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