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REPRODUCTIVE PHYSIOLOGY (5 The Mammary gland (Lactation cycle (stops at…

- - - - Germ cells migrate into genital ridge from yolk sac
      - Primordial germ cells populate sex cords
      - Mesenchyme of genital ridge contributes cell that become theca cells (Leydig cells in male)
      - Tubes derived from Mullerian ducts will become oviduct
      - Oocytes are released through the surface of ovary by rupture of tissue elements surrounding ovary (ovulation)
      - Fimbria (specialised end of oviduct) develops to enable oocyte to be removed efficiently from the surface of ovary
      - Fundamental difference between testes and ovarian development:
        i) invagination of sex cords in male continues into the medulla of embryonic gonads
        
        connections are made with medullary cords from mesonephros (primitive kidney)
        
        Wolffian duct becomes epidydimis, vas deferens & urethra - has direct connections to seminiferous tubules - male germ cells pass to exterior of animal through closed tubular system
    - - developing of genital tubular system & external genitalia is under control of developing gonad
        
        Female:
        
        Developing gonad: ovary
        
        Mullerian duct develops into oviduct, uterus, cervix & vagina
        
        Wolffian duct regresses - testosterone is absence
        
        Male:
        
        Rete testis produces Mullerian-inhibiting factor - Mullerian duct regresses
        
        Wolffian duct maintained - influence of androgens
      - Development of external genitalia follows the development and direction of gonads
        
        Female:
        
        Labia (folds of tissue) form vulva
        
        Clitoris develops
        
        Male:
        
        Androgens from testis direct formation of penis & scrotum
        
        Influence by the absence or presence of androgens
        
        Sexual differentiation of hypothalamus
        
        Male:
        
        Exposure of hypothalamus to androgens at about birth time
        
        Conversion (aromatization) of androgens to estrogens is essential for maleness, mediated by enzymes in neural tissue
        
        Female: absence of androgens
        
        Fundamental concept of organization of reproductive system with regard to genotype:
        
        female system is organized in the absence of testes
        
        if male, must have intervention by testes through the production of androgens & appropriate tissue enzymes in two circumstances:
        i) within internal genitalia for conversion to more potent androgens
        ii) within hypothalamus for conversion to estrogens
  - - - Hypothalamus:
        
        Midcentral to the base of brain
        
        divided into halves by third ventrical forming ventral & lateral walls of third ventrical
        
        has clusters of neurons (nuclei)
        
        secrete peptide hormones - control pituitary activity
        
        peptides move to pituitary either by pasage through axons of neurons / by vascular portal system
        
        pituitary responds to the hypothalamic peptides to produce hormones that are important in for control of gonads
        
        regulates appetite & temperature, integrates activity of autonomic nervous system
        
        has direct connection to neurohypophysis through neural stalk
        
        contain axons originate from neuronal cell bodies in hypothalamus
        
        2 sets of neurons:
        
        supraoptic
        
        paraventricular nuclei
        
        responsible for synthesis of vasopressin & oxytocin respectively
        
        small peptide hormones coupled to larger peptide molecules (neurophysins)
        
        transported from site of synthesis in hypothalamus (neuronal cell bodies) to site of storage & eventual release (neurohypophysis)
        
        no direct passage of axons to adenohypophysis through the neural stalk
        
        connected by venous portal system - carry hypothalamic substances that control adenohypophysis
        
        example:
        
        gonadotropin-releasing hormone (GnRH): peptide produced in medial preoptic nucleus
        ~ a critical hormone for gonadotropin release
        
        dopamine: amino acid produced in arcuate nucleus
        
        synthesis of GnRH, as with oxytocin & vasopressin, involves production of larger precursor molecule with a C-terminal region of 56 amino acids
        
        GnRH-associated peptide (GAP)
        
        2 more items...
    - - control reproductive processes
      - Pituitary gland:
        
        anterior lobe - adenohypophysis / pars distalis
        
        intermediate lobe - pars intermedia
        
        posterior lobe - neurohypophysis / pars nervosa (release oxytocin)
      - other hormones include: growth hormone (GH), corticotropins (ACTH) & thyroid-stimulating hormone (TSH)
        
        Gonadotropins & TSH:
        
        glycoproteins
        
        contain carbohydrate moieties
      - FSH & LH: synergistic in folliculogenesis (maturation of the ovarian follicle) & ovulation in ovary
        
