LOSS OF TELOMERASE ACTIVITY DURING MALE GERM-CELL DIFFERENTIATION

Although the activity of telomerase, an enzyme which synthesizes telom eres de novo and stabilizes telomere length has been demonstrated in t he testis, the precise expression of activity in different germ cell t ypes is not known. We examined telomerase activity using a PCR-based t elomeric repeat amplification protocol during development of the rat t estis from birth to adulthood. Telomerase activity was relatively high from birth to the 4(th) week of age, and then low between the 5(th) t o 10(th) week, suggesting that the type A spermatogonial stem cells ma y be the population which is expressing the highest levels of telomera se activity. To ascertain which germ cells expresses the telomerase ac tivity, purified populations of type A spermatogonia from 9-day old ra ts, and pachytene spermatocytes, round spermatids and epididymal sperm atozoa from adult rats were isolated. While type A spermatogonia expre ssed very strong telomerase activity, the fractions containing pachyte ne spermatocytes and round spermatids also expressed telomerase activi ty, but at comparatively lower levels. Telomerase activity was totally absent in epididymal spermatozoa. Thus, it appears that the telomeras e activity is expressed at high levels in the type A spermatogonial st em cells, is down-regulated during spermatogenesis, and is absent in t he differentiated spermatozoa.