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The eggs once retrieved from the ovary are cultured and assessed for their maturity and the matured MII oocytes are inseminated. The oocyte and sperm quality profoundly affects the fertilization rate and embryo development, thus evaluating their morphology on Day 0 is very important.
Post insemination within 17-20 hours the PNs are seen on Day 1 which indicates successful fertilization where in the embryo growth begins
And on Day 2 the embryo starts division in even patterns and on Day 4 the blastomere cells of embryos combine together into a mass, the stage is called as compaction or morula stage. Where in the cell lineages are decided.
On Day 5 the embryo attains the blastocyst stage where there are two differentiated cell lineages which are trophectoderm cells (TE) and inner cell mass (ICM). The trophectoderm cells will form placenta in later stages. And this is graded as per schoolkraft and David gardner grading system.
In normal conditions (natural conception), after fertilization, the embryo grows until the blastocyst stage. The blastocyst grows as the cells divide and the cavity expands, where it “hatches” from the zona pellucida to implant into the endometrium. The blastocyst is the highest degree of development that an embryo can reach in vitro.
The factors influencing the development of blastocyst invitro are the age of the woman, the number of oocytes retrieved and the intrinsic and culture conditions. The embryos that develops till blastocyst stage are lesser in number than fertilized ones. This is majorly because of the metabolic block of the embryo invitro during the cleavage stages due to maternal, paternal and embryonic genomes. This metabolic block is overcomed by blastocyst stage embryos, even though the number of embryos available for freezing may be lower but the pregnancy rates following the transfer of a blastocyst is better. In in vitro fertilization (IVF), the blastocyst culture is important to increase the success rate of IVF because of better embryo selection after better genomic activation and endometrial receptivity.