Download - Implantation (General Embryology)
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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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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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Dr. Sherif Fahmy
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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Dr. Sherif Fahmy
Ectopic Pregnancy
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Placenta Praevia
Dr. Sherif Fahmy
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.
Dr. Sherif Fahmy
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Dr. Sherif Fahmy
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
Dr.Sherif Fahmy
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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