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Spontaneous and induced fusion of intact and acrosome removed spermatozoa with mouse fibroblasts (L cells) was studied in vitro. In the experiments with acrosome-removed spermatozoa some features similar to physiological fertilization were seen during the process of spontaneous heterokaryon (Hk) formation. Even in the initial stage of co-incubation the treated spermatozoa generally adhered to the surface of somatic cells by the posterior region of the head. The penetration of acrosome-removed spermatozoa to the cytoplasm of L cells then occurred but was markedly delayed. Penetration of L cells by non-treated spermatozoa did not occur. The observed triggering of sperm chromation decondensation in the heterokaryon indicates a “true membrane reaction” during the process of fusion of acrosome-removed spermatozoa and L cells. The degree of sperm head decondensation remained incomplete in all cases detected and only 10−3 Hk contained decondensed chromatin of spermatozoal origin. It is concluded that a true membrane reaction may be induced between spermatozoa and somatic cells, after acrosome destruction by simple physico-chemical agents, and without “fusogenic” agents.

General labeling of ripe mouse spermatozoa was obtained two weeks after six injections of L-arginine-5-3H monohydrochloride, administered within two days. Besides tail labeling a very intensive nuclear labeling was obtained. It is presumed that this nuclear labeling represents almost exclusively tritiated arginine incorporation into basic nuclear protein. Mouse oocytes were fertilized in vitro with such labeled spermatozoa. It was found that the nuclear label is lost very early during male pronucleus formation, probably concomitantly with sperm chromatin decondensation. This might indicate total loss of the basic spermatozoal nuclear protein before male genome activation.