Implantation
This is the process by which the blastocyst settles into endometrium. It begins by the 6th day and completed by the 7th day after fertilization. By the 10th day, the blastocyst becomes completely imbedded into the endometrium with the point of implantation sealed up completely by the development of new epithelia lining.
Other facts associated with it include:
Proliferation of the Trophoblast into two layers i.e. the Syncytiotrophoblast (outer layer) and Cytotrophoblast (inner layer).
Hatching of the blastocyst with the shedding or disappearance of the zona pellucida by the 5th day. Note that one functions of the zona pellucida is to prevent premature implantation.
Invasion of the endometrium by the syncytiotrophoblast.
NB:
Implantation may be accompanied by slight bleeding via the vagina and this is called implantation bleeding. However, it is no longer an implantation bleeding if it is accompanied by cramping, backache or increased bleeding.
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Development of Bilaminar germ cell
By the end of Day 7 and beginning of Day 8, the inner cell mass becomes a relatively flat, circular plate of cells, called the bilaminar embryonic disc, which consists of two layers: the dorsal epiblast and ventral hypoblast.
Concurrently, a cavity develops within the epiblast, termed the amniotic cavity. This small cavity will enlarge until by the end of the fourth week, when it almost completely surround the embryo, which then "floats" within the amniotic cavity and its contained fluid, the amniotic fluid.
The epiblast is a thick layer of high columnar cells that serves as the floor of the amniotic cavity. The roof of the amniotic cavity, termed amnion, consists of cells termed amnioblasts, which originally derived from the epiblast. The epiblast is continuous at the periphery with the amnion.
The hypoblast consists of cuboidal cells. Some cells migrate from the hypoblast to line the blastocyst cavity. With the development of this lining, the blastocyst cavity becomes the exoceolmic cavity and the membrane lining it is termed the exoceolmic membrane or Heuser's membrane.
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