A cytophotometric analysis of the structure of hypothalamic cell populations in the frog, Rana temporaria (L.), with special reference to seasonal changes in chromatin status.

We used cytophotometry after the Feulgen reaction and UV cytophotometry to measure the DNA content of quiescent cells of the hypothalamic preoptic region (HPR) of adult and juvenile frogs (Rana temporaria) that had been caught in their natural habitat in winter, spring and summer. The histone-to-DNA ratio in cell nuclei was cytophotometrically determined using a combined Feulgen, heparine and alcian-blue staining procedure. The vast majority of HPR cells studied had nuclei with a diploid DNA content. However, we observed great variability in the Feulgen-DNA content of the HPR cell population, which was not detected in the diploid standard (hepatocytes). This heterogeneity in the diploid sample of the HPR cell populations was always greater in prespawning frogs and may have been due to differences in the chromatin arrangement in nuclei. About 1% of cells had a DNA content either ranging between diploid and tetraploid levels (H2C cells) or at the tetraploid level (4C and 2C x 2 cells). The proportion of these cells was not affected by the age of the animals or the annual cycle, thus suggesting that there is no age-related increase in the mean DNA content in the frog HPR. The mean DNA contents of H2C and 4C cells were much higher than those in the standard (hepatocytes). This cannot be simply attributed to the presence of different amounts of nuclear proteins, but rather indicates that at least a certain proportion of the highest DNA contents may be due to a real extra-DNA synthesis.