Implantation of Blastocyst

After fertilization in the fallopian tube, the zygote undergoes cleavage, forming a morula, and then a blastocyst, which moves into the uterine cavity. The endometrium's glands secrete fluid that aids blastulation. The blastocyst consists of a blastocoel (fluid-filled cavity), a trophoblast (outer layer forming the chorion and placenta), and an inner cell mass (developing into the organism).

The blastocyst implants itself into the uterine endometrium, typically around day 7 after fertilization. The section of the trophoblast adjacent to the inner cell mass attaches to the endometrium. The trophoblast cells release digestive enzymes to create a space for the embryo to embed.

By day 10, the embryo is entirely embedded within the endometrium. The digestive enzymes break down blood vessels, creating a temporary source of nutrition and oxygen, leading to the formation of the placenta. The umbilical vesicle and amniotic cavity also begin to form, with the amniotic cavity eventually housing the developing embryo.

Around 25 days post-fertilization, a direct connection forms between the chorion of the embryo and the uterine blood vessels, establishing the placenta for continuous nutrient and oxygen supply. The embryonic stalk develops into the umbilical cord, and the amniotic cavity expands to accommodate and protect the growing embryo with its fluid.