Developmental potential and chromosome constitution of strontium-induced mouse parthenogenones

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Abstract
The brief exposure of recently ovulated mouse oocytes to M16 embryo culture medium supplemented with strontium chloride (M16 Sr2+) for 2–10 min was observed to induce a high incidence of parthenogenesis. A lower incidence of activation and a significant rate of oocyte degeneration was observed when oocytes were incubated in M16 Sr2+ medium for 20–60 min. The majority of the oocytes exposed to this agent for 2–10 min developed as single‐pronuclear haploid parthenogenones. The incidence of this parthenogenetic class was reduced as the duration of exposure to M16 Sr2+ was increased from 2 to 30 min. Under these conditions a greater proportion of the activated oocytes developed as two‐pronuclear diploid parthenogenones, due to failure of second polar body extrusion. The activation frequency and the proportionate incidence of the pathways of parthenogenetic development observed following the exposure of ovulated oocytes to calcium‐free M16 medium differed significantly from that induced by exposure to M16 Sr2+. Cytogenetic analysis of the single‐pronuclear haploid class of Sr2+‐induced parthenogenones at metaphase of the first‐cleavage mitosis has shown that this agent did not induce a significant increase in the incidence of chromosome segregation errors during the completion of the second meiotic division. Analysis of the developmental potential of the two‐pronuclear class of diploid Sr2+‐induced parthenogenones during the preimplantation stages of embryogenesis revealed that their cell number and rate of cell division were less than those of fertilised embryos retained either in vivo or in vitro. The novel methods of activating oocytes indicated in this study present new opportunities to improve the efficiency of embryo cloning techniques with the ruminant species.