General reproductive physiology, lectures 1-3

Seminiferous tubules

*Spermatogenesis: -development of male gamete into spermatozoa -proceeds in an orderly manner from basal (interfacing with the interstitium) to apical (interfacing with tubule lumen) compartment *Sertoli cells: processes surround and separate developing germ cells ('nurse'/nourish developing germ cells) *Spermatogenesis is a continuous process, cycles begin ~ every 16 days, take ~ 64 days

Hypothalamo-Pituitary-Gonadal Axis

-GnRH stimulates LH & FSH *Estradiol in women and testosterone in men exert negative feedback @ 2 levels: 1)Hypothalamus: has an inhibitory effect upon GnRH secretion 2)Pituitary: has an inhibitory effect upon LH secretion -Separate gonadal protein products also regulate secretion by the hypothalamus and pituitary gland: *Inhibin: inhibitory effect on GnRH and FSH *Follistatin: inhibitory effect on FSH *Activate (Activin) : stimulatory effect on FSH

Male reproductive subdivisions

1 Spermatogenesis 2 Performance of the male sexual act 3 Hormonal regulation of male functions 4 Hormonal regulation of accessory cellular organs and cellular metabolism and growth

peritubular myoid cells

1) smooth muscle cells that are responsible for the contractile nature of the seminiferous tubule (move the spermatozoa and fluid to the rete testes) 2) In stage II and IV of spermatogenesis, GDNF is secreted by these cells upon testosterone binding the androgen receptor (in contrast to GDNF (Glial cell line-derived neurotrophic factor) secretion by the Sertoli cells during stages IX and I). Following this, GDNF binds GFRA1 on spermatogonial stem cells stimulating sperm stem cell renewal

Importance of GnRH pulsatility

1)LH secretion is entrained to GnRH secretion (i.e. dependent on) • Loss of pulsatility leads to receptor down regulation 2) Cyclic variation in pulse pattern (female): • Low frequency / high amplitude pulses -> FSH • High frequency / low amplitude pulses -> LH **Allows differential regulation of FSH, LH (infertility treatment - Ex GnRH administered that mimic timing, amp and F of pulses)

GnRH (gonadotropin-releasing hormone)

A deca-peptide from Arcuate Nucleus of hypothalamus (pre-optic area) Secretion = pulsatile: essential for function (maintains receptor sensitivity) Secretion regulated by: - Puberty - Gonadal feedback - Prolonged stress - CRH inhibits GnRH - Pheromones (+ / -) - Prolactin: ↓ Gonadotrophin levels during lactation

male climacteric (andropause)

A period of declining reproductive function that may be first seen in early 50s Although sperm counts decline, men can still father children throughout old age

seminal fluid

Alkaline fluid containing nutrients, motility factors, clotting factors and hormones: Prostate (~ 30%) adds citrate, Ca, Zn, phosphate ion Seminal vesicles (~60%): add fructose, PG's, citric acid and fibrinogen

Spermatogenesis : FSH & Testosterone

At puberty spermatogonia proliferate rapidly by mitosis inside testis. Spermatagonia become modified and enlarged to form primary spermatocytes. Primary spermatocytes divide by meiosis, each giving rise to two secondary spermatocytes (46 chromosomes). Each secondary spermatocyte undergoes a second meiotic division. Each giving rise to two spermatids (23 chromosomes each). Differentiation and elongation: Sperm.

Capacitation of sperm

Ejaculated sperm are incapable of fertilization, and must be 'capacitated in the female genial tract x 4-6 hrs Capacitation: is a cAMP-mediated process, involving: Ca2+ influx, leading to increased 'whiplash' motility - may enhance penetration of the ovum Removal of surface proteins and cholesterol → weakening of sperm membrane and readiness for acrosomal rxn (ability to fertilize an ovum)

Regulation of GnRH

Endorphin and prolactin (anterior pituitary) inhibit GnRH release. -Prolactin inhibition affords post-partum contraceptive effect Overproduction of prolactin via pituitary tumor can cause amenorrhea - shuts off GnRH -Treated with bromocryptine (dopamine agonist) -Surgical removal of pituitary tumour

vas deferens (ductus deferens)

Excretory duct of testis that joins epididymis with ejaculatory duct

Feedback Regulation of LH

Gonadal products inhibit GnRH secretion by the hypothalamus (indirect inhibition of LH). Gonadal steroids: ↓↓ LH secretion by the pituitary Paradoxically: (Positive feedback regulation by Estradiol) Rising Oe in the late follicular phase of the ovarian cycle has a 'priming effect' on LH synthesis: Sustained high Oe → surge in LH secretion, inducing ovulation oral contraceptive

