Reproductive System

• Gonadotropin releasing hormone (GnRH)


  Hormone released by the hypothalamus that stimulates the release of follicle stimulating hormone (FSH) and luteinizing hormone (LH) from the anterior pituitary

  
• Follicle stimulating hormone (FSH)


  Hormone released by the anterior pituitary that stimulates the growth and development of ovarian follicles in females and spermatogenesis in males

  
• Luteinizing hormone (LH)


  Hormone released by the anterior pituitary that triggers ovulation in females and stimulates testosterone production by Leydig cells in males

  
• Factors affecting puberty in males and females


  • Nutrition
  • Growth
  • Age
• Hypothalamic-pituitary-gonadal axis in males


  1. Hypothalamus secretes GnRH

  2. Anterior pituitary secretes FSH and LH

  3. FSH and LH stimulate male sex steroid production and spermatogenesis

  
• Characteristics of GnRH secretion in males


  • Secreted in tonic (continuous), low amplitude pulses
  • Secreted from arcuate nucleus (ARC) and preoptic area (POA) of hypothalamus

  
• Characteristics of LH and FSH secretion in males


  • LH and FSH released by anterior pituitary gonadotrope cells into general circulation
  • After puberty, pulses of GnRH, LH and FSH occur 4 to 8 times a day
  • Blood concentrations of FSH are lower but last longer because of a longer hormone half-life

  
• Two-cell, two-gonadotropin mechanism in males


  1. LH stimulates Leydig cells to make testosterone (T)

  2. FSH stimulates Sertoli cells to convert testosterone to dihydrotestosterone (DHT) and estradiol (E2)

  3. T and DHT are essential for spermatogenesis
• Effects of male androgens


  • Formation of male internal and external genitalia before birth
  • Growth, maturation and maintenance of the male reproductive system at puberty and thereafter
  • Promotion of libido or male sex drive at puberty
  • Male pattern of hair, antler or feather growth
  • Deepening of the voice
  • Muscle growth
  • Bone growth at puberty
  • Aggressive behavior and seeking of the female in some animals
• When too much T, DHT and E2 is produced


  Negative feedback reduces GnRH and LH/FSH secretion
• Doping with exogenous T


  Causes negative feedback on LH and FSH resulting in reduced testicular size
• Characteristics of GnRH secretion in seasonal breeders


  • GnRH secretion may be continuous or seasonal depending on species
  • Helps align with reproductive cycle in female and time birth with availability of food
• Corpus cavernosum


  Penile tissue that fills with blood from deep artery during erection, causing the tissue to swell and apply pressure
• Corpus spongiosum


  Penile tissue around the urethra that fills with blood but does not apply pressure, cushioning the urethra so it remains open during erection
• Erection and ejaculation reflex


  1. Parasympathetic dilation of penis arterioles causes tissue to fill with blood and veins to constrict

  2. Sympathetic contraction of smooth muscles in prostate, reproductive ducts and seminal vesicles helps deliver semen into urethra

  3. Contraction of skeletal muscles at base of penis increases pressure and forcibly expels semen
• Female mammalian reproductive functions


  • Development of oocytes (eggs) within the ovary (Oogenesis)
  • Release of a mature oocyte for possible fertilization (Ovulation)
  • Oocyte and sperm transport for fertilization
  • Development of young inside female (Pregnancy or Gestation)
  • Giving birth or laying eggs (Parturition)
  • Nourishing offspring with colostrum and milk (Lactation)
• Structures of the female reproductive tract


  • Ovaries
  • Oviduct or fallopian tube
  • Uterus
  • Cervix
  • Vagina
• Vulva


  External female genitalia including labia majora and minora, clitoris and vestibule
• Clitoris


  Tissue similar to the glans penis in males, involved in arousal in some species
• Ovarian structures


  • Ovarian follicles: Contain an oocyte and supportive cells that produce estradiol (E2)
  • Corpus luteum (CL): Structure that develops after a follicle has ovulated, producing progesterone (P4)

  
• Follicle growth and ovulation


  1. Prior to birth, all oocytes form and become arrested in meiosis I as primary oocytes

  2. Follicles grow and mature prior to puberty, but ovulation does not occur until puberty

  3. Antral follicles are the largest and most developed follicles, containing follicular fluid