Question

Explain the term implantation.

Answer 1

Hint: Implantation occurs once the embryo reaches the blastocyst stage, i.e. approximately 5-6 days after fertilization. In nature, 50 percent of all fertilized eggs are lost before a woman's missed menses.

Complete answer:
Implantation is the process of embedding of the blastocyst into the endometrium of the uterus. It may occur at any period between the sixth and tenth day after fertilization. Implantation leads to pregnancy. The uterus wall is made of three layers, named perimetrium, myometrium and endometrium. During the period of implantation, the uterine wall changes its thickness and vascularized to prepare for receiving the embryo.

Till the time the zona pellucida is present, the blastocyst cannot attach itself to the uterine wall. It hatches from the zona by secreting protease enzyme trypsin which lyse the matrix of the zona. Integrins of trophoblast cells bind to the uterine extracellular matrix components like laminins, collagen, fibronectin and hyaluronic acid for firm binding of the blastocyst to the uterine wall (endometrium).

The trophoblast under the influence of Wnt proteins secretes another set of enzymes such as collagenase, stromelysin, plasminogen activator which digest the extracellular matrix of uterine wall tissue and enables the embryo to bury in the uterine wall. It then sinks into the uterine connective tissue so that by ninth day after fertilization the walls of the uterus, mainly the endometrium enclose the implanted blastocyst.

Note: Occasionally, under abnormal conditions, the blastocyst may implant in the peritoneal cavity or on the abnormal site. The epithelium at the abnormal region responds with the increased vascularity so that the blastocyst starts its development. This results in an ectopic pregnancy which may sometimes threaten the life of the mother, as blood vessels at these sites are ruptured due to the growth of the embryo.