Spermatozoa

In the epididymis, sperm mature and become motile becoming this In the vas deferens, sperm stored @ 34-35C for up to several months

Hormonal Regulation of Spermatogenesis

In utero, early testosterone surge induces differentiation of germ cells into spermatogonia Process arrested until puberty by inhibin/MIF FSH - Spermiogenesis, sperm maturation LH: acts on Leydig cells → testosterone...essential for the latter stages of spermatogenesis

Seminal Vesicle

Mucoid fluid (containing nutrients and prostaglandins which aid fertilization)

arcuate nucleus of hypothalamus

Secrete neuropeptide Y (NPY), gonadotropin-releasing hormone (GnRH), agouti-related peptide (AgRP), cocaine- and amphetamine-regulated transcript (CART), kisspeptin, dopamine, substance P, growth hormone-releasing hormone (GHRH), neurokinin B (NKB), β-endorphin, melanocyte-stimulating hormone (MSH), and somatostatin. Proopiomelanocortin (POMC) is a precursor polypeptide that is cleaved into MSH, ACTH, and β-endorphin and expressed in the arcuate nucleus

Epididymis

Sperm empty from Testis into here -coiled tube, 6 meters long (small quantity stored here)

ejaculatory duct

Sperm, prostatic fluid and mucoid fluid:

Leydig Cell Function

Steroid secreting cells: Testosterone Synthesis - this has both local and distant targets Regulated by LH Have many paracrine interactions e.g. Inhibin ↑ and oestradiol ↓ testosterone synthesis

Sertoli Cell Function

Structural support Guide sperm cells toward the lumen as they advance toward the stages of spermatogenesis Sexual differentiation in utero (via Mullerian Duct Inhibition Factor (MIF)/Inhibin: regression of the embryonic mullerian duct that is programmed to give rise to the female reproductive tract) Blood-testis barrier (formed by sertoli-sertoli cell tight junctions): Controls flow of nutrients and hormones to developing germ cell, maintains an immunologically safe microenvironment for developing sperm Hormone synthesis: Oestradiol. Inhibins: FSH stimulates inhibin production by sertoli cells which negatively feeds back to the gonadotrophs (pituitary gland) to inhibit FSH production. Spermatogenesis: express the androgen receptor and FSH receptor (NOT GERM CELLS). Hormonal stimulation of these cells indirectly promotes spermatogenesis. Androgen-binging protein (ABP) synthesis - binds to testosterone regulating it's availability to germ cells (maintains high testosterone levels in adluminal space)

Reproductive hormones and growth

Testosterone and Estradiol stmulate Growth Hormone (GH)

DHT (dihydrotestosterone)

carried out peripherally (urogenital tract, genital skin, hair follicles and liver) by 5alpha-reductase. essential for masculinization of external genitalia in utero, growth during puberty, prostate gland activity etc.

Prostate Gland

collects sperm from vas deferens and adds a chemical fluid to the semen

Testis

composed of up to 900 coiled Seminiferous Tubules (spermatogenesis)

LH/FSH

for growth, development, pubertal maturation, reproduction, sex steroid hormone secretion Regulation of LH/FSH secretion is complex, regulation includes: pulsatile, periodic, diurnal, cyclical and stage of life elements Regulation differs between the sexes LH pulses are biphasic FSH pulses are uniphasic Diurnal- LH/FSH more pronounced during puberty Cyclic in females - ovarian cycle with LH surge at time of ovulation Non-cyclic in males - but constant pulses of LH cause pulses of testosterone to be produced

Two functions of testis

gametogenesis and hormonogenesis Spermatogenesis in seminiferous tubules and androgen production in interstitial leydig cells

Pubescent Regulation of GnRH Neurons

leptin permissive for GnRH release but not trigger

Maturation of HP-G axis leads to:

↑ GnRH pulses ↑ Gonadotropin pulses, LH > FSH, with nocturnal LH peak ↑↑ gonadal hormones

menopause

the time of natural cessation of menstruation; also refers to the biological changes a woman experiences as her ability to reproduce declines decreased circulating estrogen and progesteron Loss of negative feedback so FSH becomes larger than LH

Gonadotropins (Gns)

• LH, FSH, TSH and hCG • Glycoprotein hormones • alpha and β subunits • Each subunit encoded by different gene • alpha subunit is identical for all hormones ***β subunit are unique and provide biological specificity