6th lecture embryogenesis 2nd week-8th

7/27/2019 6th Lecture Embryogenesis 2nd Week-8th

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Fusion of egg and sperm nuclei

completes fertilization

Dr Eman khammas Al-sadi

Embryology lecturer


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CopyrightThe McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Fusion of egg and sperm nuclei completes fertilization


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Prenatal PeriodA. Early Embryonic Development

1.Cells undergo a period of mitosis called

cleavage, when cells become smaller and smaller.

2.The dividing mass of cells (morula)moves down the uterine tube to the uterus, where a stage

called the blastocystimplants in the lining of the uterus.


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3.The offspring is called an embryo during the first

eight weeks of development, and a fetus after that time.4.Some of the cells of the blastocyst become the

placenta which also secretes hormones.


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3 distinct layers can be recognized in the endometriumA- superficial

(impact layer

B-inter

mediate

spongy

layer

B-intermediate

spongy layerC- thin

basal layer

The human blastocyst implant in the endometrium along the posterior

or anterior wall of the uterus, between the opening of the glands


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C. Embryonic Stage1.The embryonic stage lasts from the second to the eighth week ofdevelopment, during which time the placenta develops, and all themain internal organs and major external features appear.

During the second week, the embryo is now called a gastrulaand itsinner cell mass transforms into the embryonic disc, and layers formwithin it.

These layers become the three primary germ layers and give rise to allorgan systems.


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Uterus at time of implantation

The wall of the uterus consists of three layers:A- endometrium:

Or mucosa lining the inside wall

B- myometrium:Thick layer of smooth muscle

C- perimetrium:

The peritoneal covering lining the outside wall


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2nd week of development

t :outer layer of multinucleated cells, without distinct cellsyncytiotrophpblas-2

boundaries

1-cytotrophpblast:an inner layer of mononucleated cells.

The trophoblast cells in the area over the embyroblast are differentiated in to 2

layers:

the blastocyst is partially imbaded in the endomatreal stroma

At the 7

th

-8

th

day of development


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day)th7Blastocyst (at


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Cells of the embyroblast(inner cell also

differentiated in to 2 layersmass)hypoblast + epiblast layers=flat disc

1-hypoblast layer :a layer of small

cuboidal cells adjacent to the blastocystcavity.

2-epiblast layer: a layer of highcolumnar cells adjacent to the amniotic

cavity,


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there is a small

cavity appears

with in the

epiblast enlargewith time to

become the

amniotic cavity

Amnioblast: epiblastcells adjacent to the

cytotrophoblast they

line the amnioticcavity together with

the rest of the

epiblast

the endometrimnear the

implantatin site

is edematous &

highly vascular

The gland :large

tortuous

secretes large

amount ofglycogen

&mucus


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( lacunar stage). Day 9

1-The blastocyst is deeply embedded & the penetration defect is closed by a fibrin

coagulum.

2-at the embryonic pole : vacuoles appear in the syncytium fuses to form large

lacunae so called ( lacunar stage).

3-at the abembryonic pole : the exocolemic membrane formed which is a thin layer

of flat cells lining the inner surface of the cytotrophoblast originating from the

hypoblast.

4-exocolemic cavity or the (primitive yolk sac)


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Days11-12 bilaminar disc formation

1-the blastocyst is completely embedded

in the stroma& the surfaceepithelium completely covered thedefect in the uterine wall.

2- trophoblast at the embryonic pole cchby lacunar spaces forming the intercommunicating network while at theabembyronic pole it still in the

cytotrophoblastic form.3-The syncytiotrophoblast penetrates

more & erodes the endothelial liningof the maternal capillaries(sinusoids).

4- establishing the uteroplacentalcommunication : as the lacunae

become continuous with thesinusoids, & maternal blood entersthe lacunar system.


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5-extraembyronic mesodermlayers formation:

which consist of fine loose connective tissuecells derived from the yolk sac cellsbetween the trophpblast & the

exocolemic cavity 2 layers1-extraembryonic somatopluric mesoderm.

2-extraembryonic splanchnopleuricmesoderm

6- ( chorionic cavity ) called (extraembryonic coelom).

this space surrounds the primitive yolk sac&amniotic cavity , except the connectingstalk

7-decidua reaction :

the cells of the uterus becomepolyhedral& loaded with glycogen+lipid ,intercellular spaces are filledwith extravasate& the tissue isedematous.

