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HBIO4 > Reproduction
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Reproductive System

Male

  • Testes in the scrotal sac (→extension of abdominal cavity)
    • Constant temp (body temp - 2°C) is maintained by
      • Heat exchange in arteries and veins
      • Semi-external position
      • Muscles in scrotal sac → move testes up/down against warmer abdomen
    • If testes don't descent during development → infertility
  • Tubular system
    • 1) Seminiferous tubules
    • 2) Vasa efferentia
    • 3) Epididymis
    • 4) Vas deferens
    • 5) Urethra → carries fluids from the urinary and reproductive system
  • Function
    • Seminiferous tubules are found within testes and produce sperm (gametogenesis)
      • Heads of sperm are embedded in Sertoil cells
        • Prevent destruction of sperm by immune system → sperm and body cells are genetically different!
        • Provide nutrients
      • Tails are projected into fluid-filled lumen
    • Interstitial cells are found between seminiferous tubules and produce testosterone
    • Epididymis is where sperm acquire the ability to swim and are stored
    • Prostate secretes alkaline fluid → neutralizes acidity of vaginal tract (which would kill sperm)
    • Seminal vesicles secrete fructose → main energy source for sperm
  • Semen
    • 5% sperm
    • 70% secretions from seminal vesicles
    • 25% secretions from prostate gland
  • Sperm that are not discharged are degenerated, absorbed, lost via urine

Female

  • Ovaries in the abdominal cavity produce gametes + sex hormones
    • Follicle development
      • Follicle = follicle cells surrounding an oocyte (→developing egg/immature ovum)
      • At birth, each ovary contains 2 million follicles
      • Remain in suspension until puberty → at puberty, only 400 000 remain
      • From puberty to menopause, only 400 will be released
      • Every month, 20 follicles develop BUT only one follicle ruptures
    • Corpus luteum
      • Ruptured follicle develops into corpus luteum
      • Pregnant → inhibits next menstrual cycle
      • Not pregnant → degenerates and next menstrual cycle starts
  • Fallopian tube (or oviduct)
    • Connects ovaries with uterus
    • Lined with ciliated epithelium (→motility)
    • Usually, this is the place of fertilization

Summary

  • Sperm are ejaculated outside the cervix
    • Semen is alkaline → neutralizes acidic pH of the vagina
    • Cervical mucus is thin and watery during ovulation → allows sperm to swim into uterus
    • Contraction of uterus helps sperm to move into the fallopian tubes
  • Ovulation
    • Follicle ruptures and releases a 2° oocyte into the fallopian tube
    • Sperm enters 2° oocyte
    • 2° oocyte completes meiosis II to form a haploid ovum
    • Within minutes, sperm and ovum nuclei fuse to form a diploid zygote (this is fertilisation!)
    • Zygote moves along the fallopian tube and develops into a blastocyst
    • Moves into uterus where it implants into the endometrium


Figure 49. Author: Mike Jones

Gametogenesis

  • Formation of gametes
    • Spermatogenesis → sperm
    • Oogenesis → ova
  • Stages
    • 1) Multiplication of diploid cells by MITOSIS
      • Epithelium of seminiferous tubules multiplies
      • Epithelium inside ovary of female fetus multiplies
    • 2) GROWTH of daughter cells from mitotic divisions
    • 3) MATURATION of haploid daughter cells into gametes (eggs, sperm)
      • Primary oocyte/spermatocyte divide by MEIOSIS producing haploid cells (46→23)
  • Gametogenesis differs in females
    • Primary oocytes form before birth
    • Cell division in meiosis is unequal → produces 1 ovum and smaller polar bodies (no function)
    • Meiosis is suspended at
      • Prophase I → resumed after puberty
      • Metaphase II → resumed after ovulation

