ALL-TRANS-4-OXO-RETINOIC ACID - A POTENT INDUCER OF IN-VIVO PROLIFERATION OF GROWTH-ARRESTED A-SPERMATOGONIA IN THE VITAMIN-A-DEFICIENT MOUSE TESTIS

Vitamin A deficiency leads to an arrest of spermatogenesis and a loss of advanced germ cells in male mice. In the present study, the effects of several retinoids and carotenoids on these mouse testis were inves tigated. First, the proliferative activity of the growth-arrested A sp ermatogonia in vitamin A-deficient (VAD) mice testis was determined, 2 0, 24, or 28 h after administration of 0.5 mg all-trans-retinoic acid (RA). The bromodeoxy-uridine (BrdU) labeling index of A spermatogonia in control VAD testis was 5 +/- 1% (n = 4, mean +/- SD). When RA was i njected (ip), the highest labeling index was found 24 h after RA admin istration: 49 +/- 5%. When various concentrations of RA, all-trans-4-o xo-retinoic acid (4-oxo-RA) or all-trans-retinol acetate (ROAc), rangi ng from 0.13-1 mg, were injected, the labeling index of A spermatogoni a always increased in comparison with the VAD situation. A maximum ind ex at 24 h was found when 0.5 mg 4-oxo-RA was injected: 56 +/- 3%. Thi s labeling index was even higher than those after injection of RA or R OAc, 49 +/- 5% and 34 +/- 6% respectively. The increase of the BrdU la beling index was dose dependent. After an initial increase of the labe ling indices with increasing retinoid doses, the labeling indices decr eased at a higher concentration. This decrease is likely due to a conc entration dependent timeshift of the optimum of BrdU labeling to short er time intervals after retinoid administration because a labeling ind ex of 66 +/- 1% was found 20 h after injection of 1 mg RA. At 24 h, th is labeling index was halved: 33 +/- 2%. These indices show that the d egree of synchronization of spermatogenesis is also dependent on the r etinoid dose. When the dimers of RA and 4-oxo-RA, respectively beta-ca rotene (PC) and canthaxanthin, were given, 24 h after administration B rdU-labeling indices comparable with the VAD value were found. Repeate d injection of beta C twice a week did induce a reinitiation of sperma togenesis, but compared with RA, the activity of beta C was lower and delayed. It is concluded that 4-oxo-RA is active in adult mammals in v ivo. It is at least as potent as RA in the induction of the differenti ation and subsequent proliferation of growth-arrested A spermatogonia in VAD mice testis. Furthermore, the degree of synchronization of sper matogenesis is influenced by the retinoid dose. Finally, carotenoids w ere shown to act in the induction of spermatogonial cell proliferation too but with a lower and delayed activity.