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Human Development
Fertilization
Must occur within 24hrs postovulation
 Requires capacitated sperm (6-8hrs)
 Secondary oocyte completes Meiosis II
once sperm enters oocyte
 Zygote formed with fusion of male and
female pronuclei

Cleavage
Rapid mitotic cell division of zygote
 2-cell stage; 36 hours
 4-cell stage; 48 hours
 8-cell stage; 72 hours
 Morula - solid ball of cells (blastomeres)
at 96 hours
 Takes 4-5 days to travel from uterine
tubes to uterine cavity

Implantation
Morula develops into bastocyst, a
hollow ball of cells differentiated into
inner cell mass and trophoblast
 Trophoblast embeds into endometrium

– secretes enzymes, HCG
– chorionic villi
– attachment occurs 7-8 days after
fertilization
Embryonic Development
Extends from the 2-8th week after
conception
 Inner cell mass differentiates into:
 Ectoderm - epidermis of skin, entire
nervous system
 Endoderm - functional lining of the
digestive and respiratory system
 Mesoderm - forms muscle, bone, blood
vessels, and kidneys

Fetal Membranes
Chorion - fetal part of the placenta,
outermost membrane
 Amnion - fluid filled sac covering
embryo and later fetus
 Yolk sac - source of nourishment for
embryo; early site of blood formation
 Allantois - developing umbilical cord and
developing bladder of fetus

Placenta Formation
Functions to exchange waste products
and blood gases
 Chorionic villi penetrate into the decidua
basalis of the endometrium
 Chorion - fetal portion
 Decidua basalis - maternal portion
 Placenta previa

Hormones of Pregnancy
HCG secreted by the chorion maintains
corpus luteum
 FSH repressed, cycle stops
 Progesterone maintains endometrial
lining
 Placenta secretes estrogen and
progesterone
 Corpus luteum degenerates
 Late pregnancy: relaxin secreted