Copulation and Fertilisation

Structure

  • Sperm
    • Head
      • Acrosome (enlarged lysosome → digestive enzymes → penetrate egg)
      • Haploid nucleus (n)
    • Middle → mitochondria produces ATP for tail movement
    • Tail → Flagellum for movement
  • Oocyte
    • 0.1mm (100μm) in diameter. Sperm's head is only 2.5μm across
    • Haploid nucleus is surrounded by cytoplasm
    • Yolk droplets contain proteins and lipids
    • Plasma membrane is surrounded by a  jelly coat made of glycoproteins

Capacitation

  • Occurs after ejaculation in the female reproductive tract
  • Destabilisation of acrosomal membrane
  • Achieved by removal of cholesterol and glycoproteins from the membrane
  • Membrane becomes more permeable to Ca2+
  • Allows acrosome reaction

Acrosome Reaction

  • Contact of jelly coat and sperm triggers Ca2+ to enter acrosome
  • Acrosome bursts and releases enzymes
  • Enzymes help to digest and penetrate jelly coat
  • Sperm nucleus enters oocyte, tail and middle piece remain outside
  • Cortical reaction blocks entry of more sperm
  • 2° oocyte undergoes second division of meiosis → produces ovum + second polar body
  • Nuclei (ovum + sperm) fuse forming a zygote

Implantation

  • Zygote undergoes cleavage as it moves along the fallopian tube
    • Cells divide by mitosis to form a morula
    • Moves along the fallopian tube by cilia and peristalsis
  • Morula develops into a blastocyst
    • Trophoblast → outer cell layer
    • Inner cell mass → well become the embryo
    • Fluid filled cavity → protection (absorbs shocks, resists compression)
  • Trophoblast forms microvilli
    • Secrete enzymes which digest the endometrium
    • Provide large surface area for absorption
  • Blastocyst becomes buried within endometrium
  • Microvilli are replaced by placenta

Placenta

Structure

  • Umbilical cord connects placenta with fetus
    • 2 umbilical arteries → deoxygenated blood from fetal aorta to placenta
    • 1 umbilical vein → oxygenated blood from placenta to fetal vena cava
  • FICK'S LAW: (surface area x difference in conc)/thickness of surface is proportional to rate of diffusion
  • Large surface area
    • Microvilli grow into endometrium
    • Each villi contains a network of fetal capillaries
    • Surrounded by thin pool of maternal blood
  • Max difference in concentration
    • Fetal Hb has a greater affinity for oxygen than adult Hb
    • Countercurrent flow
      • Flow of maternal and fetal blood in opposite direction
      • Uterine artery to umbilical vein
      • Maintains gradient / prevents concs from reaching an equilibrium
  • Short diffusion path
    • Fetal and maternal blood supply is separated by a thin membrane made of 3 layers
      • Capillary endothelium
      • Thin layer of connective tissue
      • Epithelium covering villi
    • Exchange surface is only one cell thick
    • Maternal and fetal blood never mixes
      • Maternal blood may be genetically different from fetal blood

Function

  • Exchange of substances between maternal and fetal blood
    • O2 and waste products (urea, CO2) cross placenta by diffusion
    • Glucose enters fetal blood by facilitated diffusion
    • Amino acids enter fetal blood by active transport
      • Placenta contains many mitochondria
    • Maternal antibodies are taken into villi by pinocytosis
      • Infant has temporary immunity to same diseases as its mother after birth
  • Secretes hCG → maintains pregnancy

Birth

  • Dilation of the cervix
    • Fetus lies with its head down against the stretched cervix
    • Weak contraction of uterus every 30min / increase in strength and frequency
    • Caused by oxytocin released by pituitary gland
  • Expulsion (also birth or parturition)
    • Contractions of mother's abdominal muscles
    • Umbilical cord shuts down, isolating baby from mother
    • Rises CO2 content of the blood / stimulates baby's first breath
  • Delivery of placenta (30 min after birth)

Lactation

  • Advantages
    • Milk is bacteria free/ contains antibodies
    • Contains essential nutrients → Ca2+ for bone growth
  • Disadvantages
    • NO fibre/iron → give solid food after 6mo