Sperm nuclear enlargement in fertilized hamster eggs is related to meiotic maturation of the maternal chromatin

Experiments were conducted to test the hypothesis that sperm nuclear expansion in fertilized hamster eggs is correlated with meiotic processing of the maternal chromatin. In vitro fertilized hamster eggs were fixed at regular intervals following insemination, stained with the DNA specific fluorochrome, Hoechst 33342, and the extent of sperm nuclear expansion measured. Sperm nuclei enlarged in multiple, distinct phases (A-E) that were temporally correlated with meiotic processing of the maternal chromosomes: phase A, metaphase II; phase B, early anaphase II; phase C, late anaphase II, and phases D and E, female pronuclear development and expansion. During phase A, sperm nuclei were unchanged but enlarged at different rates during phases B (272 microns2/min), D (106 microns2/min) and E (29 microns2/min), and condensed during phase C (rate = -102 microns2/min). Area increases of both sperm nuclei and female pronuclei during phase D were significantly less in polyspermic and polygynic zygotes. If sperm nuclear enlargement and the status/activity of the maternal chromatin were correlated, it would be anticipated that alterations in the normal progression of meiotic maturation would be manifested in sperm nuclear expansion. The following agents affected both meiotic maturation and sperm nuclear enlargement: colchicine, antimycin A, and puromycin. These observations suggest that processes attending sperm nuclear transformations, the completion of maternal meiotic maturation, and development of the female pronucleus are coupled and may be linked by common regulatory agents.