The document outlines the developmental changes occurring in the blastocyst and endometrial stroma during the second week of embryonic development. Key stages include the differentiation of trophoblasts and embryoblasts, embedding of the blastocyst within the endometrium, and the formation of various embryonic and extraembryonic structures. It also addresses clinical implications of abnormal implantation and related conditions.

2. 2ND
WEEK OF DEVELOPMENT
Dr Anber Saleem

4. Day 8
 Changes in blastocyst
 Blastocyst is partially embedded in
endometrial stroma
 Differentiation of trophoblasts into
cytotrophoblasts and
syncytiotrophoblasts
 Differentiation of embryoblasts into
hypoblast and epiblast.
 Formation of bilaminar flat disc.
 Appearance of amniotic cavity

5. Day 8

6. 2. Changes in endometrial stroma
Becomes edematous and highly vascular
Large and tortuous glands secrete abundant
glycogen and mucus

7. DAY 9
 Blastocyst is more deeply
embedded in endometrium.
 Penetration defect is closed by
fibrin coagulum.
 Lacunar stage of
syncytiotrophoblasts at
embryonic pole
 Formation of exocoelomic
membrane(Heuser’s membrane).
 Formation of exocoelomic cavity
(primitive yolk sac).

8.  Day 9

9. Day 11 and 12
 Blastocyst is completely embedded
in endometrial stroma.
 Surface epithelium almost entirely
covers the original defect in uterine
wall.
 Blastocyst produces a slight
protusion into the lumen of uterus.
 Development of trophoblasts at
embryonic pole.
 Establishment of uteroplacental
circulation

10. Day 11 and 12

11. Formation of extraembryonic mesoderm.
Formation of extraembyonic cavity (chorionic cavity).
Formation of Extraembryonic somatic and splanchnic
mesoderm.
Growth of bilaminar germ disc in comparison with
growth of trophoblasts.
Decidua reaction in endometrium

12. Day 13
 Surface defect in endometrium has
healed.
 Appearance of villous structures in
trophoblasts.
 Primary villi
 Formation of secondary yolk sac
(definitive yolk sac)
 Formation of exocoelomic cysts
 Chorionic plate
 Umbilical cord.

13. Day 13

14. Week of 2’s
Trophoblasts differentiate into 2 layers
1. Cytotrophoblasts
2. Syncytiotrophoblasts
Embryoblasts forms 2 layers;
3. epiblasts
4. hypoblasts
Extraembryonic mesoderm splits into 2 layers
5. Somatic
6. Splanchnic
2 cavities form
7. Amniotic
8. Yolk sac

16. CLINICALS

17. 1) Abnormal implantation
2) Placenta previa
3) Ectopic pregnancy
4) Abnormal blastocyst
 Hydatidiform mole
i. Trophoblasts develop and form placental membranes
ii. No embryonic tissue.
iii. High levels of HCG.
iv. Entire genome is paternal.Thus most moles arise from
fertilization of an oocyte lacking a nucleus followed by
duplication of male chromosome to restore the diploid
number.
v. Paternal gene regulates the most of the development of
trophoblasts.

20. JAZAKALLAH