        FSH: more dominant during growth of follicle
        
        LH: more dominant during final stages of follicles maturation through ovulation
  - - - if in continuous (pharmacological) manner, system can be downregulated
        
        continual occupancy of GnRH interrupts intracellular signal for synthesis & release of gonadotropins
    - - increased in follicular phase
        
        no progesterone
      - decreased in luteal phase of estours cycle
        
        presence of estrogen
      - estrogens decreases pulse ampitude
        
        progesterone decreases pulse frequency
      - increased pulse frequency + decreased pulse amplitude = nurture final growth phase of developing antral follicle
    - - increase frequency of pulsatile release of GnRH - gonadotropin secretion
        
        overcomes metabolic clearance rate
      - purpose is to induce changes within the follicle that leads to its rupture (ovulation)
        
        duration is relatively short (12-24 hours) due to main factor driving response (estrogen) decrease in concentration as the follicles respond to preovulatory gonadotropin surge
        
        effective because follicle able to signal its stage of maturity to hypothalamus & adenohypophysis by estrogen
      - hypothalamic site for positive-feedback stimulation is in the preoptic anterior hypothalamic region
    - - estrogen causes negative-feedback inhibition of gonadotropin secretion
        
        sensitivity (effective at low concentrations)
        
        rapid onset (within a few hours)
        
        affect through an effect on both pituitary gland & hypothalamus
        
        hypothalamic site for negative-feedback inhibition is arcuate nucleus (area immediately above median eminence
      - progesterone affect gonadotropin pulse frequency
        
        modulatory efect at level of hypothalamus
    - - β-Endorphin:
        
        opioid peptide produced from hypothalamic precursor molecule pro-opiomelanocortin
        
        inhibit LH secretion when pharmacologically administered systemically
      - inhibin:
        
        protein produced by granulosa cells of developing follicle
        
        inhibits gonadotropin secretion
      - prolactin:
        
        important in reproductive process mainly because of its effect on mammary gland & lactation in mammals
        
        has more control on inhition than stimulation
        
        prolactin secretion increases if pituitary gland is disconnected from hypothalamus ( either by cutting pituitary stalk or transplanting pituitary gland to another sire (e.g. kidney capsule))
        
        luteotropin
        
        potent prolactin secretion inhibitor:
        
        catecholamine dopamine
        
        produced by neurons in arcuate nucleus
        
        γ-aminobutyric acid (GABA)
        
        GAP
        
        dopamine agonist (ergot type compunds bromocriptine & cabergoline) can supress prolactin secretion in cases of hyperprolactinemia
        
        cabergoline
        
        shorten interestrous intervals in female dogs
        
        promote luteolysis in femlae dogs & female cats during latter half of pregnancy
        
        prolactin-releasing factors:
        
        thyrotropin-releasing hormone (TRH)
        
        vasoactive intestinal peptide (VIP)
        
        inhibit dopamine synthesis within hypothalamus
        
        estrogens
        
        by lactotropes by decreasing lactrotpoe sensitivity to dopamine and increasing number of TRH receptors
  - - - initial development involve growth of oocyte
      - follicle cells devide and form granulosa that is several cells thick
        
        secrete zona pellucida (lies within granulosa and immediately surrounds oocyte)
        
        maintain contact with oocyte through zona pellucida with the help of development of cytoplasmic processes
        
        interaction among granulosa cells is facilitated by development of gap junctions
        
        important as granulosa has no blood supply
        
        blood vessels are excluded at the level of the membrana propria
    - - the proximity of granulosa & theca cells allows cooperative estrogen synthesis (two-cell mechanism)
        
        theca produces androgens (testosterone & andostenedione) under influence of LH
        
        has limited capacity for producing estrogens
        
        diffuses across membrana propria into granulosa
        
        androgens are transformed into estrogen (estradiol-17)
        
        granulosa unable to form androgens (the precursors of estrogen biosynthesis)
        
        effect of estrogen:
        
        formationof additional FSH receptors as follicle development proceeds
        
        antral follicle becomes increasingly sensitive to FSH as it develops and able to grow under a relatively steady state of FSH secretion
    - - FSH & estrogens initiate formation of LH receptors on granulosa
        
