Hormonal basis of reproduction

Introduction-Hormone – It is the Chemical messenger produced by

a ductless gland i.e. Endocrine glands or tissue and carried in the blood to a target organ where it effects a change in cellular activity.

Cells communicate with each other via these chemical messengers . Hence These are the modes of intercellular communication.  

Endocrine glands of cow-

Classifications of hormones-

Classifications of reproductive hormones-Based on mode of action--primary hormones of reproduction : FSH & LH-secondary hormones of reproduction : ACTH &

TSH Based on biochemical structures-proteins-polypeptide hormones having MW

of 300-70000 dalton Eg- oxytocin –glycoprotein in nature FSH & LH-polypeptide in nature

Cont…steroids-derived from cholesterol & have

MW of 300 – 400 daltons Eg- testosteroneFatty acids- derived from arachidonic acid

& have MW of about 400 daltons eg-prostaglandinsAmines- derived from tyrosine or

tryptophan eg- melatonin

Diff. Modes of intercellular communication-4 basic modes1)Neural communication- neuro transmitters

are released at synaptic junctions from nerve cells & act across narrow synaptic clefts between as neurotransmitters.

2)Endocrine communication- hormones transported through blood circulation typical of most hormones.

3)Paracrine communication- products of cell diffuse through extracellular fluid to affect neighbouring cells that are at a distance . i.e. prostaglandins

4)Autocrine communication- cells secrete chemical messengers that bind to receptors on the same cell that secreted messenger.

Regulation of hormone secretion-Nervous system plays essential role in regulation

of gonadal activities by means of Endocrine feedback mechanismNeuroendocrine reflex Immunoendocrine control.Endocrine feedback mechanismA target gland hormone (i.e. estrogen) can

influence secretion of tropic stimuli that causes its own release ( i.e. FSH)

Feed back control occurs at the level of hypothalamus & pituitary gland.

Depending upon conc. In blood steroid hormones may exert stimulatory (+ve) or inhibitory feed back(-ve).

Cont…A. Inhitory or negative feedback - Stimulation of ovary increases estrogen

secretion , FSH levels decline. When pituitary hormones reach a certain

level hypothalamic nuclei respond by decreasing the production of releasing hormones thereby decreasing production of pituitary tropic hormone & lower level of target gland function.

B. Stimulatory or positive feedback- Increase level of estrogen during

preovulatory phase increase release of pituitary LH leading to ovulation.

Cont…1. Long loop feedback- involves interaction among

gonads,pituitary & hypothalamus.2. Short loop feedback- permits pituitary gonadotropins to

influence secretory activity of releasing hormones from hypothalamus without mediation of gonads.

General features-Group 1 Group 2

Types Steroids,iodothyronines, calcitriol ,retinoids

Polypeptides,proteins, glycoproteins, catecholamines.

Solubility Lipophilic Hydrophilic

Transport protein Yes No

Plasma half life Long (hours to days) Short(minutes)

Receptor Intracellular Plasma membrane

Mediators Receptor – hormone complex

cAMP ,cGMP,Ca 2+, metabolites of complex phosphoinositols, kinase cascades.

Primary hormones of reproduction-Regulate various reproductive processes

i.e.spermatogenesis,ovulation,sexual behavior , fertilization,implantation,maintenance of gestation , parturition,lactation & maternal behavior.

Secondary or metabolic hormones-

These indirectly influence reproduction.

Hypothalamic hormones- GnRH-

I. Decapeptide (10 aminoacids)II. MW 1183 daltonIII. Synthesised & stored in medial basal

hypothalamusIV. In response to neural signals pulses of GnRH

released into hypophyseal portal system for release of FSH & LH from anterior pituitary.

PRH- (Prolactin releasing hormone) Stimulate release of prolactin hormone

PIH-(Prolactin inhibiting hormone) Inhibit release of prolactin hormone It is a catecholamine i.e.dopamine which is a

low MW amine synthesised from L-Tyrosine & secreted from nerve terminals in median eminence.

Adenohypophyseal hormones-FSH-

Glycoprotein hormone having MW 32000 daltonConsists of α & β subunitsFunctions-

Stimulates growth & maturation of ovarian follicle i.e. Graafian follicle

In presence of LH stimulates estrogen production In males it is responsible for spermatogenesis( upto

secondary spermatocyte stage)

LH-Glycoprotein composed of α & β subunits with

a MW of 30000 daltonsBiological half life is 30 mins

Cont…In conjuction with FSH induce estrogen

secretion from ovarian follicle.Preovulatory LH surge responsible for

rupture of follicle wall & ovulation.In males stimulate interstitial cells (Leydig

cells) to produce androgens.PROLACTIN-

Polypeptide hormone consisting of 198 aminoacids & having MW of 24000daltons.

