Hormonal and genetic control of germ cell apoptosis in the testis.

Programmed cell death is an evolutionarily conserved cell death process that plays a major role during normal development and homeostasis. In many cases, the ordered execution of this internal death programme leads to typical morphological and biochemical changes that have been termed apoptosis. The crucial role of this mode of cell death in the pathogenesis of diverse human diseases including cancer, acquired immunodeficiency syndrome, neurodegeneratives disorders, atherosclerosis and cardiomyopathy is now supported by a wealth of data. In adult mammals, including humans, germ cell death is conspicuous during normal spermatogenesis and plays a pivotal role in sperm output. Withdrawal of gonadotrophins and testosterone further enhances the degeneration of germ cells in the testis. The availability of a quantitative method for analysing the testicular DNA fragmentation and in situ methods to localize specific germ cells undergoing apoptosis, either spontaneously or in response to a variety of death triggering signals, opens new avenues in the understanding of the significance of germ cell apoptosis during normal and abnormal states of spermatogenesis. A growing body of evidence demonstrates that both spontaneous (during normal spermatogenesis) and accelerated germ cell death triggered by deprivation of the gonadotrophic support or moderately increased scrotal temperature in adult rats occur almost exclusively via apoptosis. Although there has been spectacular progress in the understanding of the molecular mechanisms of apoptosis in various systems other than spermatogenesis, elucidation of the biochemical and molecular mechanisms by which germ cell apoptosis is regulated has only just begun. It is likely that germ cell apoptosis is controlled in a cell-type specific fashion, but the basic elements of the death machinery may be universal. In addition, there is increasing evidence that homozygous disruption of a number of genes in mice results in infertility through accelerated germ cell apoptosis. Manipulation of spermatogenesis by survival factor(s) deprivation or increases in extrinsic death signals in loss-of-function or gain-of-function mouse models provides a basis for further attempts to define the intrinsic regulation of various death-related genes by external death signals. Such information is crucial for effective management of male factor infertility as well as more targeted approaches to male contraception.