Origin and maturation of centrioles in association with the nuclear envelope in hypertonic-stressed sea urchin eggs

Unfertilized sea urchin eggs were parthenogenetically activated via a prolonged hypertonic treatment. The continuous subjection to the osmotic stress turned on the cell cycle and brought about the very slow development of mature centrioles. Within 1 h of the exposure, large osmiophilic aggregates were detected at the nuclear surface and were interpreted as early centriolar precursor forms. Ultrastructural examination of eggs during the 8 h of treatment revealed that the precursor forms systematically converted into mature centrioles which then produced daughter centrioles. All centriolar precursor bodies were associated with microtubules; cartwheel structures were the earliest morphological feature of centrioles detected within the osmiophilic bodies. Nascent centrioles appeared at the nuclear surface and, during their development into centrioles, continually maintained intimate associations with the nuclear envelope. We suggest that the resultant modification of the intracellular environment via the hypertonic stress promotes the activation of centriolar 'information-bearing-residues' or 'seeds' located at the nuclear envelope. The oocyte centrioles, before disappearing from the maturing egg, may have produced these 'seeds' and left them at the nuclear surface.