Male Reproductive Physiology

Jeremy Johnson D.O.

November 26, 2008

Male Reproductive Physiology Male Reproductive Axis Testis Epididymis Spermatozoa Vas Deferens

Male Reproductive Axis 3 tiers of organization: Hypothalamus, Pituitary gland, Testis Hypothalamus: GnRH (gonadotropin-releasing hormone) Pituitary gland: LH (luteinizing hormone), FSH (follicle-stimulating

hormone) LH: stimulates Testosterone production by Leydig cells in

interstitium FSH: supports spermatogenesis by stimulating Sertoli cells in the

seminiferous epithelium

Inhibin: secreted by Sertoli cells, suppresses FSH secretion by gonadotropes; ? Use of Inhibin B as marker for impaired testicular function

Activin: secreted by Sertoli cells, stimulate transcription of FSH B subunit

Male Reproductive Axis Hypothalamus: GnRH - 3 types of rhythmicity

Seasonal (in months) – peaks in Spring Circadian (in hours) – highest Testosterone levels in AM Pulsatile (in minutes) – peaks occur every 90 - 120 minutes

Melatonin: modifies seasonal & circadian rhythms from inputs from pineal gland (seasonal) & neural connections (circadian) from suprachiasmatic nucleus (mammalian 24-hr clock)

Precursors of GnRH neurons migrate to hypothalamus from olfactory placode during development

Kallman’s Syndrome: congenital hypogonadotropic hypogonadism, failure of normal migration of GnRH neurons -> hypothalamus unable to secrete GnRH; anosmia/other midline defects + hypogonadism is diagnostic

Male Reproductive Axis Pituitary gland Anterior Pituitary

(Adenohypophysis) – regulated by bloodborne factors Gonadotropes (secrete LH &

FSH)* Corticotropes (secrete ACTH)? Lactotropes (secrete PRL)* Somatotropes (secrete GH)* Thyrotropes (secrete TSH)

*significant effects on male reproductive function

?unknown effects on male reproductive function

Posterior Pituitary (Neurohypophysis) – regulated by neural stimuli Oxytocin Vasopressin (ADH)

Male Reproductive Axis

Steroid Feedback

Testosterone exerts negative feedback suppression on the release of GnRH at the level of hypothalamic neurons & pituitary; T is not the only active steroid in the target cells

Testosterone –(aromatase) Estradiol Testosterone –(5 alpha reductase) DHT Testosterone acts primarily to feedback at the hypothalamus; Estrogens

primarily feedback to the pituitary gland In males:

LH secretion is regulated primarily by Testosterone FSH secretion is regulated primarily by Estradiol

Male Reproductive Axis Development of male reproductive axis

7 weeks gestation: 1st identifiable step differentiating ovarian from testicular pathways is movement of primordial germ cells into medullary cords

SRY (Sex-Determining Region on Y c’some) controls early testis differentiation SRY gene product (a TF) acts w/ other TFs (WT-1, SOX-9, DAX-1) to initiate

male sexual differentiation 10% of 46 XX males have no identifiable SRY gene

Sertoli Cells: secrete MIS (Mullerian Inhibiting Substance aka: Anti-Mullerian Hormone); causes female reproductive structures to regress

Leydig Cells: secrete Testosterone which induces differentiation of the Wolffian duct system (epididymis, vas deferens, sex accessory glands)

Male Reproductive Axis Endocrinology of Testis Leydig cell differentiation

1st wave - 7 weeks gestation: stimulated by hCG from placenta; androgens appear in circulation

2nd wave - 2-3 months after birth: stimulated by gonadotropin production from neonate’s pituitary; briefly elevates Testosterone

Androgens produced during first 2-6 months of life are thought to hormonally imprint hypothalamus, liver, prostate, phallus & scrotum

Leydig cells of infants then regress & testes are dormant until puberty

Puberty Hypothalamus generates pulses of GnRH around 12th year of life Onset of GnRH pulses typically occurs at night, due in part to gradual

decrease in nocturnal melatonin secretion from pineal gland Also influenced by nutritional status of body and growth rate

