Hydroxyurea exposure alters mouse testicular kinetics and sperm chromatin structure

The effects of hydroxyurea (HU) on testicular cell kinetics and sperm chromatin differentiation were investigated in mice. Whole testis, minced testicular cell suspensions and caudal epididymal sperm cells were obtained at 8 and 29 days after i.p. injections containing 0, 25, 50, 100, 200, 400 and 500 mg/kg HU x 5 days. Testis weights were unaffected by 25 mg/kg HU while 500 mg/kg caused up to a 50% loss of testicular weight by 29 days. Flow cytometrically measured acridine-orange (AO) stained testicular cells revealed altered population ratios at the highest dosages at 8 days and for all dosages except 25 mg/kg HU at 29 days. At 8 days, 400-500 mg/kg HU caused a near depletion of tetraploid cells. Flow cytometry of AO stained sperm, previously treated with acid to potentially induce DNA denaturation, was used to follow the shift from normal chromatin structure to an abnormal form with increased sensitivity to DNA denaturation in situ. The extent of DNA denaturation was quantitated for each cell by the computer-derived value alpha t, alpha t = [red/(red+green) fluorescence]. The flow cytometry measures, standard deviation of alpha t (SD alpha t), mean of alpha t (X alpha t) and cells outside the main peak of alpha t (COMP alpha t), gave similar dose response curves to the sperm head morphology assay. SD alpha t was more sensitive than the X alpha t as a measure of HU-induced alteration of chromatin structure. The major conclusions reached are that HU inhibits DNA synthesis, probably by inhibiting ribonucleotide reductase, causing maturation depletion of pachytene spermatocytes and, subsequently, depletion of meiotic daughter cells and differentiated cell types leading to mature sperm. This inhibition of DNA synthesis is related to an alteration of sperm chromatin structure and abnormal sperm head morphology.