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1st & 2nd Weeks of development

ByDr. Abdul Waheed Ansari

Chairperson & Prof. Anatomy,RAKCODS. RAKMHSU

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Embryonic changes during first week of pregnancy

• Fertilization takes place in the uterine tube.• The zygote formation is the result of fertilization.• The zygote divides continuously to form a ball of cells.• The zona pellucida ruptures after due to increase in cell

population.• The cells organize into two layers, inner cell mass and outer

cell mass.• The inner cell mass are embryonic cells, going to form

embryo proper.• The outer cell mass are trophic cells, going to form the

placenta.

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Zygote

• 1. polar body• 2. perivitelline space• 3. zona pellucida• 4. cell membrane

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Implantation

• The ball of cells is called as morula, a cavity appears inside, it is renamed as blastula.

• The morula migrates and moves towards the uterine cavity and the outer cell mass penetrates the layers of uterus and buries the developing embryo into the walls of the uterus.

• It is called as implantation.

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Normal & abnormal sites of implantation

• Normal implantation zone of the blastocyst is in the superior and posterior wall of the uterine cavity.

• The fertilized human oocyte migrates through the tube and implants itself on the 6th day in the endometrium.

1. Ovarian2. Infundibular3. In the fallopian tube (most frequent)4. Interstitial5. In the lower uterus segment (later placenta praevia)6. Abdominal7. In the pelvic area

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As implantation is underway

• There is much activity inside and around the blastocyst. • A structure called the yolk sac forms from the cells of

the hypoblast.• The yolk sac is a structure that provides nutrients from

the mother to the embryo before the placenta is ready to function.

• A membrane called the amnion forms from the epiblast. • The epiblast also gives rise to the entire head and body

of the embryo.

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Four celled stage

• 1. perivitelline space• 2. zona pellucida• 3. blastomeres

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Morula

• 1. zona pellucida• 2. perivitelline space

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Blastocyst

• 1. cavity within blastocyst

• 2. zona pellucida

1. zona pellucida 2. embryo escaping from the zona (called hatching) 3. blastocyst cavity 4. perivitelline space

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Inner cell mass-embryoblasts1. inner cell mass2. blastocyst cavity• The inner cell mass will going to form

the entire embryo.• The inner cell mass will rearrange into

two layers-the bilaminar germ disk-tall columnar cells lines the amniotic sac and cuboidal cells will form the roof of yolk sac.

• These are the changes occurring during the second week of development.

• The tall columnar cells are going to form the epiblast cells and the low cuboidal cells will form the hypoblast cells.

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During 2nd week of development

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The trophoblasts are the outer cell mass

• The trophoblast cells initially single layered. Later they form double layered structure.

• The inner cells are called as cytotrophoblast cells.

• The outer cells are called as syncitiotrophoblast cells.

• The primary villus will form together-the precursor of placenta during the second week of development.

• The secondary villus is the result when extraembrionic mesoderm is invading the primary villus.

• With the migration of fetal blood vessels into the secondary villus, it is renamed as tertiary villus.

• The syncitiotrophoblast cells manufacture the hormone, HCG (Human chorionic gonadotropin) and released into the urine of mother.

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Pregnancy test

• Approximately 8 days after fertilization, cells from the growing embryo begin producing a hormone called human chorionic gonadotropin or hCG.

• This hormone is present in a pregnant woman’s blood and urine almost immediately and is the substance detected by most pregnancy tests.

• HCG is commonly called the pregnancy hormone and its detection is the most frequent way of confirming pregnancy.

• Just 8 to 9 days after fertilization.

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Establishment of utero-placental circulation (Days 11 to 12)

• The syncytial cells penetrate deeper in the stroma of the uterine wall, they finally erode the endothelial lining of the maternal capillaries.

• The syncytial lacunae then become continuous with the sinusoids and maternal blood enters the lacunar system.

• As the trophoblast continues to erode more and more sinusoids, maternal blood begins to flow through the trophoblastic lacunar system, thus establishing the primitive utero-placental circulation.

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References

• http://www.ehd.org/dev_article_unit2.php• http://

teleanatomy.com/alaqah-BilaminarGermDisc.html

• http://www.embryology.ch/anglais/gnidation/etape02.html