        FSH receptors begin to diminish
        
        increasing secretion of estrogen by antral follicle results in initation of preovulatory surge of gonadotropins
        
        follicle falls progressively under control of LH
- - - - Luteolysis = structural and functional degradation of the corpus luteum (CL), which occurs at the end of the luteal phase of both the estrous and menstrual cycles in the absence of pregnancy
        
        pattern of synthesis and release of PGF2α is essential to its luteolytic effect
        
        must be pulsatile (pulse occuring at about 6-hour intervals)
        
        minimum of 4-5 pulses within 24 hours is required
        
        if pulse intervals increase significantly before complete luceolysis (e.g. 12 hours, CL can recover and continue function, even if at lower level of steroind activity
      - follicular cycles are distinct
        
        follicle regression (atresia) begins before onset of growth of next follicle
        
        first dominant follicle regresses about midluteal phase
        
        a second dominant follicle begins growth immediately
        
        produce substances that inhibit development of other antral follicles to maintain status as dominant follicle
        
        inhibin:
        
        peptide hormone produced by granulosa
        
        inhibits secretion of FSH
      - duration required for development of antral follicle to the ovulations about 10 days in domestic animals
        
        two phases in final antral follicle development
        
        a relatively slow phase (4-5 days)
        
        accelerated growth phase (4-5 days) ; terminates in ovulation
        
        can be initiated during luteal phase
        
        occur under influence of relatively slow pulse rate of gonadotropin release that occurs during luteal phase
    - - preovulatory surge of LH I24 hours before ovulation) initiates critical changes in follicle that affect its endocrine organ status
        
        result in release of oocyte
        
        oocyte-inhibiting factor prevents oocyte from resuming meiosis
        
        luteinizing-inhibiting factor prevents granulosa from prematurely being changed into luteal tissue
        
        cause granulosa to produce substances, such as relaxin & prostaglandin F2α
        
        affect continuity of CT of thecal layers of follicle
      - estrogen is used by follicles to:
        
        stimulate growth & development of granulosa
        
        signal hypothalamus & anterior pituitary as to the readiness of follicles for ovulation
  - - - prepares uterus for initiation & maintenance of pregnancy
    - - breakdown of tissues surround granulosa (membrana propria)
        
        hemorrhage into cavity can occur from vessels in theca
      - the folds of tissue that protrude inward into cavity contain granulosa, theca cells & vascular system that support cell growth & differentiation
    - - luteotropin
        
        gonadotropic hormone secreted by the anterior pituitary
        
        in females it stimulates growth of the mammary glands and lactation after parturition.
    - - important so that animals reenter a potentially fertile state asap
      - lifespan of CL after ovulation must be of sufficient duration to allow a newly developing conceptus
        
        to synthesize & release factors that allow CL to be maintained
        
        must be relatively short so nonpregnant animal can return to a potentially fertile state
      - uterus is responsible for control of duration of the life span of CL
      - countercurrent transfer system
        
        involve movement of molecules across blood vascular system from higher concentrations in the venous effluent (utero-ovarian vein) to an area of lower concentration (ovarian artery)
      - general systemic transfer
        
        involves passage of molecules through general circulatory system
    - - presence of an embryo results in blockage of PGF2α synthesis and a continuance in luteal activity
      - in nonpregnant large domestic animals, inflammatory responses of endometrium caused by bacterial contamination can result in significant synthesis & release of PGF2α
        
        premature luteolysis
        
        shortening of the estrous cycle
        
        pathognomonic (specifically characteristic or indicative of a particular disease or condition) for uterine infection
  - - - ovulatory process is governed by internal mechanism
        
        estrogen from antral follicle initiates the ovulatory release of gonadotropins
      - copulation replaces estrogen as stimulus that induces ovulatory release of gonadotropins
        
        require exposure to elevated estrogen concentrations before they can respond to copulation by the release of gonadotropins
        
        have follicle growth pattern (in the absence of coitus)
        
        cohorts of follicle develop, are maintained in a mature state for a few days then regress
        
        can be distinctly separated
    - - life span of CL is only 1-2 days in the absence of copulation
      - copulation initiates the release of prolactin
        
        prolongation of luteal activity for up to 10 or 11 days in no pregnancy (pseudopregnancy)
- - - - limited periods of estrus (/ sexual receptivity)
        
        onset of proestrus defines the start of cycle
        
        proestrus begins within 48 hours after end of luteal phase
        
        except pig; proestrus not occuring for 5-6 days
      - behavioural events
        
        described by indicating whether animals are in estrus
        
        sexual behaviour
        
        equine Cl is relatively difficult to identify by palpation per rectum
        
        difficulty of determining ovarian status
      - gonadal events
        
        if differentiation of follicles & CL is possible
    - - sexually receptive during most of the reproductive cycle
        
