IMPLANTATION
Implantation is the process where the embryo becomes attached to a nutritional substrate. The
term is applied generally for those embryos which become associated intimately with the
uterine wall. Moreover, it is well known that the embryos of elasmobranch and teleost fishes
as well as those of reptiles, birds and prototherian mammals become attached to yolky
substrate of the egg. Here we shall be discussing the implantation of mammals and it includes
the attachment of the blastocyst to the epithelial lining of the uterus, the penetration of the
blastocyst through the epithelium and its embedding in the compact layer of the
endometrium.
Mechanism of implantation
After ovulation, the freshly released ovum may be enveloped by few or many follicular cells
forming corona radiata. In Fallopian tube the numerous sperms reach through vagina, uterus
etc. secretes an enzyme named hyaluronidase to fertilize the ovum. It dissolves the follicular
cells or corona radiata that envelop the ovum. Before entering the uterus, the fertilized egg is
almost devoid of any follicular cell. Here the blastocyst remains free within the uterus for
about four days. Thus, the blastocyst comes into direct contact with the uterine epithelium
and makes further growth possible.
At the same time the cells of the uterine epithelium in the area of attachment begins to break
down, apparently as the result of some digestive enzyme secreted by the trophoblast. This
erosion creates a gap in the epithelium through which the invading trophoblast advances and
comes into relation with the connective tissue beneath. The implantation site for man (and
