IVF - babies for those who cannot conceive naturally

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