2ND WEEK OF EMBRYONIC DEVELOPMENT
IMPLANTATION (2ND WEEK) Implantation of the blastocyst begins at the end of the first week and is complited by end of the second week. The cellular and molecular events related to implantation are complex. It involves a receptive endometrium and hormonal factors Estrogen progesteron prolactin growth factors HOX genes T he Zona pellucida degenerates (day 5). Its disappearence results from enlargement of the blastocyst and degeneration caused by enzymatic lysis. The lytic enzymes releases from the acrosomes of the sperm that surround and partially penetrate the zona pellucida. The blastocyst adheres to the endometrial epithelium mainly seen at (DAY 6) The trophoblast differentiate into two layers syncytiotrophoblast cytotrophoblast (Day 7) The syncytiotrophoblast erodes enometrial tissues and the blastocyst starts to embed in the endometrium (day 8) Blood filled lacunae appear in the syncytiotrophoblast (day 9) The blastocysts sinks beneath the en...