PREGNANCY AND EMBRYONIC DEVELOPMENT

• Pregnancy - time from fertilization until infant is born
• Conceptus - developing offspring
• Embryo - period of time from fertilization until week 8
• Fetus - week 9 until birth
• Gestation period - from date of last period until birth (approx. 280 days)
• For fertilization to occur, sexual intercourse must occur no more than 2 days before ovulation and no later than 24 hours after.
• Zygote - first cell of a new individual
• Cleavage - rapid series of mitotic divisions that begin with the zygote
• Embryonic Development:

  • Zygote - first cell of a new individual; begins cleavage 24 hours after fertilization
  • Morula - a ball of 16 cells that forms 3 days after ovulation
  • Blastocyst - a ball of 100 cells; secretes human chorionic gonadotropin (hCG) to induce the corpus luteum to continue producing hormones; attaches to the endometrium 7 days after ovulation
• Functional areas of the blastocyst:

  • Trophoblast - large fluid-filled sphere
  • Inner cell mass - cluster of cells to one side
• Primary germ layers of the blastocyst:

  • Ectoderm - outside layer, gives rise to nervous system and epidermis
  • Mesoderm - middle layer, gives rise to everything else
  • Endoderm - inside layer, forms mucosae and associated glands
• Amnion - fluid-filled sac that surrounds the embryo
• Umbilical cord - blood vessel-containing stalk of tissue that attaches the fetus to the placenta
• Placenta - forms a barrier between mother and embryo; delivers nutrients and oxygen