Chapter 16: Development

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Chapter 16 Development

Ode to the worm! Nobel Prize 2002
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Complexity increases with Development. 4 stages
in this image.
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How do we develop?

• On ovulation day, egg and sperm fuse to form
  zygote.
• Zygote divides, implants onto uterus and grows
  into Embryo and hangs out for about 9 months.
• Embryo decides it is time to breathe air, fetal
  adrenal glands trigger contractions and out comes
  baby.
• Baby grows grows grows into child, child
  undergoes puberty and becomes adult.
• Adult lives, works, reproduces (perhaps), gets
  gray hair and croaks.

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Fertilization

• Egg must develop and be released on ovulation
  day.
• Egg must be correctly positioned in the oviduct
  and attract sperm.
• Vaginal tract must activate sperm.
• Hormonal levels must be exact.
• Ensure only one sperm joins with egg.

• Sperm must undergo capacitation--process of
  activation by substances in female vaginal tract
  fluids.
• Sperm motor from vagina up through cervix,
  uterus, to the oviduct.
• Many sperm attempt fertilization, only one
  succeeds (except for twins).

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Egg and Sperm

• Egg--
• Corona Radiata Cells from follicle that
  nourished egg in ovary.
• Zona Pellucida layer that covers plasma
  membrane--will form fertilization envelope
• Plasma Membrane cell membrane around egg
• Egg Nucleus contains DNA

• Sperm--
• Head Contains sperm nucleus and acrosome.
• Acrosome Contains enzymes.
• Mid Piece Contains Mitochondria
• Tail Flagella made from Microtubules

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REMEMBER!!!!!!!!!

• If viable sperm contact an egg at the time of
  ovulation fertilization will occur.
• This typically occurs on day 14. Remember Day
  1 is first day of menstruation.
• The fertilized egg will implant on day 6.
• The new embryo will begin to produce HCG--Human
  Chorionic Gonadotripin.
• HCG maintains the corpus luteum and allows the
  production of progesterone and estrogen until the
  placenta takes over this task.

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Dev before Implantation

• Fertilization
• Cleavage successive rounds of cell division. A
  one cell zygote--2 cell--4 cell--8 cell--.
• Cleavage occurs in the oviduct.
• Morula 16 cell stage--enters the uterus.
• Key cell differentiation step
• Trophoblast
• Inner Cell Mass

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Blastocyst

• Hollow ball of cells.
• Each cell is called a blastomere.
• Inner cell mass--become the embryo.
• Trophoblast--Incredible Altruistic Cells!
• Escape from the Zona Pellucida
• Digest through Endometrium
• Initiate HCG secretion
• Form the Placenta

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Dev before Birth

• Cleavage--process of cell division.
• Morphogenesis--shaping the embryo.
• Differentiation--how do different cells get their
  identity?
• Growth--increase in size--cell divisions continue
  and cells themselves grow.

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Gastrulation

• Truly the most important day of your life!
• Process of forming 3 germ layers--this process
  requires cell movement.
• Each germ layer forms specific tissues and organs
• Ectoderm--(blue)--will form skin and nervous
  system.
• Mesoderm--(red)--will form muscles, kidneys,
  connective tissue, and reproductive organs.
• Endoderm--(yellow)--will form digestive tract,
  lungs, liver and bladder.

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Extraembryonic Membranes

• Establishing extraembryonic membranes is
  critical. These membranes protect the embryo and
  link embryo to mother
• Amnion--provides fluid environment for fetus.
• Chorion--becomes the placenta--site of gas and
  nutrient exchange with mother.
• Allantois--becomes unbilical blood vessels

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The Placenta

• Nutrient and Gas Exchange between fetus and
  mother.
• Fetal side--from chorion.
• Maternal side--from uterine tissue
• Blood of fetus and mother do not mix.
• Fetal chorionic villi project into maternal
  blood.
• Exchange occurs across membranes.
• Umbilical cord stretches between placenta and
  fetus.

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Birth Defects

• 1 in 16 newborns (6.25 out of 100) born with
  birth defect. Many minor, but some serious or
  fatal.
• 20 of defects (3.125 out of 1000) are genetic.
• Causes
• neural tube closure problems--folic acid.
• drugs--aspirin, caffeine, alcohol, vitamin A
  creams, cigarette smoke, cocaine, heroine,
  thalidomide, DES.
• pathogens--rubella, HIV, STDs, listeria, others.

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Genetic screening

• Amniocentesis--remove fluid from amniotic cavity,
  analyze cells for genetic abnormalities.
  Performed 15th -17th week of pregnancy.
• Chorionic villi sampling--remove villi by
  suction, test for genetic abnormalities.
  Performed 5th to 12th week of pregnancy, chance
  of risk for fetus.
• Screening eggs--obtain eggs and test a polar body
  (eggs clone). If polar body is normal,
  fertilize and implant the egg.

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Parturition--Hormonal Control

• Fetus--HypothalamusCortisol Releasing Hormone
  
• Fetus--Anterior pituitary --ACTH
• Fetus--Adrenal Gland produces Cortisol and DHEAS.
• Cortisol from fetus converted to prostaglandins
  in placenta--these begin contractions.
• DHEAS from fetus converted to estriol in
  placenta--these promote oxytocin in mother.
• Oxytocin (from Posterior pituitary) in mother
  begins labor.
• Cervical stretching--positive feedback.

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Parturition--Stages

• Stage I water breaks, cervix dilates.
• Stage II contractions increase to every 1-2
  min, baby emerges. Episiotomy (cut vaginal
  orifice) can prevent ripping. Baby emerges,
  umbilical cord cut.
• Stage III Placenta is delivered about 15min
  after birth. Remember our altruistic trophoblast
  cells!

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