        onset of menstruation designated as the start of cycle
        
        begins within 24 hours of the end of luteal phase
    - - time of ovulation differs for both phases
        
        luteal & follicular phases are separated
        ∴ ovulation occurring at a minimum of 12-13 days after onset of menstruation
        
        follicular phase overlaps luteal phase
        ∴ ovulation occurs relatively earlier in estrous cycle
        
        ovulation is easier to predict
        
        estrus is tightly coupled to preovulatory release of gonadotropins & ovulation
  - - - the onset of reproductive life (the time of first ovulation)
      - involves an increase in the synthesis & release of gonadotropin-releasing hormone (GnRH) from hypothalamus
        
        drives gonadotropin secretion in pulstile form and follicle growth
      - changes in photoperiod
        
        important to allow lambs enter puberty
        
        must expose to a long photoperiod during prepubertal development
        
        1-2 weeks (under experimental conditions)
        
        minimal interval from end of photoperiod to onset of puberty is 10 weeks
        
        secretion of gonadotropins cause significant follicle growth
        
        due to decreased sensitivity of hypothalamus to estrogens produced by growing follicles
        
        appearance of preovulatory-type surge of gonadotropins
        
        production of luteal structure
        
        1 more item...
        
        termination (occurs with the summer soltice) allows sensitivity of hypothalamus decrease in response of -ve estrogen feedback
        
        have suppressive effect on the timing of puberty in animals whose ovarian cyclicity is controlled by light
        
        influences timing of puberty onset in macaque monkeys
        
        depends on physiological maturity of individual
        
        nutrition & growth are likely determinants of the earlier time for onset of puberty
      - establishment of cyclical ovarian activity within a relatively short period
        
        starts later and ends earlier for ewe lambs
        
        earlier cessation (ending) of ovarian activity results from an earlier response to -ve estrogen feedback
        
        takes longer in pubertal primates
        
        first significant follicle growth usually ends in ovulatory failure
        
        refractoriness to long photoperiod experienced by animals during spring & summer
    - - occurs because of ovarian inadequacy, not gonadotropin secretion inadequacy
      - menopause = end to ovarian activity
        
        depletion of oocytes
        
        occurred throughout reproductive life of individual
        
        ovarian failure
        
        involves cyclical irregularity caused by failure of follicle development & ovulation
        
        ovarian follicle activity stops
        
        estrogen concentrations decrease
        
        in no -ve-feedback inhibition, gonadotropin concentrations increase dramatically
      - not recognized
        
        partly because some species have shortened lives for economic / humane reasons
      - no menopause
  - - - testosterone causes masculinization of sexual centers in hypothalamus (medial preoptic area)
    - - androgens essential for maintenance of libido in males
        
        castrated males are able to maintain libido despite lower concentration of androgens of adrenal origin
    - - onset of sexual receptivity is tightly coupled to onset of preovulatory gonadotropin surge
      - incresed GnRH secretory activity, increased sexual receptivity
  - - - controls the occurrence of reproductive cycles in a number of domestic animals
        
        annual period (continuous/cyclical ovarian activity)
        
        period of no ovarian activity (anestrus)
      - response is different among species
        
        positive
        
        occurs relatively soon after occurrence of summer or winter solstice
        
        negative
        
        requires longer duration for an effect
      - pineal gland
        
        produces melatonin in response of darkness
    - - suppressive effects on ovarian activity
        
        in sow:pig:: complete; do not come into estrus until after piglets are weaned
        
        :cat: can have ovarian activity suppressed throughout lactation, although occasionally come into estrus during latter part of lactation
        
        in lactating beef cow:cow:: suppressed; first estrus & ovulation not occurring before day 45 postpartum
        
        suckling process important to ovarian suppression
        
        dairy cows are not suppressed by lactation unless involves large nutritional deficit
        
        involve the importance of suckling with its related stimulation of prolactin synthesis
        
        inhibiting factor:
        
        dopamine
        
        GnRH-associated peptide
        
        sensory input from suckling suppresses production of inhibiting factors
        
        essential links in gonadotropins synthesis
        
        decreased output results in decreased ovarian activity through decreased gonadotropin synthesis & release
    - - chemical compounds that allow communication among animals through olfactory system
      - are called sex pheromones if sexual behaviour is affected
        
        have musk type of odor
      - most prominent tissue sources:
        
