Ultrastructural changes in hamster spermatogenic cell nuclei after incorporation into homologous oocytes by electrofusion

Isolated hamster spermatogenic cells at various stages of spermatogenesis were examined by thin-sectioning techniques after electrofusion with activated homologous oocytes. The nuclei and attached organelles of the cells remained almost unchanged within the ooplasm three to five hours after the fusion pulse, by which time the oocytes had developed to the pronuclear stage. Only the spherical nuclei of primary spermatocytes and early spermatids in the Golgi and cap phases underwent modifications in their fine structures. They gained the morphological characteristics of well-developed mammalian pronuclei; e.g., electron-dense round nucleolus-like bodies and blebbing of the nuclear envelope. In contrast, the elongated nuclei of later spermatids in the acrosome and maturation phases retained their original features, except that their acrosomes were deformed. Thus, ooplasm-mediated transformation within activated oocytes at the pronuclear stage occurred only in nuclei containing dispersed chromatin and having nuclear pores in the envelope. Mol. Reprod. Dev. 52:66-73, 1999. (C) 1999 Wiley-Liss, Inc.