Chapter 28 - Development

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Chapter 28 - Development
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SPERM MIGRATION

• In upper 1/3 of fallopian tubes within 24 hrs of
  ovulation 48 hrs of insemination.
• Only about 3000 of original 300 million make it.

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CAPACITATION

• Even after arrival at egg, sperm cant fertilize.
  
• Fluids in female soften plasma membrane and
  dilute inhibitory factors that prevent the
• acrosome from working
• Ca diffuses in and enhances tail lashing

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CAPACITATION

• Timing sperm live only 6 days, so cant get
  pregnant more than a week before ovulation, nor
  more than 14 hours after egg wont be viable
  long enough

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Fertilization

• Several sperm needed to enzymatically penetrate
  cell layers around the ovum acrosome reaction
  exocytosis of acrosome
• Only 1 sperm enters the ovum - tail midpiece
  disintegrate, egg releases enzymes to destroy
  sperm surface receptors prevents polyspermy.

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Fertilization
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MEIOSIS

• Oocyte now completes meiosis.
• 23 male chromosomes 23 female chromosomes join
  to form zygote.

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TRIMESTERS

• 1 - Fertilization through week 12 most
  sensitive time-over half die
• 2 - Weeks 13 through 24 complete most organ
  development near end IS possible to survive
  birth
•  

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TRIMESTERS

• 3 - 25 weeks birth - growth continues to point
  where survival is more likely brain liver
  kidneys have to develop further AFTER birth.
• Single births usually 40 weeks, twins 35 weeks 

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CLEAVAGE/PRE-EMBRYONIC DEVELOPMENT

• Mitotic cell divisions produce identical copies.
• Morula - solid ball of cells.
• Blastocyst - hollow ball - cavity is called
  blastocoel.
• Made of trophoblast outer layer Inner cell
  mass

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Human Morula
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Human Blastocyst
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CLEAVAGE/PRE-EMBRYONIC DEVELOPMENT

• Becomes implanted in endometrium - inner cell
  mass facing endometrial wall. TAKES ABOUT 1
  WEEK.
• Trophoblast gives rise to chorion - forms
  placenta with chorionic villi secretes hCG to
  maintain corpus luteum.

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CLEAVAGE/PRE-EMBRYONIC DEVELOPMENT

• Inner cell mass forms embryo and other
  extra-embryonic membranes

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IMPLANTATION 6 days after ovulation

• Trophoblast cells facing wall fuse ?
  syncytiotrophoblast grows into uterus
• Uterus responds by growing over and burying
  entire blastocyst
• Ideally blastocyst implants HIGH on the wall.
• Trophoblast grow into placenta

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IMPLANTATION 6 days after ovulation

• Placenta - nutritive bridge between mother
  fetus by end of 3rd month
• Produces large amounts of hCG - peaks at wk 8-9,
  drops to constant level at wk 16, stimulates
  corpus luteum to continue secretion of ES and P
• Later, the placenta secretes its own P and ES to
  maintain pregnancy, and relaxin to aid in
  delivery and prevent premature contractions

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EXTRA-EMBRYONIC MEMBRANES

• Chorion - outer -derived from trophoblast of
  blastocyst.
• Forms placenta.
• Secretes estrogens, progesterone, relaxin.
• Amnion - for protection.
• Filled with amniotic fluid.
• Shock absorber for fetus.
• Repository for fetal urine, etc.

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EXTRA-EMBRYONIC MEMBRANES

• Allantois - blood vessels to placenta and
  umbilicus.
• Later becomes ligament attached to urinary
  bladder.
• Yolk sac - has little yolk, but source of early
  blood cells primordial gametes.

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EMBRYOGENESIS

• Embryoblast flattens into embryonic disc in
  amniotic cavity 2 layers epiblast toward
  cavity, hypoblast away from
• Primitive streak forms along midline with
  primitive groove running down it ? bilateral
  symmetry, front and back defined

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EMBRYONIC DEVELOPMENT - Gastrulation

• Results in 3 germ layers - ectoderm, endoderm,
  mesoderm.
• Epiblast will become ectoderm skin, etc. and
  CNS
• Migration of epiblast cells into and through the
  primitive groove to replace hypoblast cells with
  endoderm will be gut lining

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EMBRYONIC DEVELOPMENT - Gastrulation

• Then a third layer migrates in between the two
  mesoderm will become mesenchyme basis of
  connective tissues, and muscle