Functions- Initiates & maintains lactation luteotropic in rodents i.e. maintains the C.L.

Neurohypophyseal hormones-These hormones are secreted from

hypothalamus & stored in posterior pituitary until they are needed.

Transferred from hypothalamus to pituitary along the axons of nervous tissue.

Oxytocin-An octapeptideFunctions-

Stimulate uterine contraction Facilitate spermatozoal transport Induction of parturition (Ferguson’s reflex)Reflex release of milk upon suckling & milking

stimulation

Release of oxytocin-

Melatonin-It is N-Acetyl-5-methoxy tryptamine synthesised

in pineal gland. Synthesis-Blood vessel Tryptophan

pineal

parenchyma serotonin light N-Acetyl

TransferaseN-Acetyl serotonin

dark Hydroxy

indole- o-methyl

transferase Melatonin Blood vessel Melatonin

Gonadal steroid hormones-Ovaries,testes & to some extent adrenal &

placenta secrete these steroids.Major hormones are

androgen,estrogen,progestin.Have common nucleus called

cyclopentanoperhydrophenanthrene ring i.e. consisting of 3 six membered fully hydrogenated (perhydro) phenanthrene rings & 1 five membered cyclo pentane ring.

18 –c steroid has estrogen activity,19-c steroid has androgen activity,21-c steroid has progestogen activity.

In blood plasma mostly bound to albumin protein.

Structures of steroid hormones-Natural steroids-

Synthetic steroids-

Synthesis-

Synthesis-

Cont…..

Conversion of testosterone to estradiol-

Mechanism of action-

Estrogen-Estradiol is biologically active.Zeronal is a compound having estrogenic

activity is produced by mold.DES is synthetic non-steroidal estrogen used

for growth promoting purpose in bovines.Functions-

Behavioral estrous signs in animalsPotentiate effect of oxytocin & PGF2αDevelop female secondary sexual

characteristics like development of mammary gland.

Control release of FSH & LH.

Progestogens-Functions-

Prepare endometrium for implantation & maintains pregnancy

Inhibit uterine motilityInhibit estrus & ovulatory surge of LH

Androgens-19-c steroids with –OH or an oxygen group at

position 3 & 17 & a double bond at position 4.Called as 17-keto steroid when oxygen present at

position 17.

Cont..Secreted by leydig cells of testes & limited

amount by adrenal cortex.Functions-

Stimulate late stage of spermatogenesis & prolong lifespan of epididymal sperm

Promote growth ,development, secretory activity of accessory sex glands

Maintains secondary sexual characteristics & sexual behaviour in males

Hormones in males-

Other gonadal hormones-Relaxin-

Secreted by CL of pregnancyPolypeptide hormone consisting of α & β

subunits connected by 2 disulfide bonds having MW of 5700 dalton.

Major function is dialation of cervix & vagina prior to parturition.

Inhibin-Secreted from sertoli cells in male & granulosa

cells in female.Protein hormones consisting of α & β subunits

connected by 2 disulfide bridge Inhibit release of FSH without altering LH

release.

Cont…..Activin-

Stimulates release of FSH.Consists of two β subunits.Present in follicular fluid & rete testes fluid.

Follistanin-A proteinInhibits secretion of FSHBinds with activin thereby neutralises its

biological activities.

Placental hormones-PMSG or eCG –

Is a glycoprotein with α & β subunits containing more sialic acid than FSH & LH that accounts for its long half life.

Has both FSH & LH biologic actionshCG-

Glycoprotein consisting of α & β subunits & has MW of 40000daltons. α subunit have 92 aninoacids & 2 carbohydrate chain.β subunit has 145 AA & 5 carbohydrate chains.

It has luteinizing & luteotropic & little FSH properties.Placental lactogen-

Protein with MW 22000-23000 daltons & has 192 AA Has prolactin like properties

Pregnancy specific protein B(PSPB)- Prevent destruction of CL in early pregnancy.

Prostaglandins-Secreted from almost all body tissueAre 20-c unsaturated hydroxy FAs with a

cyclopentene ring.Arachidonic acid ,an essential FA is precursor of

PGF2α & PGE2 those are more closely associated with reproduction.

Functions-Luteolytic agent that ends CL of estrous cycle &

start a new cycleEnds early pregnancyContraction of smooth msl in reproductive tract

there by helps in erection,ejaculation,sperm transport etc

Ovulation,formation of CL ,parturition & milk ejection.

Synthesis of prostaglandins-

Action of PGF2α on CL-

Structures-

Figure 28.25

The Hormonal Regulation of Ovarian Activity-

In females-

Follicular development in ovary-

Figure 28.12

Hormonal Feedback and the Regulation of the Male Reproductive Function

In male animals-