Nuclear and microtubule reorganization in nuclear-transferred bovine embryos

We studied the nuclear and microtubule dynamics in nonactivated and pre-activated chromatin-removed oocytes following transfer of nuclei from bovine fibroblast cells. Immediately after fusion between membranes of oocytes and fibroblasts, a microtubule aster containing a gamma-tubulin spot was seen near the transferred nucleus in most oocytes regardless of activation conditions. Most fibroblast nuclei transferred into nonactivated oocytes underwent premature chromosome condensation (PCC) and divided into two masses of chromosomes. In contrast, fibroblast nuclei in pre-activated oocytes rarely underwent PCC, but formed a swelled pronuclear-like structure. Under nonactivation condition, microtubular spindles surrounded condensed chromosomes during the division of two nuclear structures. Gamma-tubulins were detected in the vicinity of condensed chromosomes, suggesting transient spindle formation. Two pronuclear-like structures near the microtubular aster containing gamma-tubulin spot(s) later formed a syngamy-like nuclear structure. While 20% of reconstructed oocytes under nonactivated conditions developed to morulae and blastocysts, only 4% of reconstructed oocytes under pre-activated conditions developed to morulae and blastocysts. These results suggest introduction of a foreign centrosome during somatic cell nuclear transfer, which probably plays a role in nuclear remodeling and subsequent development.