D 13


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Day 13

1-the surface defect is in theendometrium is usually healednearly completely.

2-primary villous formation :

from the trophoblast cells, Columns ofproliferative cytotrophoblastic cellssurrounded by syncytium

3-2ndry yolk sac formation ordefinitive yolk sac : from the

hypoblast cells that migratesalong the inside of the exocoelomicmembrane , they proliferate &form a new cavity.(much smaller insize)


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4-exocoelomic cysts:

Large portions of exocoelimic cavity was

pinched off in the chorionic cavity orin the extraembryonic coelom .

5-chorionic plate formation:

The extraembryonic mesoderm lining

the inside of the cytotrophoblast is

known as chorionic plate

connecting stalk :-6

t he space where the extraembryonic

mesoderm traverse the chorionic

cavity . Later with the development

of the blood vessels it becomes the

umbilical cord

B H l Ch d i P


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B. Hormonal Changes during PregnancyThe outer layer of cells (trophoblast) of the blastocyst stage

secrete the hormone human chorionic gonadotropin(hCG ,Levels of hCG remain high until the placenta

can produce enough hormones on its own to maintainthe pregnancy.


placental lactogenfor breast development and estrogens.

.Other hormonal changes during pregnancy include

increased secretions ofaldosterone (promotes fluidretention) and

parathyroid hormone (to maintain a high calcium level in

the blood).


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Clinical correlates

Abnormal implantation

The syncytiotrophoblast is responsible for

Hormonal production including human

chorionic gonadotropin(hCG)

detected by (RIA) radioimmunoassay at the end

of the 2nd week (PT)=pregnancy test


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Implantation rejection-1

What are the factors which prevent embryo implantation

rejection by the maternal system?

Combination of factors include:

1-production of immunosuppressive cytokines& proteins2-expression of an unusual major histocompatable

complex that block recognition of the conceptus as

a(FB)

If the mother has an autoimmune disease i.e. SLEantibodies generated by the disease may attack the

conceptus & reject it


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2-Abnormal implantation sites:-Abnormal implantation sites:

a-placenta previa

:in which the blastocyst implants closeto the internal os of the cervix later

on the placenta bridge the opening &causes severe & even life threateningbleeding in the 2nd part of pregnancy& during delivery.

b-Ectopic pregnancy:

Implantation takes place outside theuterus i.e. in the abdominal cavity,ovary, uterine tube mainly in theampulla. mostly the baby dies aboutthe2nd month of gestation causing

severe hemorrhage& abdominal painin the mother

C b l bl t t


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C-abnormal blastocyst:

Some consist syncytium only, others

showed trophoblastic hypoplasia, or

absent embryo, or abnormal germ

disc. they usually show no signs of

pregnancy bec( C L)could not have

persisted--- so aborted.

D- hydatiform mole:

Trophoblast develops & form placental

membrane with a little or no embryo

tissue, mole secrete high level of

(hCG)or may change to benign or

malignant tumors(invasive mole or

chorionic carcinoma)


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3-Functional difference

Genomic imprinting

a Phenomenon in which there isdifferent expression ofhomologous alleles or

chromosome regions dependingon the parent from whom thegenetic material is derived ,imprinting involve autosomes &sex chromosomes(one xchromosome is inactivated in

somatic cells & form thechromatin positive body (Barrbody Functional difference inmaternal & paternal genes areprovided by the following2examples

3-Functional difference


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1- in mole cases:

paternal genes regulate most of the development of the

trophoblast in mole ,fertilization occur in oocyte lacking a

nucleus followed by duplication of the male

chromosomes to restore the diploid number

2-microdeletion on chromosome 15 from a father produce

Prader-willi syndrome ,While if from the mother it result

in Angelman syndrome


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Preimplantation &post implantation-4

failure

occurs often even in some fertile women's &under

optimal conditions for pregnancy


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Q1-Why bleeding occur some times?

because of increase blood flow in to the lacunarspaces at the implantation site.

Q2-why there is inaccuracy in determining the

expected date of delivery?

Because the bleeding from the implantation siteoccur s near the 28th day of the menstrual cycle&

might be missed as normal cycle.


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6th lecture embryogenesis 2nd week-8th

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