Early G1 in the male rat meiotic cell cycle is hypersensitive to N-methyl-N-nitrosourea-induced micronucleus formation.

Abstract

The effects of the known carcinogenic and teratogenic agent N-methyl-N-nitrosourea (MNU) were studied on male rat meiosis. To examine possible cell-cycle delay, an immunohistochemical technique based on 5-bromo-2'-deoxyuridine (BrdU) labelling of S-phase cells was developed. BrdU tablets were implanted subcutaneously in adult male rats. A single i.p. injection of 10 mg/kg of MNU was given simultaneously. After 16-22 days, preparations of stage I of the seminiferous epithelium were made and stained immunohistochemically using anti-BrdU antibodies. MNU did not cause any significant meiotic delay, but did cause a slight non-significant reduction of the percentages of BrdU-labelled step 1 spermatids at 18 days (80%) compared to controls (95%). In addition, the induction of meiotic micronuclei was studied after short (1-3 days: late meiotic stages) and long (16-22 days: early spermatocytes and B spermatogonia) exposure times. The peak induction occurs between 21 and 20 days, indicating that the M-G1 transition or the very beginning of G1 of the cell cycle of primary spermatocytes are the most sensitive stages of the action of MNU. The number of step 1 spermatids decreased dramatically in animals treated for 22 days, denoting a highly toxic effect on type-B spermatogonia. No unscheduled DNA synthesis was detected in any meiotic stage of spermatogenesis by using this BrdU labelling method. The results indicate that the spermatid micronucleus test based on microdissection of seminiferous tubules can accurately point out the most sensitive stage for chemically induced clastogenesis. Moreover, the BrdU-immunohistochemical application enables the simultaneous study of cell cycle kinetics.(ABSTRACT TRUNCATED AT 250 WORDS)