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The effect of prolonged exposure to a hypertonic medium on human lymphocytes during mitogenic stimulation with phytohemagglutinin was investigated. The process of chromatin decondensation during the first 24 hrs stimulation (G0 to G1 transition) and the changes in kinetic parameters and the occurrence of chromosome aberrations from 48 hrs to 72 hrs of stimulation were studied. In HT medium, lymphocyte transition from G0 to G1 was slowed; there were fewer S-phase cells, after 48 hrs PHA stimulation, whereas after 72 hrs the resistant cells showed the same frequency of S-phase cells as the controls. The mitotic index was always smaller, and the frequency of G0/G1 cells larger. No significant increase in the frequencies of chromosome aberrations were found. These findings suggest that human peripheral lymphocytes can survive and grow in a hypertonic medium; chromosome damages, if not repaired, may be lethal, and only lymphocytes with normal karyotypes can survive for long times in the HT medium, although with modified kinetic characteristics.

A method is proposed to evaluate the amount of DNA resistant to the C-banding pretreatments (C-heterochromatic-DNA) in metaphase chromosomes. Measurements were performed by microfluorometry on propidium iodide stained metaphases of man, gorilla and mouse; in these species, C-heterochromatin exhibits significant differences of both base composition and distribution along the chromosomes. The amount of C-heterochromatic-DNA was found to be about 16%, 28% and 58% of the total DNA content (genome size) in man, gorilla and mouse, respectively. The areas of C-bands after Giemsa staining were also assessed by microdensitometry, and corresponded to about 8%, 15% and 14% of the total karyotype area of man, gorilla and mouse respectively. No direct relation thus exists between C-band areas and the amount of DNA resistant to the C-banding pretreatments. In man and gorilla, the amount of C-heterochromatic-DNA accounts for the differences observed in genome size.