bio 13

• Fertilization


  • Of the 200 million sperm released, only 200 reach the secondary oocyte
  • Genetic information from haploid sperm and egg merge to form diploid zygote
  • Oocyte undergoes meiosis II when sperm enters the cytoplasm of the secondary oocyte

  
• Fertilization


  1. Occurs in the fallopian tube within 12 – 24 hours after ovulation

  2. Occurs between day 7 and day 21 of cycle

  3. Ovulation may not occur exactly on day 14

  4. Secondary oocytes are viable for about 24 hours after ovulation

  5. Sperm are viable for 48 hours after deposition in the vaginal canal

  
• If fertilization occurs, corpus luteum does not degenerate
• Production of estrogen and progesterone is maintained by corpus luteum until placenta produces these hormones prevents menstruation
• Zygote


  Fertilized diploid egg (1 cell)
• Early cleavage

  1. Begins ~24 hours after fertilization

  2. Rapid cell division with no growth phase (G1)
• Morula


  Embryo is a solid mass of cells not larger than the original zygote
• Blastocyst


  • Embryo becomes hollow, with an inner cell mass and trophoblast
  • Blastocyst implants in the Endometrium, lining of the uterus
• Implantation of Blastocyst to Endometrium of Uterus


  1. ~6 days after fertilization the blastocyst binds to the endometrium (implantation)

  2. Inner Cell Mass: develops into embryo

  3. Trophoblast (outer layer of cells): develops into fetal portion of the placenta

  
• 2nd week of Development

  1. Inner cell mass becomes: Yolk sac, Amnion, Embryo

  2. Trophoblast becomes: Cytotrophoblast, Syncytiotrophoblast

  3. Both will then become part of the chorion

  
• Four Embryonic Membranes


  • Chorion: Will develop into the fetal half of the placenta
  • Yolk sac: The first site of blood cell formation
  • Allantois: Will develop into the umbilical blood vessels
  • Amnion: Fills with amniotic fluid and protects embryo during development

  
• 3rd Week of Development

  1. Gastrulation: Blastocyst folds inwards

  2. Formation of 3 primary germ layers: Endoderm, mesoderm, ectoderm
• 3 primary germ layers

  • Ectoderm: outer layer; becomes skin, brain, nervous tissues
  • Mesoderm: middle layer; becomes the muscles, bones, blood
  • Endoderm: develops into lining of the digestive system, respiratory system and lining of other organs
• Placenta


  • Develops at the site of implantation and connects the fetus to the mother via an umbilical cord
  • Functions in the exchange of gases, nutrients and waste between mother and fetus
  • Stores nutrients (carbs, proteins, iron etc) to be released when the fetus requires it
  • Eventually produces enough progesterone to prevent menstruation or miscarriages (replaces the role of the corpus luteum)
• Umbilical Cord


  • Connects fetus to the placenta: 2 umbilical arteries and 1 umbilical vein
  • Umbilical arteries carry deoxygenated blood from the fetus to the placenta
  • Umbilical vein carries oxygenated blood from the placenta to the fetus

  
• Embryonic Period (Weeks 4-8)


  1. Embryo grows slightly from ~0.4cm to 1.9cm

  2. Limbs begin to appear

  3. Brain continues to develop

  4. Major organs continue developing

  5. Heartbeat can be detected

  6. Resembles a small human at the end of the embryonic period

  
• High levels of progesterone and hCG lead to morning sickness
• Human chorionic somatomammotropin (hCS)


  Involved in mammary gland preparation for lactation
• Corticotrophin-releasing hormone (CRH)


  • Responsible for the timing of birth
  • Indirectly contributes to development of fetal lungs by increasing cortisol levels
• Developing fetus pushes upward


  • Intestine (constipation)
  • Liver, stomach (heartburn)
  • Diaphragm (shortness of breath)
  • Compresses bladder (urgency and frequency of urination increases)
  • Increases heart rate and cardiac output
• Labor and Delivery: Hormones

  1. Increases in estrogen: Stimulates the release of prostaglandins, Stimulates oxytocin receptors on uterus

  2. Increases in oxytocin: Stimulates uterine contractions, Stimulates the placenta to secrete prostaglandins

  3. Prostaglandins: Stimulate uterine contractions and the production of enzymes to breakdown collagen fibers in the cervix which assists in dilation
• Stages of Labor and Delivery: Dilation

  Stage 1 - Dilation: Onset of contractions, Duration 6-12 hours, Regular contractions of the uterus, Rupturing of the amniotic sac (water breaking), Full dilation of cervix (10 cm)
• Stages of Labor and Delivery: Expulsion and Placental Birth

  1. Stage 2 - Expulsion: Duration: 10 minutes – several hours, Delivery of the baby

  2. Stage 3 – Placental birth: Duration: 5 - 30 minutes, Expulsion of the placenta, Constricts torn blood vessels