animal reproduction

• Reproduction


  Generation of new individuals from existing individuals
• How reproduction occurs in animals


  • Asexual versus sexual reproduction
  • Internal versus external fertilization
  • Specialized reproductive organs
• Testes


  Produce sperm

  
• Ovaries


  Produce egg

  
• Endocrine regulation


  1. P-G axis
• Both asexual and sexual reproduction occur in animals
• Sexual reproduction


  Creation of zygote by fusion of male gamete (sperm) and female gamete (egg)
• Asexual reproduction


  Creation of offspring without the fusion of egg and sperm
• Asexual reproduction methods


  • Fission and Fragmentation
  • Budding
  • Parthenogenesis
• Unique reproductive adaptations


  • Eusocial insects (bees, wasps, termites)
  • Hermaphroditism (sequential, simultaneous)
  • Parthenogenesis with 'mating'
• Reproductive cycles


  Related to changing seasons, controlled by hormones and environmental cues

  
• Internal fertilization


  Eggs are fertilized by sperm in the female reproductive tract, requires behavioral interactions and compatible copulatory organs

  
• External fertilization


  Eggs are fertilized by sperm in the external environment, requires aqueous environment and precise timing

  
• Gonads


  Organs that produce gametes
• Cloaca


  Common opening between the external environment and the digestive, excretory, and reproductive systems in nonmammalian vertebrates
• Mammals have 2 or 3 openings for the digestive, excretory and reproductive tracts
• Human female reproductive anatomy


  • Egg cell production, fertilization, and embryo development
  • Egg cells are produced in the ovaries
  • Fertilization occurs in the oviducts
  • Embryo develops in the uterus

  
• Human male reproductive anatomy


  • Sperm and semen production
  • Sperm are produced in seminiferous tubules in the testes
  • Sperm are propelled via the muscular vas deferens and combine with additional fluid from the seminal vesicles, prostate, and bulbourethral glands to form semen during ejaculation

  
• Gametogenesis


  Production of gametes
• Spermatogenesis


  Production of small, motile sperm, occurs in seminiferous tubules of testes

  
• Oogenesis


  Production of (relatively) large eggs, occurs in ovaries

  
• Differences between spermatogenesis and oogenesis


  • After meiosis, four cells develop into sperm while only one becomes an egg in oogenesis (polar bodies are discarded)
  • Spermatogenesis occurs throughout adolescence and adulthood; all primary oocytes are present at birth in females
  • Sperm are produced continuously without the prolonged interruptions in oogenesis

  
• Oogenesis


  • Primary oocytes (2n) are arrested in prophase I very early during development
  • Every month, a single primary oocyte is released from an egg follicle (ovulation)
  • Immediately prior to ovulation, meiosis I completes and the secondary oocyte is arrested at metaphase II
  • After fertilization, meiosis II completes

  
• Spermatogenesis


  • Testes
  • Seminiferous tubules
  • Primary spermatocyte (2n)
  • Secondary spermatocyte (n)
  • Spermatids and sperm cells (n)

  
• H-P-G axis

  Hypothalamus secretes gonadotropin-releasing hormone (GnRH), anterior pituitary secretes follicle stimulating hormone (FSH) and luteinizing hormone (LH), receptors for FSH and LH in the gonads (ovaries or testes) trigger the release of sex hormones (androgens and estrogens) which cause gametogenesis and mediate reproductive behaviors

  
• Female reproductive cycle

  • Hypothalamus
  • Anterior pituitary
  • LH and FSH
  • Follicle growth
  • Ovulation triggered by surge of estradiol and LH
  • Corpus luteum
  • Estradiol and progesterone
  • Thickening of the endometrium

  
• Menstrual cycles


  Endometrium is shed from the uterus in a bleeding called menstruation, behavioral sexual receptivity is not limited to a time frame
• Estrous cycles


  Endometrium is reabsorbed by the uterus, behavioral sexual receptivity is limited to a "heat" period, the length and frequency of estrus cycles vary from species to species
• Male reproductive cycle

  • Hypothalamus
  • Anterior pituitary
  • LH and FSH
  • Testosterone and spermatogenesis
• Fertilization

  Occurs in the oviduct and meiosis II of the egg cell completes, zygote undergoes cleavage and implants in uterus

  
• Pregnancy, ~ 38 weeks in humans

  • First trimester: Placenta forms and allows exchange of nutrients, gasses, and waste between mother and embryo, Organogenesis: development of body organs; occurs at 8 weeks, major structures present; called a fetus
  • Second trimester: The fetus grows and becomes active (mother can feel movement), Uterus grows to accommodate body growth
  • Third trimester: Results in labor, the process by which childbirth occurs, Mediated by hormones