42. Individual development. Blastogenesis. Segmentation. Formation of the blastocyst.

Question 1

Blastogenesis - Segmentation

Answer 1

Duration: 3-4 days

Early Division:

30 hours: 2 blastomeres
40-50 hours: 4 blastomeres
60 hours: 8 blastomeres

Totipotent Stage:
Up to 4-8 cells, each capable of developing into a separate embryo.

Blastogenesis (Segmentation):
3rd Day: 16 blastomeres (morula).
-Outer Cell Mass: Light blastomeres.
-Inner Cell Mass: Dark blastomeres.

Results of Segmentation:
-Increased cell number (no G1 phase).
-Decreased cell size (zygote size preserved).
-Densification (morula formation).
-Differentiation into pluripotent cells (future embryo and integuments).

Question 2

Formation of the Blastocyst

Answer 2

Outer Cell Mass (“Light” Blastomeres):
-Rapid division, form a single layer.
-Develop into trophoblast (connects embryo to mother, provides nutrition).
-Connected by E-cadherin.

Inner Cell Mass (“Dark” Blastomeres):
-Forms embryoblast (develops into the embryo).

Blastocoel Formation (3.5-4th day):
-Fluid-filled cavity forms between outer and inner cell masses.
-Morula transitions to blastocyst stage.

Blastocyst Stage:
-3.5-4th Day: 58 cells, in the fallopian tube.
-5th Day: 107 cells, enters uterine cavity.
-Embryoblast Location: Embryonic pole.
-Zona Pellucida: Degenerates for uterine lining penetration.
-Endometrium: In secretory phase, ready for embryo implantation.