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2. Tissues of the body Early Embryonic Development Series Lecture 2 /4
First Year students
College of Medicine/University of Kufa
Dr. Ali Ibrahim M.B.Ch.B.; M.Sc.
Tissues of the body Early Embryonic Development Series Lecture 2 /4
College of Medicine / University of Kufa
Dr. Ali Ibrahim College of Medicine University of Kufa

3. Objectives: to understand
Cleavage.
Morula formation.
Blastocyst formation.
Implantation.
Important events in the second week of development (the week of twos).
Some important clinical correlates.
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4. Week1: Cleavage: begins 30 hours after fertilization
Results in 2 blastomeres of equal size (Day2)
Cells become smaller with each division ?
Day 3: morula (mulberry). Each cell at this stage of development is TOTIPOTENT, i.e. has the capacity to become ANY cell type.
Day 4: Compaction, formation of the first cavity (blastocyst): the cells after compaction are PLURIPOTENT i.e. has the capacity to become MANY cell type “multilineage potential”.(Inner cell mass → embryo proper. Outer cell mass → trophoblast → placenta)
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Day 5: hatching
blastocyst hatches from zona pellucida.
no longer constrained → now free to enlarge.
can now interact with uterine surface to implant.
Day 6-7: implantation begins
the conceptus now has 107 cells (of these 8 will make the embryo and 99 begin the development of the fetal membranes----at the very earliest stages placenta has priority???).
The ideal site of implantation is posterior uterine wall.
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Blastocyst (4-5 Days)
Implanting Blastocyst (6 Days)
Syncytiotrophoblast
Cytotrophoblast
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By the end of 1st week: 1.Zygote → morula → blastocyst. 2.Begin implantation in uterine mucosa.
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12. Second Week of Development: Bilaminar Germ Disc
Second week of development is known as week of twos :-
Trophoblast: cytotrophoblast and syncytiotrophoblast.
Embryoblast: epiblast and hypoblast.
Extraembryonic mesoderm: somatoplure and splanchnoplure.
Cavities: amniotic and yolk sac.
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Day 8:
Blastocyst → partially embedded.
Trophoblast → cytotrophoblast and syncytiotrophoblast.
Inner cell mass (embryoblast) → epiblast and hypoblast.
Amniotic cavity ??? amnioblast ???
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Day 9:
Blastocyst → more deeply embedded.
Penetration defect → fibrin coagulum.
Syncytium → vacuoles → vacuoles fuse →lacunae (lacunar stage).
Hypoblast → exocoelomic (Heuser) membrane.
Which is with hypoblast → exocoelomic cavity (primitive yolk sac).
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Days 11:
Blastocyst → completely embedded.
Lacunar spaces → intercommunication network (embryonic pole).
Abembryonic pole → still mainly cytotrophoblasts.
primitive yolk sac membrane is pushed away from cytotrophoblast layer by acellular extraembryonic reticulum (fine, loose connective tissue) → reticulum later converted to Extraembryonic mesoderm by cell migration.
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Day 12:
Maternal capillaries → congested and dilated called sinusoids.
Maternal sinusoids invaded by syncytiotrophoblast.
Lacunae + sinusoids → uteroplacental circulation (support of the embryo changes from histiotrophic to haemotrophic ???).
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Day 13:
Formation of secondary (definitive) yolk sac:
Cells of hypoblast migrate → new cavity (secondary or definitive yolk sac) and exocoelomic cyst (pinches off from primitive yolk sac).
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Day14:
Surface defect:
Usually → healed.
Occasionally → bleeding at implantation site due to increase flow to lacunar space → bleeding near 28th day → confused with menstrual bleeding → inaccuracy expected delivery date.
Trophoblast → villous structure ?
Cellular column + surrounded syncytium → primary villi.
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Day14: continued
Extraembryonic mesoderm → large cavities → confluent → Extraembryonic coelom (chorionic cavity) → surround → primitive yolk sac + amniotic cavity → except → where germ disc connected to trophoblast by → connecting stalk (The only place where extraembryonic mesoderm traverses the chorionic cavity. With development of blood vessels, the stalk becomes the umbilical cord).
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Day14: continued
Extraembryonic mesoderm → somatopleuric and splanchnopleuric.
Chorionic plate: extraembryonic mesoderm lining the inside of the cytotrophoblast.
Note: growth of bilaminar disc relatively slow compared with that of trophoblast → the disc remain very small ( mm).
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Clinical correlates:
Pregnancy test:
The syncytiotrophoblast is responsible for hormone production including human chorionic gonadotropin (hCG).
By the end of the second week, quantities of this hormone are sufficient to be detected by radioimmunoassays, which serve as the basis for pregnancy testing.
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No rejection of the conceptus (50% → father) ?????
Placenta previa (can cause haemorrhage in pregnancy) ??? (requires C-section delivery)
Abnormal implantation sites (ectopic pregnancy)???
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Hydatidiform mole ??? genomic imprinting ???
Mole may produce benign or malignant (invasive mole, choriocarcinoma) tumors.
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Early pregnancy loss 􀂄approximately 50% of all zygotes are lost in the first 2 -3 weeks. 􀂄15% diagnosed pregnancies will miscarry. 􀂄1% women suffer from recurrent miscarriage (miscarriage in 3 consecutive pregnancies). Other conditions linked to implantation defects: 􀂄IUGR ??? 􀂄Pre-eclampsia ???
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