Second week of development

Second week of development in embryonic life

1. Second Week of Embryo D evelopment 1 Prepared by: Anish Dhakal (Aryan) MBBS Student Patan Academy of Health Sciences

2. OBJECTIVE To describe different changes during 2 nd week of development 2

3. Ending of first week 3

4. Day 7 4

5. Beginning of 2 nd week 5

6. Contd … The blastocyst is partially embedded in the endometrial stroma Trophoblast differentiated into layers viz. cytotrophoblast and syncytiotrophoblast Embryoblast also divides into two layers viz. Hypoblast and epiblast (bilaminar disc) Hence 2 nd week is also known as week of twos A small cavity appears in the epiblast , in future known as amniotic cavity so epiblast lines the amniotic cavity Tortuous glands secrete abundant glycogen and mucus. 6

7. 9 th day 7

8. 9 th day contd … Penetration defect repaired by fibrin coagulum At embryonic pole, vacuole appears in syncytium which later fuse to form lacuna hence known as lacunar phase At abembryonic pole, exocoelomic (a.k.a. Heuser’s ) membrane form a flattened cell layers, and this lines the inner membrane of cytotrophoblast This membrane and hypoblast lines exocoelomic cavity or primitive yolk sac 8

9. 11 th and 12 th day contd … The blastocyst is completely embedded in stroma Lacunar space forms an intercommunicating network Syncytotrophoblast penetrates deeper in stroma and erode sinusoids and becomes continuous with them Uroplacental circulation is established by maternal blood entry into lacunar system Extraembryonic mesoderm and extraembryonic coelom/chorionic cavity appear Extraembryonic mesoderm consists of extraembryonic somatopleuric mesoderm and extraembryonic splanchnopleuric mesoderm Decidua reaction is also seen 9

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11. 12-13 th day 11

12. 13 th day contd … Surface of endometrium is usually healed Sometimes bleeding may occur because of increased blood flow Primary villi i.e. cellular column with syncytial covering are seen Secondary yolk sac a.k.a. definitive yolk sac is formed Exocoelomic cysts are seen in chorionic cavity 12

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14. 14-15 th day 14

15. Uteroplacental Circulation Nutrient and oxygen at first week by simple diffusion Maternal sinusoids anastomose with trophoblastic lacunae Formation of Primary and Secondary stem villi which later in third week gets converted to tertiary stem villi 15

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17. Thank you 17