Translocation of the subplasmalemmal cytoplasm in human blastomeres: possible effects on the distribution and inheritance of regulatory domains

The subplasmalemmal cytoplasm of the mouse and human oocyte contains proteins, high-polarized mitochondria and for the mouse, mRNA that appears to have regulatory roles in early development. For the human, the irreversible loss of certain proteins and high-polarized mitochondria into small fragments that arise spontaneously and incorporate the subplasmalemmal cytoplasm has been suggested to have developmental consequences for the affected cell(s). Time-lapse studies have shown that fragments of this type can be resorbed during cleavage and in certain instances, their disappearance largely restores phenotypic normality to the human embryo. Time-lapse microscopy was used to examine this type of fragmentation and describe behaviours associated with their fusion or resorption, or both, in early human embryos. The transfer of fragments containing high-polarized mitochondria (ΔΨmHIGH) labelled with a potentiometric fluorescent stain between embryos, and to oocytes and parthenogenetically activated eggs, demonstrates fragment resorption by incorporation of high-polarized mitochondria in the subplasmalemmal cytoplasm. The results indicate the potential for spontaneous transposition of subplasmalemmal regulatory elements within and between cells exists in early human embryos and that loss or gain of function may have different developmental impacts on affected cells. The location of incorporated mitochondria confirms their stability in subplasmalemmal domains, and suggests the possibility that a high polarity may be a factor in spatial specification.