Question

The list below describes (in no particular order) some early events of the pre-embryonic period.

1 – Cortical reaction

2 – Blastocoel forms

3 – Yolk sac and amnion start to form

4 – Dichorionic-Diamnionic twins

5 – Four celled pre-embryo

6 – Human chorionic gonadotropin present in urine

The 6 steps above are not in the correct order.

Answer 1

1. The cortical reaction is a process initiated during fertilization by the release of cortical granules from the egg, which prevents polyspermy, the fusion of multiple sperm with one egg.

2. Four celled pre-embryo- The mitosis continues and the first two cells divide into four cells, then into eight cells and so on.

3. Blastocoel forms- In a process called cavitation, the trophoblast cells secrete fluid into the morula to create a blastocoel, the fluid-filled cavity.

4. Human chorionic gonadotrophin can be detected in the urine from 7-9 days post-fertilisation as the embryo attaches and implants in the womb; it forms the basis of most over-the-counter and hospital pregnancy tests

5. The yolk sac and amnion develop simultaneously, which begins during days 8 through 14 of embryogenesis. The yolk sac has a lining of extra-embryonic mesoderm. Although yolk sac formation occurs during the second week of development, it cannot be visualized clinically on ultrasound until five weeks of gestation.

6. Dichorionic-Diamnionic twins- A dichorionic diamniotic (DCDA) twin pregnancy is a type of twin pregnancy where each twin has its own chorionic and amniotic sacs.