Download - 2nd Week of Development
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IMPLANTATION &
SECOND WEEK OF DEVELOPMENT
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Embryogenesis
Fertilization zygote (cleavage)
Organogenesis
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Embryogenesis development of primary
germ layers and their locations.
Organogenesis formation of the different
organs.
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Stages of Embryogenesis
1.Cleavage
2. Gastrulation
3. Neurulation
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Copyright 2009, John Wiley & Sons, Inc.
Implantation
About 6 days afterfertilization attaches toendometrium
Orients inner cell masstoward endometrium
7 days after fertilizationattaches more firmly andburrows in
Endometrium becomesmore vascularized andglands enlarge
Decidua modified portion
of endometrium afterimplantation
Regions named relative tosite of implantation
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Fertilization to Implantation
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Copyright 2009, John Wiley & Sons, Inc.
Relationship of a blastocyst to the
endometium of the uterus at implantation
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Week 1 post conception
Zygote divides repeatedly moving down tube toward
uterus (cleavage) The daughter cells are called blastomeres
Morula: the solid cluster of 12-16 blastomeres at
about 72 hours
Day 4: late 60 cell morula enters uterus, taking up
fluid becoming blastocyst
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Blastocyst stage
Two distinct types of cells
Inner cell mass: forms the embryo
Trophoblast: layer of cells surrounding the cavity whichhelps form the placenta
Floats for about 3 days
Implantation on about day 6 post conception
Trophoblast erodes uterine wall Takes 1 week to complete
If inner cell mass of a single blastocyst divides:monozygotic (identical) twins
_____inner cell mass
___trophoblast
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Week 2
Inner cell mass divides into
epiblast and hypoblast
2 fluid filled sacs Amniotic sacfrom epiblast
Yolk sac from hypoblast
Bilaminar embryonic disc: area
of contact
(gives rise to the whole body)
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Week 2 Pregnancy becomes
established
Formation of:1. Fluid-filled amniotic cavity
2. Yolk sac
3. Embryo from embryonic disc
Chorion (placenta)
* at the end of this period,
a woman will just have
missed her period!
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Copyright 2009, John Wiley & Sons, Inc.
Second week of development
Development of trophoblast About 8 days after fertilization, trophoblast develops
into 2 layers in region of contact between blastocyst and
endometrium : cytotrophoblast & syncytotrophoblast.
Become part of chorion Day 9 : lacunar stage as lacunae appears in syncyto.
Blastocyst becomes buried in endometrium and inner
1/3 of myometrium
Secretes human chorionic gondadotropin (hCG) thatmaintains corpus luteum so it continues to secrete
estrogens and progesterone
Maintains uterine lining
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Copyright 2009, John Wiley & Sons, Inc.
Principal events in the second week of
development
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Bilayer embryonic disc
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Copyright 2009, John Wiley & Sons, Inc.
Second week of development (cont.)
Development of bilaminar embryonic disc Cells of embryoblast also differentiate into 2 layers
around 8 days after fertilization
Hypoblast (primitive endoderm)
Epiblast (primitive ectoderm)
Small cavity enlarges to form amniotic cavity
Development of amnion
Amnioblast forms roof of amniotic cavity and epiblastforms floor
Amnion eventually surrounds entire embryo
Amniotic cavity filled with amniotic fluid
Fluid derived from maternal blood and later fetal urine
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Development of yolk sac
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The outer layer of the blastocyst forms a membrane
that protects and nourishes the developing embryo.
The embryo is known as the amniotic sac.
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Copyright 2009, John Wiley & Sons, Inc.
Second week of development (cont.)
Development of yolk sac (exocoelomic cavity) Also on 8th day after fertilization, cells at edge of
hypoblast migrate to cover inner surface of blastocyst
wall
Form exocoelomic membrane
Yolk sac hypoblast and exocoelomic membrane
Relatively small and empty since nutrition derived from
endometrium
Several important functions
supplies early nutrients, sourceof blood cells, contains primordial germ cells that migrate to
gonads to form gametes, forms part of gut, functions as shock
absorber, prevents desiccation
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Copyright 2009, John Wiley & Sons, Inc.
Second week of development (cont.)
Development of sinusoids (lacunar stage)
9th day after fertilization, blastocyst completely embedded inendometrium Syncytiotrophoblast expands and spaces (lacunae) develop 12th day lacunae fuse to form lacunar networks Endometrial capillaries dilate to form maternal sinusoids
Development of extraembryonic coleom - about 12th day afterfertilization
Fuse to form single large cavity Lined by extraembryonic splanchnopleuric mesoderm & E.
somatopleuric mesoderm.
Development of chorion Formed by extraembryonic mesoderm and 2 layers or trophoblast Becomes principal embryonic part of placenta Protect embryo from immune responses of mother Produces hCG Connecting (body) stalk connects bilaminar embryonic disc to
trophoblast will become umbilical cord
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Day 13
Decidua reaction begins on day 12.
Surface defect of implantation is healed.
Primary villi formation. Secondary or definitive yolk sac.
Exocoelomic cysts.
Connecting stalk.
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Development of sinusoids.
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Week 2 The Two-Layered Embryo
Bilaminar embryonic disc inner cell mass
divided into two sheets
Epiblast and the hypoblast
Together make up the bilaminar embryonic disc
QuickTime and a
TIFF (LZW) decompressor
are needed to see this picture.
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Week 2 The Two-Layered Embryo
Amniotic sac formed by an extension of
epiblast
Outer membrane forms the amnion
Inner membrane
forms the amniotic saccavity
Filled with amniotic fluid
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Week 2 The Two-Layered Embryo
Yolk sac formed by an extension of
hypoblast
Digestive tube forms from yolk sac
Tissues around yolk sac
Gives rise to earliest blood cells and blood vessels
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