        sebaceous glands
        
        reproductive tract
        
        urinary tract
      - Whitten effect
        
        involve synchronization of estrus through sudden introduction of male
        
        stimulates synthesis & release of Gn
        
        use to manipulate estrous cycles
        
        use to influence onset of puberty in pigs
      - Bruce effect
        
        involve blockage of pregnancy development by introduction of a different (strange) male in proximity to a recently bred female
        
        block the release of prolactin
      - important for the attraction of male to female at time of sexual receptivity
      - dormitory effect:
        
        synchronization of menstrual cycles in roommate women
    - - affect in postpartum period
- - - - spermatozoa that concentrated and stored in epididymis gradually change from oxidative (aerobic) to glycolytic (anaerobic) metabolism
      - sperm are ejaculated into vagina
        
        movement through cervis is aided by estrogen-induced changes in cervical mucus
        
        form channels that facilitate movement
        
        emphasized in primate
        
        thinning of mucus occurs just before ovulation
        
        sperm
        
        transport is rapid
        
        not involved in fertilization
        
        sperm get damaged
        
        undergo changes within female genital tract that are prerequisite for fertilization
        
        capacitation
        
        removal of glycoproteins from sperm cell surface
        
        1 more item...
        
        deposition of sperm before ovulation is preferred timing for producing maximal fertility
      - environment of female genital system is generally inhospitable to survival of sperm
        
        wbc quickly attracted to uterine lumen because sperm cells are foreign
        
        special reservoirs evolved to aid in survival of sperm during transport
        
        include cervix & oviduct, latter involving areas at the uterotubule junction and within ampulla
        
        progressively filled
        
        when full, able to release a few sperm on a continuous basis
        
        so fertilization can occur shortly after arrival of oocytes within oviduct
      - prerequisite of oocyte is that is must undergo first meiotic division before fertilization
      - once fertilization occurred, embryo develops to morula or early blastocyst stage within oviduct before moving to uterus
        
        4-5 days
        
        affords uterus time to finish inflammatory response involving removal of spermatozoa
        
        allows endometrial glands time to secrete nutrients under influence of progesterone from developing CL
    - - domestic animals
        :cow2::goat::racehorse::pig2::sheep:
        
        luteal activity is controlled by uterus
        
        modification of uterine PGF2α synthesis & release is important for establishment of pregnancy
        
        embryo produces substances that modify uterine production PGF2α
        
        way for endometrium be informed regarding presence of embryo (pregnancy recognition)
        
        estrogen synthesis
        
        trophoblastin produced before day 14 of pregnancy in sheep and cattle
        
        has close structural relationship to molecule interferon
        
        movement of embryo in the tract
        
        in mare:
        
        moves throughout both horns before being fixed at day 16
        
        in pigs:
        
        minimal number of embryos need to be present (~4)
        
        to occupy a sufficiently large area of endometrium
        
        in litter-bearing animal:
        
        use transuterine migration
        
        maximize opportunity for fetal development
      - cats
        :cat:
        
        CL lasts for 35-40 days after ovulation regardless of the presence of pregnancy
        
        early modification of luteal activity is not essential for establishment of pregnancy
        
        implantation occurs at ~day 13
        
        allows fetoplancental unit to influence & extend luteal activity that is compatible with pregnancy maintenance
        
        relaxin
        
        placental hormone produced beginning at about day 20 of gestation
        
        synergizes with progesterone for the support of pregnancy
        
        causes ligaments & associated muslces surrounding pelvic canal to relax
        
        placental lactogen
        
        have both somatotropic & lactogenic effects on the basis of growth hormone-like and prolactin-like properties
      - primates
        
        rescue of CL at the onset of pregnancy involves the production of luteotropin called chorionic gonadotropin (CG)
        
        produced by trophoblastic cells (syncytiotrophoblasts) of the embryo
        
        must have intimate contact with interstitium of endometrium (implantation called interstitial)
        
        embryo penetrates endometrium at ~8-9 days after fertilization
        
        begins 24-48 hours after implantation, with immediate enhancement of luteal progesterone production
    - - produce progesterone
        
        in cattle, goats, pigs: progesterone is never produced enough to support pregnancy
        
        production of estrogen require interaction between fetus & placenta
  - - - progesterone maintain inactivity of myometrium and promote tightly contracted cervix
      - estrogen
        
        influence uterine muscle by stimulating production of contractile protein & formation of gap junctions
        
