Implantation (General Embryology)

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Cleavage & Migration

Transcript of Implantation (General Embryology)

Page 1: Implantation (General Embryology)

Cleavage & Migration

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Dr.Sherif Fahmy

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Dr. Sherif Fahmy

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Dr.Sherif Fahmy

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Dr. Sherif Fahmy

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Formation of blastocyst• By the end of 4th day, zona pellucida starts

degeneration and disappears by the end of 5th day.• The blastomeres that remain after rupture of zona is

called morula.• Fluid enters between cells and forms a cavity

(blastocele) which separates cells into outer cell mass (trophoblast) and inner cell mass (embryoblast). This stage is called blastocyst which is formed at the 6th day.

• Embryoblast forms embryo and then fetus while trophoblast form placenta, amniotic cavity and umbilical cord.

Dr. Sherif Fahmy

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2-cell stage

4-cell stage

8-cell stage

Morula

Early blastocyst

Blastocyst

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Blastocele

Inner cell mass (Embryoblast) Outer cell mass

(Trophoblast)

Embryonic pole

Abembryonic pole BlastocystDr.Sherif Fahmy

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Dr. Sherif Fahmy

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Dr. Sherif Fahmy

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Second Week of Pregnancy

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Implantation• Dif.: It is the process by which

blastocyst is embedded in the endometrium.• Timing: Starts at 7th day and

completed at 11th day.• Site: usually at upper part of

posterior wall of uterus near fundus.Dr. Sherif Fahmy

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Steps of Implantation• Blastocyst fixes its embryonic pole

to site of implantation.• Syncytiotropholast erodes the

compact layer of the endometrium to form implantation cavity through which the blastocyst reaches spongy layer of endometrium.

Dr. Sherif Fahmy

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-Endometrium of pregnancy is called decidua.-Site of penetration is closed by fibrin clot (coagulum) at 9th day.-Surface epithelium overgrows the fibrin clot at 11th day.

Dr.Sherif Fahmy

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Inner cell mass (embryoblast)

Outer cell mass (trophoblast)

Endometrial arteriol

Endometrial gland

Blastocyst

Fixation of embryonic pole to implantation site

Endometrium

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Dr. Sherif Fahmy

EndometriumEndometrial arteriol

SyncytiotrophoblastEmbryoblast

Trophoblast

Blastocyst

Cytotrophoblast

Endometrial gland

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Dr. Sherif Fahmy

Gland Arteriol

Syncytiotrophoblast

Cytotrophoblast

EpiblastTrophoblastHypoblast

Amniotic cavity

Primary yolk sac

Endometrium

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Dr. Sherif Fahmy

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Abnormal Sites of Implantation• 1- Ectopic pregnany: They are tubal,

ovarian or omental (peritoneum).• 2- Placenta praevia: Parietalis,

marginalis and centralis.

Dr. Sherif Fahmy

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Ectopic Pregnancy

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Placenta Praevia

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Placenta praevia parietalis

Normal implantation site

Placenta praevia marginalis

Placenta praevia centralis

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Decidua• It is endometrium after implantation which

is sheded after birth of fetus.• Character of endometrium:• Increased secretory function of uterine glands.• Decidual cells are stromal cells filled with

glycogen.• Arteries become more spiral with arterio-

venous anastomosis.

Dr. Sherif Fahmy

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Parts of decidua:• Decidua basalis: It is the part of decidua

between blastocyst and myometrium. It forms the fetal part of placenta.

• Decidua capsularis: It covers the blastocyst except embryonic pole and separates it from uterine cavity.

• Decidua parietalis: It is the rest of endometrium that lines the rest of uterine cavity.

Dr. Sherif Fahmy

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Decidua basalisDecidua capsularis

Decidua parietalis

Uterine cavity

Dr.Sherif Fahmy

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Fate of decidua:• Decidua basalis shares in the

formation of placenta.• Decidua capsularis and parietalis

fuse together and shedded with placenta after delivery.

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Decidua basalis

Decidua parietalis

Decidua capsularis

Uterine cavity

Fused decidua parietalis and capsularis

Decidua basalis

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Changes of Blastocyst in the Second Week of

Pregnancy

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7th day:

Dr. Sherif Fahmy

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8TH Day of PregnancyEndometrium

Syncytiotrophoblast

Cytotrophoblasts Hypoblasts

Amniotic cavity

Amnioblasts

Epiblast

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• 8th day:• Blastocyt is partially implanted to the endometrium

which is called decidua. • Trophoblastic cells proliferate rapidly to be

differentiated into 2 layers: cytotrophoblast (inner cellular layer) and outer syncytiotrophoblast (sheet of protoplasm).

• Formation of amniotic cavity between epiblast and amnioblasts.

• Cells of inner cell mass (embryoblast) become arranged into 2 layers: Hypoblast (primary endoderm) and epiblast (primary ectoderm), so forming a bilaminar germ disc.

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9th & 10th days

Fibrin clotPrimary yolk sac

Heuser’s membrane

Lacunar spacesSyncytio-trophoblast

Endometrial arteriol

Hypoblast

Amniotic cavity

Epiblast

Amnioblast

Cyto-trophoblast

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9th day:• Implantation is completed and the penetration

defect in endometrium is closed by fibrin clot.• Formation of primitive yolk sac (exocoelomic

cavity). This occurs by proliferation of cells from hypoblast which descend to line the trophoblast at the original blastocele called Heuser’s membrane.

10th day:- Lacunar spaces appear in syncytiotrophoblast.

Dr. Sherif Fahmy

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11th & 12th daysBlood inside lacunae

Endometrial sinusoid

Syncytio-trophoblast

Endometrial epithelium

Extraembryonic mesoderm

Primary yolk sac

Large spaces

Amniotic cavity

Cytotrophoblast

Bilaminar embryonic disc

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11th & 12th daysSyncytio-trophoblast

Chorionic Vesicle

Cyto-trophoblast

Somato-pleuric (Somatic)

Splanchno-pleuric (Splanchnic)

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11th & 12th days:• Implantation defect is covered by epithelium.• Blastocyst produces a slight protrusion into uterine

cavity.• Lacunar spaces become filled with maternal blood

due to deeper penetration of cyncytiotrophoblast to the endometrium.

• Formation of extraembryonic mesoderm in the inner aspect of cytotrophoblast. Now trophoblasts is called chorion and blastocyst is called chorionic vesicle.

• Large spaces appear in the mesoderm which form extra-embryonic coelom except at part connected to caudal end of embryo called connecting stalk.

Dr. Sherif Fahmy

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13th day

Endodermal cells

Secondary yolk sac

Exocoelomic cyst

Somatic mesoderm

Connecting stalk

Somatic mesoderem

Splanchnic mesoderm

1ry chorionic villi

Extra-embryonic coelom Chorionic cavity)

Dr.Sherif Fahmy

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Chorionic Vesicle

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13th day:• Formation of secondary yolk sac which

becomes lined with endodermal cells and reduced in size due to punching off a part.

• Formation of extra-embryonic coelom and connecting stalk.

• Formation of primary chorionic villi.• Development of blood vessels in connecting

stalk.• Formation of prochordal plate in which the

ectoderm and endoderm are fused together.Dr. Sherif Fahmy

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Summary of the Second Week of Pregnancy

• Through this week, the following structures are formed:–Two cavities: amniotic and yolk.–Two layers of embryonic disc: Primary

ectoderm and primary endoderm.–Two layers of trophoblast: inner

cytotrophoblast and outer cyncytiotrophoblast.

Dr. Sherif Fahmy