2nd week of Human development .embryology

event occurred in the second week of human development. embryological Slide. Anatomy

1. Mithilesh Chaurasiya Asst. Professor Department of Anatomy Dadeldhlura Medical College, Dadeldhura, far western province, Nepal.

2. Rewind : Fertilization

3. Day 1 Day 2 Day 3 Day 5 Day 4 Rewind cleavage 30hr 40-50 hr 40-50 hr 12 cell stage 96 hr 16 cell stage

4. Blastocyst – Two distinct cell types Blastocele (cavity) STRUCTURE OF LATE BLASTOCYST Embryoblasts Trophoblast Bastocyst is formed during 32-64 cell stage at 4 or 5 th day after fertilization about 107 cell stage at 5 day, only 8 cell from embryoblast.

5. Identify the structure A B C D

6. Day -6-7

7. 2 nd WEEK Week of duos/Twos (2) –why-- ? 1) trophoblast differentiate in to two layer Inner- cytotrophoblast Outer- syncytiotrophpblast 2) embryoblast ---two layer epiblast hypoblast 3) two layer of extraembryonic mesoderm Inner splanchnopleura Outer somatopleura 4) two cavity present--- amniotic cavity yolk sac cavity bilaminar disc

8. Blastocyst partially embedded in the endometrial stroma.

9. Development of trophoblast (8 th day)

10. Formation of bilaminar disc primary primary primary primary

11. Inner cell mass or embryoblast differentiate into - Epiblast or Primary ectoderm – a layer of tall columnar cells on the trophoblastic side of the embryoblast. Hypoblast or Primary endoderm – a layer of flattened or cuboidal cells adjacent to blastocyst cavity . The area of contact between these two layer is like disc and forms the bilaminar disc. Formation of bilaminar disc (8 th day)

12. Epiblast cells which are initially attached to the inner aspect of the cytotrophobast , gradually separates from the latter to form a cavity k/a amniotic cavity . Amniotic cavity – floor - is lined by epiblast cells Rest is lined by flattened amniogenic cells (or amnion ) from cytotrophoblast. Further development

13. On further development, the fattened mesothelial cells are formed from cytotrophoblast at abembryonic pole and this forms the Heuser's membrane . This Heuser's membrane enclose a cavity k/a Primary Yolk sac (AKA exocoelomic cavity). Primary Yolk sac – Roof – is lined by Hypoblast cells . Rest is lined by Heuser’s membrane . Day 9 According to some authority, the heuser’s membrane is formed by cells from hypoblast.

14. Formation of extraembryonic mesoderm (day 10-11) At later part of 2 nd week , a new population of cells appear between cytotrophoblast externally and amnion & primary yolk sac internally . They are called extra-embryonic mesoderm They are probably formed from cytotrophoblast cells.

15. Formation of extra-embryonic coelom A no. of small cavities appear in the extra embryonic mesoderm Later they all become confluent to one another to form a new cavity k/a extra-embryonic coelom/cavity or chorionic cavity .

16. Formation of extra-embryonic coelom (day 12-13) The extraembryonic cavity spits the extraembryonic mesoderm into two – 1. extraembryonic somatopleuric mesoderm – lines cytotrophoblast & amnion. 2. extraembryonic splanchnopleuric mesoderm- lines covering of the yolk sac.

17. Formation of definitive or secondary yolk sac (day 13) In succeeding stage, the primary yolk sac undergo much reduction in size Because a large portion of the sac is pinched off by the traction of the mesodermal strands which connect it to the abembryonic pole of chorion. The pinched off part is k/a exocoelomic cyst .

18. Formation of definitive or secondary yolk sac (day 13)

19. Formation of definitive or secondary yolk sac (day 13) The reduced sac thus formed is lined by new wave of cells that migrate from hypoblast along the inner surface of the Heuser’s membrane. & is converted into secondary yolk sac or simply Yolk sac.

20. Few terms : 1. Amnion - flattened cell membrane that forms roof of the amnionic cavity (excluding the epiblast layer). They are probably derived from the cytotrophoblast

21. Few terms : 2. chorion - Multilayer structure consisting of Somatopleric extraembyonic mesoderm Cytotrophobast . 3. connecting stalk – Unsplitted part of extraembryonic mesoderm Connects embryo with trophoblast. Forms the future umbilical cord.

22. Embryo at 14 day - Bubble on a Bubble in a Balloon By the end of 2 nd week - Blastocyst cavity now called chorionic cavity contains Two hollow vesicles ( amniotic & yolk sac ) Suspended from the wall of chorion by the connecting stalk . The area betn floor of aminiotic and roof of yolk sac forms the bilaminar germ disc or embryonic area. Implantation is completed by 12 th day.

23. bilaminar disc

24. Thank you