GH & IGF-1 stimulate reproductive function Leptin determines size of fat stores in body - ? Role in puberty

Male Reproductive Axis Aging of Hypothalamic/Pituitary Axis

Testosterone: levels decline at > 50 years of age

LH: basal levels increase in older men; LH pulsatility is blunted

Leydig cells: steroidogenic capacity decreases

Spermatogenesis: lower fecundity at > 40 years, 50% lower probability of achieving pregnancy w/in 1 yr compared to men < 25 years of age

Testis Gross structure & vascularization

Volume: 15 - 25 ml Longitudinal length: 4.5 - 5.1 cm 3 layers of capsule: outer visceral layer of tunica vaginalis, tunica albuginea, innermost layer

of tunica vasculosa Tunica albuginea contains smooth muscle; smooth mm. provides contractile capability to

testis as well as affects blood flow into testis Testicular arteries penetrate tunica albuginea & travel inferiorly on post. surface w/ branches

passing anteriorly; also major branches present on inferior pole (potential for injury during orchiopexy/biopsy); medial and lateral midsection of testis have fewer vessels

Capsule separated by septa; between septa are seminiferous tubules & interstitial tissue Seminiferous tubules: developing germinal elements & supporting cells (Sertoli cells) Interstitial tissue: Leydig cells, mast cells, macrophages, nerves, blood/lymph vessels (20 –

30 % of total testicular volume

Testis

Innervation: No somatic innervation Autonomic innervation from intermesenteric nn. & renal plexus; travel along testicular artery

Arterial supply: Internal spermatic testicular Deferential vasal External spermatic (cremasteric)

Countercurrent exchange of heat between pampiniform plexus & arteries Intratesticular temps 3 – 4 degrees C lower than rectal temps

Variability in # of arteries entering testis exists 1 artery: 56%; 2 arteries: 31%; 3 or more arteries: 31%

Testis Cryoarchitecture & Function

Interstitium: blood/lymph vessels, fibroblastic supporting cells, macrophages, mast cells & Leydig cells

Leydig cells: responsible for steroid production; Testosterone is synthesized from cholesterol & is principle steroid produced in human testis

3 sources for cholesterol: external (bloodborne), de novo (acetate), stored cholesterol esters

LH: regulates Testosterone production; generates cAMP & initiates transport of cholesterol into mitochondria

Testosterone peaks: 12-18 wks gestation 2 months of age 3rd decade of life (max concentration)

Testis Seminiferous tubules – germinal elements & supporting cells Germinal elements: spermatozoa Supporting cells: sustentacular cells (basement membrane) & Sertoli cells Sertoli cell functions:

Creates specialized microenvironment of adluminal compartment of seminiferous epithelium

Supports germ cells through gap junctions between Sertoli & germ cells Facilitates migration of differentiating germ cells into seminiferous tubule

Testis

Blood-Testes Barrier 3 levels:

1.) Tight junctions between Sertoli cells & spermatogonia from other germ cells

2.) Endothelial cells in capillaries 3.) Peritubular myoid cells

Spermatogonia & young spermatocytes are outside blood-testes barrier in basal compartment; mature spermatocytes & spermatids are above barrier in adluminal compartment

Blood-Testes Barrier functionally develops at onset of spermatogenesis

Testis Germinal Epithelium – 123 x 106 spermatozoa/day (21 – 374 x 106)

Phases of spermatogenesis

Proliferative: spermatogonia divide to replace their numbers; or produce daughter cells committed to becoming spermatocytes Type A spermatogonia; Ad (dark) – stem cell renewal; Ap (pale) – produce daughter cells

Meiotic: reduction division resulting in haploid spermatids Type B spermatogonia

Spermiogenic: spermatids undergo changes to form mature spermatozoa Round Sa spermatid

Entire process requires 64 days (Ap spermatogonium spermatozoon)

