Nat Sci 179 Lecture Notes Chapter 29 Prenatal Development

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prenatal period

Back 1

-38 weeks in length
-stages are cleavage, embryonic, fetal

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zygote

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fertilized egg
-fertilization usually occurs in uterine tube

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cleavage embryo

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-cleavage begins 30 hours later
-this is mitosis (1 cell  2 4 8 etc.)
-little growth occurs, so cells become smaller with each division
-organelles are produced, but little cytoplasm
-blastomeres = cells resulting from cleavage
-cleavage embryo = the tiny ball of cells after the first cell divisions

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cleavage embyro travels

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down uterine tube to uterus
-takes about 3 days to reach uterus

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morula

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16 cell stage
-floats freely in the uterine cavity for another 3 days
-cell division continues
-ball starts to become hollow
-these cells are the stem cells (multipotent cells

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blastocyst

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hollow ball of cells
-begins to implant in the endometrium (lining of the uterus)
-this lining is shed monthly if no pregnancy occurs

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implantation

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-occurs at about end of the first week
-blastocyst secretes enzymes to digest a bit of endometrium
-it enters the endometrium and becomes surrounded by it

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placenta development begins

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-vascular structure
-made from some outer cells of the blastocyst and some cells of endometrium
-regulates exchange of nutrients, gases, wastes between blood of mother and embryo

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Embryonic Stage

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-weeks 2 through 8

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gastrula stage

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-threeprimary germ layers of tissue form
-ectoderm layer becomes nervous tissue, skin
-mesoderm layer becomes muscle, bone, blood vessels
-endoderm layer becomes linings of various tracts (respiratory, digestive, urinar

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chorion develops

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a membrane surrounding the embryo

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chorion projections

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extend from chorion, called chorionic villi

site of CVS, chorionic villus sampling

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CVS, chorionic villus sampling

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a few cells of chorionic villi are removed via the mother’s cervix

-can be tested for evidence of genetic abnormalities in fetus
-can be done as early as 10th week of development

-is an alternative to amniocentesis, which is usually done at 16th week
-later in development, the chorion becomes part of the placenta

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amnion also develops at this time

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-a second membrane that will surround the embryo
-amniotic fluid forms inside the amnion (aka amniotic sac)
-surrounds fetus
-acts as a protective cushio

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yolk sac also forms at this time

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these cells later give rise to sex cells, blood and bone marrow stem cells

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allantois also forms at this time

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-another sac-like structure
-later forms blood cells, blood vessels

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Fetal Stage

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-called fetus from ninth week to birth
-period of rapid body growth
-birth usually occurs at about day 266

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neonate

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newborn

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conceptus

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general term for any stage of development of a fertilized egg

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Timeline of Development

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zygote cleavage embryo morula

blastocyst gastrula embryo

fetus neonate