While at earlier cleavage stages embryos resemble an accumulation of solitary blastomeres with a rudimentary level of biosynthesis, compaction phase (beginning on day 3) is characterized by increased biosynthetic rates and the capacity to metabolize glucose more efficiently. In addition, the compacting embryo is capable of actively regulating ionic gradients, thus controlling its internal environment (98).
Compaction is due to the formation and the number of tight intercellular junctions (e.g., desmosomes, gap, and tight junctions) causing blastomeres to become closely apposed (76,99). Due to this highly interactive cell mass, blastomeres loose their totipotent characteristics.
In humans, compaction begins around eight-cell stage probably following an intrinsic developmental clock. Precocious compacting at day 2 could result in formation of trophoblastic vesicles leaving no predecessor cells of inner cell mass (6). On the other hand, 16-cell embryos without the slightest evidence of compaction are of reduced capacity and will hardly reach blastocyst stage (99).
Tao and co-workers (100) successfully tried to predict implantation scoring embryos at the compaction stage. These authors showed that the implantation potential is positively related to the proportion of blastomeres undergoing compaction. Consequently, embryos had the worst prognosis if less than half of the blastomeres were involved in the compaction process. Blastomeres and fragments that are unable to form appropriate contacts are generally excluded from the compaction process and remain within the empty zona pellucida after hatching (99).
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