First Cell Cycle
Abbildung 1: Timing of the first cell cycle of a human zygote. The upper figure line indicates the biological occurrence in oocytes from fertilization to first cleavage. Following sperm penetration, the second meiotic division is completed. After first polar body extrusion, the G1-phase of the first cell cycle begins, in which male and female pronuclei appear. The G1-phase is followed by the S-phase in which the chromosomes replicate. It is followed by the G2-phase in which DNA synthesis cannot be observed any more. The M-phase of the first cell cycle begins with the disappearance of pronuclei and lasts until completion of the first cell division. 0PN = no pronucleus; 1PB = one polar body; G1, S, G2, M: phases of cell cycle [Author permission granted by P. Fancsovits]


Keywords: cell cycle,  Schema,  scheme,  Zellzyklus,  Zygote,  Zygote

Slow-cleaving - Fast-cleaving
Abbildung 2: Comparison of the development of slow- and fast-cleaving embryos from fertilisation to the 4-cell stage. Pronuclear breakdown and first cleavage occur earlier in faster-developing embryos than in those with slow development. The time-span between the development of faster- and slower-developing embryos can be up to 8–12 hours. It can easily occur that faster- and slower-developing embryos have the same number of blastomeres at the time of morphological assessment performed before embryo transfer. 0PN = no pronucleus; 2PN = two pronuclei; 1PB = one polar body; 2PB = two polar bodies; 1C, 2C, 4C = 1-, 2- and 4-cell stage of the embryo; *Too early assessment of pronuclear breakdown does not help to distinguish early- and late-developing embryos. **Morphological assessment in the indicated period does not allow differentiation of early- and late-developing embryos. Assessment of early pronuclear breakdown or early cleavage can help to determine viability in case of embryos with similar morphological characteristics. [Author permission granted by P. Fancsovits].


Keywords: 4-cell-stage,  4-Zell-Stadium,  Embryo,  Embryo,  Fertilisation,  Fertilisation,  Schema,  scheme