New ultrastructural observations of human oocyte smooth endoplasmic reticulum tubular aggregates and cortical reaction: update on the molecular mechanisms involved

Abstract

IntroductionThe ultrastructure of the human mature metaphase-II oocyte before and after the cortical reaction has been previously described. However, we found a new vesicular aggregate associated with smooth endoplasmic reticulum tubular aggregates (aSERT) and new observations regarding the cortical reaction. ObjectivesTo describe at the ultrastructural level a novel vesicle aggregate associated with aSERT and new observations regarding the cortical reaction. Materials and methodsDonated, surplus mature oocytes were processed for transmission electron microscopy immediately after recover. Calcium detection was performed using the pyroantimonate technique. The cortical reaction was artificially induced by ionophore treatment. ResultsWe observed an accumulation of small vesicles at the periphery of aSERT. At high magnification these were composed by small pale vesicles coated by tiny vesicles, with associated dense materials, giving a rosette-like appearance. Adjacent to these there was another group of small dense vesicles incompletely coated by similar tiny vesicles. Using calcium detection at the ultrastructural level, antimonate deposits were observed in the tubules of aSERT and in the surrounding mitochondria but not in the rosette-like structures. Regarding cortical vesicles, although most previous studies described their contents as homogeneous dense, others referred the presence of another type of cortical vesicles whose contents were moderate dense. Using ionophore oocyte activation, we observed that moderate dense cortical vesicles may correspond to a progressive swelling of the dense cortical vesicles prior to exocytosis. ConclusionsWe describe a novel vesicle aggregate associated to aSERT and new observations on the remodeling of cortical vesicles before exocytosis.