Embryonic Development
A brief description explaining the process of a fertilised oocyte (zygote) over a period of approximately one week, from ovulation via fertilisation to nidation or implantation into the uterus:
Description of the about one week-long journey of a zygote from ovulation via fertilisation to nidation in the uterus.
Day 1: Shortly before ovulation, the oocyte completes the first meiotic (maturation) division. As soon as the spermatocyte penetrates the oocyte, the second meiotic division is completed. In the oocyte, male and female genomes become visible as so-called pronuclei. The fusion of the pronuclei marks the end of fertilisation and the zygote begins to divide (cleavage). From this moment on, it is referred to as an embryo.
Day 2-4: During the transport through the Fallopian tube, cell divisions continue, successively producing four-cell, eight-cell and then multi-cellular stages.
Day 5-6: When the cell number reaches between 32 and 58, the blastocyst begins to form: the outer cells differentiate into the surrounding trophoblast and the inner cells into the embryoblast (inner cell mass).
Day 7-9: Before its nidation into the endometrium, the blastocyst detaches from the protective zona pellucida, a process called "hatching".
For an overview of the course of early embryonic development with particular regard to the ethically and legally relevant phenomena of totipotency and pluripotency, see Beier 2000.
Beier, Henning M. (2000): Zum Status des menschlichen Embryos in vitro und in vivo vor der Implantation (On the status of the human embryo in vitro and in vivo prior to implantation). In: Reproduktionsmedizin 16(5), 332-342.