Impact of genetic engineering on the understanding of spermatogenesis.

To date, about 100 genes have been found, by genetic engineering, to be implicated in spermatogenesis. Primordial germ cells, spermatogonia, spermatocytes I and elongating spermatids are particularly sensitive. Transgenic and knockout mice permit an approach to be made to the question of genetic factors involved in DNA damage repair, thermal injury, sperm chromatin compaction and sex-specific recombination. Knockout mice reveal unexpected functional redundancies of testis-specific genes. This review considers how functional divergences can exist among homologous genes from different species, and to what extent the phenotypes of knockout mice can be similar to those from spontaneous mutations. Additional anomalies in reproductive function have frequently been found in these mice, as were found factors leading to tumour susceptibility and/or various diseases. Finally, knockout mice remind us that, in nearly all cases, hemizygous individuals retain a fertility and a wild-type sperm phenotype, although half of the spermatozoa share a genetic defect. The findings strongly emphasize the importance of understanding epidemiology in male infertility, to identify hereditary forms of impaired spermatogenesis, and to create DNA and pathological germ cell banks.