The Stages of Prenatal Development
The Stages of Prenatal Development
• Induction– During prenatal development differences in
cytoplasmic composition trigger changes in genetic activity
• Gestation periods– Three trimesters
Embryonic and Fetal Periods
SECTION 29-4 The First Trimester
• Cleavage – Zygote becomes a preembryo then a blastocyst
• Implantation – Blastocyst burrows into uterine endometrium
• Placentation – Blood vessels form around blastocyst and placenta develops
• Embryogenesis – Formation of a viable embryo
The First Trimester
• A series of cell divisions that subdivides the cytoplasm of the zygote– Trophoblast – outer layer of cells – Inner cell mass – cluster of cells at one end of
blastocyst
Cleavage and blastocyst formation
Figure 29.2
Cleavage and Blastocyst Formation
• Occurs about 7 days after fertilization• Trophoblast enlarges and spreads
– Maternal blood flows through open lacunae
• Gastrulation– Embryonic disc composed of germ layers
• Endoderm• Mesoderm• Ectoderm
Implantation
Stages in Implantation
Figure 29.3
Figure 29.4
The Inner Cell Mass and Gastrulation
• Gastrulation – By day 12 surface cells move toward the
primitive streak – A third germ layer forms
• The three germ layers are:– Ectoderm – superficial cells that did not migrate– Endoderm – cells facing the blastocoele– Mesoderm – migrating cells between ectoderm
and endoderm
Germ layers
• Four extraembryonic membranes:– Yolk sac – Amnion– Allantois– Chorion
Extraembryonic Membranes
Figure 29.5a, b
Extraembryonic Membranes and Placenta Formation
Figure 29.5c, d
Extraembryonic Membranes and Placenta Formation
Figure 29.5e
Extraembryonic Membranes and Placenta Formation
• Yolk sac– Important site of blood cell formation
• Amnion– Encloses fluid that surrounds and cushions
developing embryo
• Allantois– Eventually becomes bladder
• Chorion
Embryo Anatomy