Testis

Hormonal Regulation of Spermatogenesis Intratesticular Testosterone levels are 100 x greater than serum levels Testosterone will initiate & qualitatively maintain spermatogenesis in humans

Genetic Basis of Spermatogenesis AZF (azoospermia factor) region on long arm of Y c’some implicated in deletions

resulting in azoospermia Paternal centromere: appears to organize embryonic mitotic activity; viable

embryo cannot be produced w/out this contribution

Epididymis Gross structures: Tubule: 3-4 meters in length 3 regions: Caput, Corpus, Cauda Contractile tissue (myofilaments) Innervation: intermediate spermatic

nerves (hypogastric plexus); inferior spermatic nerves (pelvic plexus); sympathetic fibers increase in # proximally

Vascularization: Testicular artery (Caput & Corpus), Deferential artery (Cauda); collateral circulation exists from Deferential & Cremasteric aa

Histology: Ciliated cells Principal cells: absorptive/secretive

processes Basal cells

Epididymis Function

Sperm transport: 2 -12 days; transport time influenced by daily testicular sperm production; 2 days in men w/ high sperm counts vs 6 days in men w/ low sperm counts; recent emission reduces transit time thru Cauda by 68%; principal mechanism for moving spermatozoa thru epididymis is probably due to spontaneous rhythmic contractions of cells surrounding epididymal tract

Sperm storage: 50% of total # of epididymal spermatozoa are stored in Cauda (capable of undergoing motility & have capacity to fertilize); fate of unejaculated spermatozoa is unknown

Epididymis Function Sperm motility maturation: increase motility observed during transit

Efferent ducts: 0% Caput: 3% Proximal Corpus: 12% Distal Corpus: 30% Cauda: 60%

Epididymis Function Sperm fertility maturation: Testicular spermatozoa are incapable

of fertilizing eggs (unless injected) Maturation is achieved at level of distal

Corpus or proximal Cauda

Biochemical changes: Increased capacity for glycolysis Changes in intracellular pH & calcium

content Modification of adenylate cyclase

activity Alterations in cellular phospholipid &

phospholipid-like fatty acid content

Spermatozoa Mature spermatozoa stored in Cauda epididymis & Ductus deferens 60 micrometers in length Head: measures 4.5 micrometers in length & 3 micrometers in width Oval sperm head: consists mostly of a nucleus which contains highly compacted

chromatin material & an acrosome which contains enzymes necessary for penetration of outer membrane of female egg

Middle piece: helically arranged mitochondria surrounding a set of fibers & characteristic 9 + 2 microtubular structure of axoneme

Mitochondria contains enzymes required for oxidative metabolism & production of ATP (primary energy source for cell)

Axoneme contains enzymes & structural proteins necessary for transduction of ATP into mechanical movement resulting in motility

Outer dense fibers are rich in disulfide bonds & thought to provide sperm tail; fibers surround middle & principal piece, terminate at end piece

Plasma membrane envelops spermatozoan (except at end piece); regulates transmembrane movement of ions & other moleculs; at head, specialized proteins in membrane participate in sperm-egg interactions during early stages of fertilization

Spermatozoa Effects of Sex Accessory Gland Secretions on Spermatozoal Function

Human ejaculate maintains an ability to coagulate initially & is liquefied by proteases from prostate (specifically PSA)

Unknown as to whether or not coagulum provides to maintain spermatozoa w/in vagina Spermatozoa must traverse cervical mucus into uterus & finally into oviduct where

fertilization occurs Uterine transport in woman takes 5 – 68 minutes Spermatozoa must undergo capacitation prior to oocyte fertilization ; capacitation

occurs at different rates for each spermatozoan Many changes occur during capacitation; most notably, the acrosome reaction &

development of hyperactivated motility occurs Unknown as to whether or not prostatic or seminal vesical secretions contribute to

capacitation Ejaculate:

Fructose – produced in seminal vesicle, provides energy for spermatozoa Albumin – supports & stimulates spermatozoa Antioxidants – enzymes (Glutathione peroxidase, Superoxide dismutase, Catalase), molecules

