2nd Week of Development

7/28/2019 2nd Week of Development

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IMPLANTATION &

SECOND WEEK OF DEVELOPMENT


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Embryogenesis

Fertilization zygote (cleavage)

Organogenesis

2


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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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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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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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Principal events in the second week of

development


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Bilayer embryonic disc


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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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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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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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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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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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2nd Week of Development