Developmental changes in calcium dynamics, protein kinase C distribution and endoplasmic reticulum organization in human preimplantation embryos.

Developmental changes in the Ca2+ dynamics of human zygotes and preimplantation embryos were related to changes in the distribution of endoplasmic reticulum (ER) and protein kinase C (PKC). The fertilization-induced Ca2+ oscillations were typically observed over > 5 h, were ryanodine-sensitive and showed a periphery-to-centre propagation of Ca2+ waves. At the same time, ER and PKC were accumulated in the cell periphery. After the appearance of pronuclei, ryanodine-sensitive Ca2+ oscillations of lower amplitude and frequency were observed until the pronuclear breakdown. However, Ca2+ waves then began in the perinuclear region, in the area of ER and PKC accumulation and spread towards the cell periphery. During the second to fourth cell cycle, small sinusoidal Ca2+ fluctuations were observed; sparse higher-amplitude Ca2+ spikes, superimposed on these basal fluctuations, appeared shortly before cell division. The sinusoidal Ca2+ fluctuations were asynchronous in individual blastomeres and disappeared progressively in arrested embryos. The direction of Ca2+ wave propagation and the distribution of ER and PKC were similar to the situation observed in pronuclear zygotes. In contrast to the zygotes, ryanodine did not arrest the Ca2+ oscillations but augmented their amplitude and frequency. These data suggest that human pre-embryos use different mechanisms of Ca2+ signalling in the early post-fertilization period, during the pronuclear development and during cleavage.