Chapter 18 Development

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Chapter 18Development
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Fertilization, Cleavage, and Implantation
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Fertilization

• Sperm and egg unite 6-24 hours after
  intercourse, sperm survives days in female
  reprod. tract
• One sperm fertilizes one egg
• Occurs in upper oviduct
• Acrosome contains digestive enzymes
• Sperm digests thru follicle cells and zona
  pellucida
• Sperm membrane fuses with egg membrane
• Zona pellucida made impermeable to other sperm

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Early Development

• Cleavage involves cell division without cell
  growth, up to about day four
• Function is to restore cell size to normal (i.e.
  smaller than the egg cell)
• Produces a morula a solid ball of cells

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Blastocyst

• Inner cell mass cells form embryo proper
• Source of embryonic stem cells
• Outer (trophoblast) cells form placenta
• Secrete hCG (human chorionic gonadotropin) after
  implantation
• Extends life of corpus luteum
• Basis of pregnancy test

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Implantation
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Implantation
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Gastrulation

• Cells migrate to produce a three layered embryo
• Ectoderm outermost layer
• Forms nervous system, skin, etc.
• Mesoderm middle layer
• Muscle, bones, blood, gonads, etc.
• Endoderm innermost layer
• Viscera, gut, etc.

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Neurulation Forms Neural Tube

• The neural tube forms the CNS (brain spinal
  cord)
• Inductive interactions between tissues
• Chemical signals influence development of nearby
  tissue
• Ex Neural tube induced by notochord

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Somites form bone muscle
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Extraembryonic Membranes and Placenta

• Extra-embryonic membranes
• Amnion lines amniotic cavity, filled with
  amniotic fluid
• Chorion makes human chorionic gonadotropin (hCG)

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Placenta

• Functions
• Nutrient exchange with mother
• Umbilical cord connects fetus to placenta
• Filters nutrients, waste, and provides antibodies
  for fetus
• No mixing of mother and fetal circulations
• Endocrine organ produces hormones
• hCG
• Estrogen and Progesterone

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Factors That Contribute to Labor

• Estrogen Progesterone ratio
• High P promotes uterine quiescence (early
  pregnancy)
• High E promotes uterine contractility (late
  pregnancy)
• Oxytocin promotes uterine contractions via
  neuroendocrine reflex

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Lactation

• During Pregnancy
• High E and P during pregnancy promote breast
  development but inhibit milk prodn.
• After Childbirth
• E P levels plummet
• Prolactin promotes milk synthesis
• Oxytocin induces milk ejection via neuroendocrine
  reflex

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Environmental Disruptions During the Embryonic
Period Cause Major Birth Defects
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Lower limbs
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Spina bifida
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Spina bifida baby