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O. Understanding the stages of
development after fertilization to the
gastrula stage.
Take out a piece of paper and split some
playdough with shoulder partner.
Reminders: Registration forms due
tomorrow. Test Wednesday.
When Harry met Sally
Stages of development
Fertilized ovum
12-24hr
fertilization
Cleavage
30hr-day 3
mitosis
Morula
day 3-4
solid ball of cells
Blastocyst
day 5-week 2 formation of cell layers
inner cell mass forms
implantation
Gastrulation
week 2->
cell layers form:
endoderm
mesoderm
ectoderm
Stem cells
 Totipotent stem cells – “totally man” ,
totally capable of becoming any cell type
in the body
 Pluripotent – have a limited number of cell
types that they could become.
Think about it…
Totipotent = freshman in college
Pluripotent = junior in college
Zygote forms
Fertilization
 Fertilization is the joining of sperm and egg.
 Meiosis II in the egg is completed at the time of
fertilization, forming one egg and one polar body.
 Following fertilization, chemical reactions occur
preventing additional sperm from entering the egg.
Cleavage
 cleavage
 zygote divides mitotically a day after
fertilization
Early development: ovulation to
implantation
Figure 3.14
Morula
 morula (solid ball)
 forms after several cell divisions, which
hollows to form a blastocyst (hollow ball).
Blastocyst – implants in the uterine lining
The developing embryo
becomes a hollow ball of
cells and is called a
blastocyst.
 Group of cells within the
hollow space forms the inner
cell mass (ICM).
 develops into the
embryo.
 The cells around the ICM
become the extra embryonic
membranes
 role in implantation
 supports embryo’s
growth
Outer and inner cell mass
 outermost cells (trophoblast)
 secrete hCG, which prevents menstruation.
 hCG can be detected in a woman’s blood or
urine and is an indicator of pregnancy.
Gastrulation:
Germ
layers:
endoderm,
mesoderm
and
ectoderm
Germ layers
Ectoderm the outermost germ layer develops
Skin – outer layer
nervous system
eye lens
Mesoderm the middle germ layer develops
muscle
connective tissue + inner layer of skin
blood vessels
Kidneys
Endoderm the innermost germ layer develops
lining of GI tract
liver
pancreas
thymus
The Embryo Forms
 During week 2 of pregnancy the amniotic cavity
forms; development of
 primary germ layers: ectoderm, endoderm, and
mesoderm
 During week 3, the primitive streak appears,
followed rapidly by development of the central
nervous system, heart, notochord, neural tube,
limbs, digits, and facial features.
 By week 8, all organs have begun to develop
and the embryo becomes a fetus.
Make your baby