Microinsemination and nuclear transfer using male germ cells.

Microinsemination has been widely used in basic reproductive research and in human-assisted reproductive technology for treating infertility. Historically, microinsemination in mammals started with research on the golden hamster; since then, it has provided invaluable information on the mechanisms of mammalian fertilization. Thanks to advances in animal genetic engineering and germ-cell technologies, microinsemination techniques are now used extensively to identify the biological significance of genes of interest or to confirm the genetic normality of gametes produced by experimental manipulations in vitro. Fortunately, in mice, high rates of embryo development to offspring can be obtained so long as postmeiotic spermatogenic cells are used as male gametes-that is, round spermatids, elongated spermatids, and spermatozoa. For some other mammalian species, using immature spermatogenic cells significantly decreases the efficiency of microinsemination. Physically unstable chromatin and low oocyte-activating capacity are the major causes of fertilization failure. The youngest male germ cells, including primordial germ cells and gonocytes, can be used in the construction of diploid embryos by nuclear-transfer cloning. The cloned embryos obtained in this way provide invaluable information on the erasure and reestablishment of genomic imprinting in germ cells.