin is antigonadotropic and is related with onset and maintenance of lactation. Prolactin also acts on CNS to induce maternal behavior i.e., nesting behavior and care of new born.
Gonadal steroid hormones
Basic nucleus of all steroid hormones is a Cyclopentano-perhydro-phenanthrene nucleus. Biological action of steroids can be predicted from number of carbons present.
Estrogen
LH stimulates theca cells to secrete testosterone which is subsequently aromatized to estrogen in granulosa cells under FSH stimulation. Target tissue is hypothalamus, reproductive tract and mammary gland.
Functions:
Action on CNS/hypothalamus/anterior pitutary: Estrogen acts on CNS and creates desire for mating in females known as behavioral estrus. Some amount of progesterone is required for inducing estrus in bovines. First ovulation at puberty is without behavioral estrus because of the fact that progesterone is not available. Estrogen has negative feedback on tonic center of hypothalamus and positive feedback effect on surge (preovulatory) center to control FSH/LH release. a) Reproductive functions: Responsible for secondary sex characters. Reproductive tissue depends upon estrogen for growth, e.g., uterus, ovary and mammary gland. Estrogen causes growth of duct system of mammary gland. b) Specific actions on genital tract: 1) Vulva: edema and relaxation. 2) Vagina: goblet cells in ant vagina secrete mucus during estrus and late pregnancy, estrogen causes growth and keratinisation of vaginal epithelium especially in dog and cat known as “Mitogenic effect”. 3) Cervix: causes cervical relaxation, induces goblet cells to secrete mucus, induces opening of cervix. 4) Uterus: Estrogen increases uterine vascularity and turgidity, causes development of endometrial glands, induces migration of leucocytes thus increasing phagocytosis and giving protection against infections. 5) Oviduct: Estrogen increases oviductal contractility thus helping in gamete transport, under estrogen influence fimbriae become turgid which is suitable for ovum pickup, induces tube locking in sheep i.e. fertilized ovum cannot move to uterus until latter is ready. 6) Ovary: induces contractility of ovarian musculature and helps in ovulation. 7) Ligaments: induces relaxation.
Clinical applications:
This hormone is rarely used due to side effects like induces drastic decrease in milk yield and disrupts hypothalamic endocrine system, estrogen has luteotropic action in sows thus can be used for estrus synchronization in pigs and estrogen can be used for induction of lactation in barren dairy animals. Clinical preparation available is synthetic estrogen known as diethyl stilbesterol (DES).
Progesterone
Progesterone is secreted from luteal cells, placenta and adrenal glands. LH stimulates its secretion. Target tissue is uterine endometrium, myometrium, mammary gland and hypothalamus. Functions: progesterone is known as hormone of maternity. Specific actions on genital tract: 1) Endometrium: induces secretion from endometrial glands which were previously prepared by estrogen, induces mucosal thickening, coiling of endometrial glands and edema of stroma. 2) Myometrium: inhibits contractions and inhibits response to oxytocin. Thus, progesterone prepares uterus for implantation and maintenance of pregnancy by increasing secretory glands in endometrium and inhibiting the activity of myometrium. 3) Mammary glands: develop secretary tissue of mammary gland along with help of estrogen. 4) Nidation and pregnancy: essential for this function. 5) Estrus symptoms: slight amount is required. During second ovulation, progesterone from regressing corpus luteum act synergistically with estrogen to produce estrus symptoms. 6) Progesterone is necessary for maternal behavior.
Clinical applications:
Progesterone prevents abortion, used in birth control pills (prevents LH surge /ovulation), used for synchronization of estrus cycle, used for induction of lactation with estrogen. Clinical preparations available are Duraprogen and proluton depot as injections in the form of implants as PRID (progesterone releasing intravaginal device) and CIDR (controlled internal drug release). PRID is a silicone coil impregnated with progesterone.
Gonadal peptide hormones
Inhibin
It is a glycoprotein which is released from granulose cells of follicles. Its function is to inhibit the release of FSH from ant pit without altering the release of LH from ant pit. Target tissue is gonadotroph cells of anterior pitutary.
Uterine hormones
Prostaglandins
PGs are 20-C unsaturated hydroxyl fatty acids with cyclopentane ring. The precursor for PGs is arachidonic acid. PGF2 alpha is released from endometrial glands whereas PGE2 is released from ovary, uterus and embryonic membranes. Target tissue for PGF2 alpha involves corpus luteum, uterine myometrium, ovulatory follicle whereas for PGE2 involve early corpus luteum Functions: PGF2 alpha: helps in ovulation, CL regression induced by PGF2 alpha initiated constriction of blood vessels, aids in gamete (sperm) transport in females PGE2: stimulates uterine contractions, dilates blood vessels, helps in ovulation, no luteolytic action and assists in maternal recognition of pregnancy in mare. Clinical application: PGF2 alpha: estrus synchronization, luteal cyst treatment. Clinical preparations available are Vetmate, Iliren and Lutalyse.
Placental hormones
PMSG/eCG (Pregnant mare serum Gonadotropin/equine chorionic Gonadotropin)
A glycoprotein with alpha and beta subunits similar to LH and FSH but has higher sialic acid content responsible for long half life. It is secreted from endometrial cups of mare from day 40 to day 150 of gestation. PMSG is isolated from pregnant mare’s blood and is not present in urine. Target tissue of PMSG is ovary. Functions: causes growth of bunch of follicles which either ovulate or get luteinized leading to formation of accessory corpora lutea. PMSG has mainly FSH like activity. Clinical applications involve use during superovulation and anestrus. Clinical preparations available are Folligon
hCG (human chorionic gonadotropin)
A glycoprotein with alpha and beta subunits, secreted from chorion (trophoblast) of blastocyst. hCG can be detected as early as day 6 after conception in humans. hCG has predominantly LH like activity and less FSH like activity. Major advantage of hCG over pituitary LH is its longer half life which increases its effectiveness for induction of ovulation. hCG is isolated from urine. Function of hCG is to facilitate production of progesterone by ovary. Clinical applications involve pregnancy diagnosis in humans and used for treatment of ovulatory defects in bovines. Clinical preparation available is Chorulon.
Placental lactogen
Placental lactogen is a protein hormone which is secreted from placenta of human, goat, sheep and cow. This can be detected in serum in last trimester of pregnancy. Target tissue is mammary gland of dam and function involves mammary stimulation of dam.
Protein B
Protein B is isolated from bovine placental tissue as early as day 22 after conception. It has half life of 7 days. Protein B is a signaling agent transmitting massage not to destroy corpus luteum. It has potential to be developed as a tool for early pregnancy detection in cattle during coming years.
hMG (human menopausal Gonadotropin)
Menopause is a stage when no follicle or CL formation is there on ovary and concentrations of progesterone and estrogen are decreased. This happens usually after 50 years of age. hMG is released from ant pit because no negative feedback control of estrogen and progesterone from ovaries is present because in menopause lady ovarian activity stops. Biological action is like FSH and less like LH.
Hormones play a central role in orchestrating the complex and highly regulated processes of reproduction in farm animals. Understanding the functions and interactions of these hormones is essential for implementing effective breeding strategies, optimizing fertility, and ensuring the success of livestock production. As advancements in reproductive technologies continue, the knowledge of hormonal regulation remains a cornerstone for sustainable and efficient breeding practices in the agriculture industry.
Compiled & Shared by- This paper is a compilation of groupwork provided by the
Team, LITD (Livestock Institute of Training & Development)
Image-Courtesy-Google
Reference-On Request.
