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Development
Chapter 29
Fertilization
• Zygote undergoes a series of mitotic cell divisions called cleavage
• zygote 2-cell stage 4-cell stage 8-cell stage morula blastocyst gastrula
Morula = solid sphere of cells (day 5)
Blastocyst = hollow ball of cells (days 6-7), it produces human chorionic gonadotropin (hCG) hormone which can be detected in the woman’s urine during a pregnancy test; source of embryonic stem cells
Gastrula = differentiation starts; contains three germ layers (ectoderm, endoderm, mesoderm; days 7-10)
Late Gastrula
Mesoderm
Endoderm
Ectoderm
Three germ layers in gastrula:Ectoderm – gives rise to the nervous system and skin
Endoderm – gives rise to lining of the gut and internal organs
Mesoderm – gives rise to skeleton, muscles, circulatory system, reproductive system
Pregnancy becomes established during days 10 -14:• Fluid filled amniotic cavity starts to form • Yolk sac starts to form (will make blood cells, germ cells) • Embryo starts to form from embryonic disc • Chorion (placenta) starts to form and will supply the
embryo with nutrients and eliminate wastes• Woman misses period
Development of the Placenta
• Embryo = the term used until the 8th week; from Greek “swelling within”
• Fetus = the term used from the 8th week until birth; from Greek “offspring”
Pregnancy can be divided into three trimesters:• First trimester = body systems form (during this
time the embryo is extremely sensitive to smoking & drugs)
• Second & third trimesters = continued body growth
BIRTH
3 parts:
1. Dilation of cervix
2. Expulsion of the baby
3. Expulsion of the placenta