Implantation:
The attachment of the blastocyst to the endometrium of the uterine wall which occurs 7-8 days after fertilization is called implantation.
Site of Implantation:
1. A fertilized ovum starts dividing rapidly to form a morula (16-cell stage) and form it blastocyst (100-cell stage). The blastocyst has an inner cell mass called a trophoblast surrounded by zona pellucida. This trophoblast cell layer has great sticking properties to the epithelial cells of the fallopian tube.
2. Within the next 3-4 days blastocyst is transported into the cavity of the uterus. In that cavity blastocyst first floats for some time and after that zona pellucid disappears and the trophoblast cell layer is exposed.
3. Due to its high sticking property trophoblast cell layer come in contact with the hormonally prepared endometrium of the uterus. Trophoblast has an inner layer called cytotrophoblast and an outer layer syncytiotrophoblast from which later the placenta will develop.
4. Now, syncytiotrophoblast secretes proteolytic enzymes that digest and liquefy the endometrium cells, so that blastocyst erodes and burrows into the endometrium resulting in implantation.
5. The blastocyst goes deepen into the uterus until the whole of it lies within the endometrium.