What is implantation; how is it brought about?
Implantation refers to the attachment of the developing blastocyst into the uterine wall. This is necessary for two reasons -1. To fix the blastocyst to the uterine wall so than it does not slip down and 2. To absorb nutrition from the uterine wall which is necessary for its growth until the time of parturition (birth)? Implantation usually takes place about 5 to 7 days after ovulation. The part of the blastocyst which has the embryonal knob lies against the endometrium (inner wall) of the uterus. By now the zona pellucida around the blastocyst disappears enabling the blastocyst to come into direct contact with the endometrium.
This contact triggers rapid division of the rophoblast cells which destroy the endometrium cells. The destruction is partly physical and partly enzymatic. As a result of this, the blastocyst sinks into a small depression formed in the endometrium. Subsequently the blastocyst will be completely surrounded by the endometrium which grows over the blastocyst. The blastocyst produces a number of projections all around to obtain the nourishment.
The endometrium which overgrows the blastocyst becomes rich in vascular supply and appears spongy. It is now called decidua (to shed) and refers to the pregnant endometrium. The different regions of the decidua are assigned different names and these are called-
1. Decidua basalis (decidua serotina). This refers to that part of endeometrium which is directly underlying the embryo.
2. Decidua capsularis (decidua reflexa). This is the thin portion of the endometrium that covers the implantation region.
3. Decidua parietalis (decidua vera). This refers to the endometrium lining the uterus other than the implantation region.
The pattern of implantation of the blastocyst varies in different mammals. In human beings it is said to be interstitial as the blastocyst burrows into the endometrium. The other two types of implantation are eccentric and superficial.