(Taurine, Hypotaurine, Tyrosine) all provide anti-oxidant protection for sperm; oxidative effects on sperm include lower sperm motility & increased damage to sperm DNA

Ductus Vas Deferens Derived from Mesonephric (Wolffian) duct 30 – 35 cm in length Begins at Cauda epididymis & terminates at ejaculatory duct near prostate gland Outer diameter: 2 -3 mm; Lumen diameter: 300 – 500 micrometers 5 portions

Sheathless epididymal portion w/in tunica vaginalis Scrotal portion Inguinal division Retroperitoneal (pelvic) portion ampulla

Outer adventitia (blood vessels, small nerves) Muscular coat (middle circular, inner/outer longitudinal) Mucosal inner layer (epithelial lining)

Ductus Vas Deferens Vascularization & Innervation

Blood supply: Deferential artery via Inferior vesicle artery

Nervous supply: sympathetic & parasympathetic inputs

Parasympathetic (cholinergic) is of minor importance in motor activity of vas deferens

Sympathetic (adrenergic) nerves provide rich supply to vas deferens; sympathetics derived from Hypogastric nerves via Presacral nerve; vas deferens also receive a special type of short adrenergic nerve which are present in all 3 layers of the muscle layers of vas deferens (greatest concentration of these nerves are present in outer longitudinal layer)

Ductus Vas Deferens Cryoarchitecture of Ductus Deferens

Lined by pseudostratified epithelium Height of epithelium decreases along the length of ductus Longitudinal folds of epithelium are simple in proximal region & more

complex at distal segments Muscle thickness gradually decreases along the length of the ductus Pseudostratified epithelium is composed of basal cells & 3 types of

columnar cells (Principal cells, Pencil cells & Mitochondrian-rich cells) Columnar cells all show steriocilia & irregular convoluted nuclei Principal cells more prominent in proximal portion of ductus Pencil & Mitochondrian-rich cells more prominent in distal portion of ductus

Ductus Vas Deferens Spermatozoal transport

Ductus exhibits spontaneous motility, has capacity to respond when stretched & contents of ductus can be propelled into urethra by strong peristaltic contractions elicited by stimulation of hypogastric nerve or adrenergic neurotransmitters

Immediately before emission, rapid & effective transport of spermatozoa from distal epididymis & proximal vas deferens occurs (apparently related to sympathetic stimulation)

This efficient transport of spermatozoa has revealed that the ductus deferens has the greatest muscle-lumen ratio (10:1) of any hollow viscus in the human body

Epididymal spermatozoal reserves: 182 million (26% caput, 23% corpus, 52% cauda); transit times in days (0.7 caput, 0.7 corpus, 1.8 cauda)

Ductus spermatozoal reserves: 130 million; storage site for spermatozoa

Ductus Vas Deferens Spermatozoal transport

“During the sexual rest, epididymal contents were transported distally through the vas deferens into the urethra in small amounts and at irregular intervals”

Urethral disposal is a mechanism for ridding the epididymis of excess spermatozoa

After sexual stimulation and/or ejaculation: contents of ductus can be propelled towards proximal ductus & cauda epididymis because distal portion had increased contractility compared to proximal portion of ductus

Refluxing was noted to reverse w/ sexual rest

Ductus Vas Deferens Absorption & Secretion Suggested that ductus deferens may have absorptive & secretory functions Principal cells have characteristics typical of cells that are capable of

synthesizing & secreting glycoproteins Stereocilia, apical blebbing, primary & secondary lysosomes of Principal

cells are characteristic of cells involved in absorptive functions Rat models have shown that terminal region of ductus possesses the ability

to phagocytose & absorb spermatozoa; unknown if significant portion of human ductus deferens possesses sufficient spermiophagy

Structure & function of ductus deferens probably depends on androgen stimulation Human ductus deferens converts Testosterone to DHT Castration causes atrophy of ductus deferens; Testosterone treatment causes

restoration of ductus deferens Castration and/or Testosterone treatment alters adrenergic contractions of ductus