        induces oxytocin receptor formation in myometrium
      - fetal adrenal cortex
        
        maturation initiate parturition
        
        progressively sensitive to fetal adrenocorticotropic hormone
        
        fetal cortisol induce placental enzymes that direct steroid synthesis away from progesterone to estrogen
- - - - develops through proliferation of epithelium
        
        arise from primary mammary cord
        
        form alveoli
        
        release fat into milk
        
        involves constriction of plasma membrane around fat droplet
        
        most important energy source in milk
        
        begins at puberty
        
        remains relatively undeveloped until pregnant
    - - cistern in cow and doe
        
        able to synthesize & store larger amounts of milk
    - - closely apposed to each other (udder) in domestic animal
        
        suspensory system is developed
        
        avoid mammary gland system break down from the weight of milk
  - - - lactogenesis occurs without milk removal
    - - passive immunity
        
        immunoglobulins
        
        can be absorbed through gut in the first 24-36 hours
  - - - release together with manipulation of teat
      - inhibited by dopamine
      - stimulated by vasoactive intestinal peptide
      - short-lived surge occurs immediately after onset of milk removal
      - other major hormone: growth hormone
        
        promote additional milk production
    - - release through exocytosis
      - lactose is the main carbohydrate
      - main proteins are caseins
        
        can be removed as a curd
        
        through curding / coagulation
        
        albumins & globulins remain as whey
    - - myoepithelial cells surround alveoli & duct
        
        responsive to oxytocin
        
        cause contraction
        
        milk letdown
  - - - can stand a longer interval to first suckling
  - - - to prepare for next lactation
      - involution
        
        requires at least a month
    - - acts on postabsroptive nutrients
      - results in mobilization of body fats used in milk formation
  - - - neoplasia
        
        dog is most susceptible
        
        exposure of mammae to estrogen & progesterone
        
        mammary tumor
      - mastitis
        
        cause by injury to teat canal
        
        repeated stretching
        
        effect formation of CT within the udder
        
        limits area into which ducts & alveoli can proliferate
        
        reducing milk-producing potential
    - - transfer of RBC agglutinating antibodies to fetus through milk
        
        common in horse
        
        hemolytic crisis between 24-48 hours after delivery
        
        die unless given therapy
      - hypocalcemia
        
        increase movement of calcium into blood
        
        lose ability to maintain normal muscle activity
- - - - acting in concert producing spermatozoa
        
        involve neuroendocrine & genital system
      - 2 testes
        
        suspended within scrotum
        
        pivotal organ
        
        functions influenced by neuroendocrine system
        
        interstitial tissue compartment
        
        basal compartment
        
        contain spermatogenia
        
        adluminal compartment
        
        steroidgenesis
        
        production of androgens in Leydig cell
        
        spermatogenesis in seminiferous tubule
        
        temperature dependant
        
        lengthy orchestrated process
        
        diploid stem cells divide by mitosis
        
        to maintain their own number
        
        cyclically produce progeny that undergo meiotic division & differentiation into haploid spermatids
        
        1 more item...
        
        3 major events
        
        spermatocytogenesis
        
        mitotic division of type A spermatogonia
        
        1 more item...
        
        type A become type B
        
        meiosis
        
        only during processes of oogenesis & spermatogenesis
        
        spermiogenesis
        
        newly formed spermatids continue to differentiate without dividing forming mature spermatids
        
        formation of acrosome
        
        condensation & elongation of nucleus
        
        formation of flagellum
        
        extensive shedding of cytoplasm
        
        require high local concentrations of androgens within testis
      - penis
        
        copulatory organ
        
        erection
        
        surrounded by tunica albugenia
    - - release of spermatozoa & accessory gland fluid into pelvic urethra
        
        sympathetically mediated thoracolumbar reflex contraction
    - - forceful expulsion of semen from urethra
        
        parasympathetically mediated sacral reflex
  - - - hypothalamus
        
        synthesise & secrete GnRH
        
        in pulsatile manner
        
        acts directly on gonadotropic cells in pituitary
        
        synthesise & secrete FSH & LH
        
        LH binds to membrane receptors on Leydig cell
        
        stimulates them to convert cholesterol to testosterone
  - - - low level of gonadotropin secretion rapidly decreases when sex steroids are admninstered
  - - - undesirable side effects
        
        affects pituitary function
        
        leads to long-lasting impairment of testicular endocrine function
        
        may lead to incomplete development of hypothalamic-pituitary-gonadal axis
        
        reduce seminal quality, daily sperm output & production, and testicular size
        
        result from negative-feedback mechanism on Gn release from pituitary