Abstract
BUdR at low levels (10 −7 M) can completely but reversibly inhibit the expression of differentiation by chondrocytes grown as single cell derived clones in a modification of Ham's medium F10. The effect of BUdR can be reversed by thymidine and uridine. Uridine itself stimulates differentiation, while the lack of thymidine and the drug aminopterin, which inhibits thymidine synthesis, interfere with expression of differentiation. Uridine reversal of the BUdR inhibition is blocked by aminopterin. Neither BUdR, aminopterin, or thymidine deprivation have significant effects on plating efficiency of the cells. BUdR-treated cells grow as apparent “fibroblasts” in the presence of the drug, but can revert to differentiated chondrocytes upon its removal. It is suggested that there exists a type of DNA synthesis necessary for cell differentiation and different from that involved in S phase cell DNA replication. Selective gene amplification such as is found in dipteran salivary chromosomes and during nucleolar production in oogenesis is suggested as possibly playing an important role in the maintenance and expression of the differentiated state in growing cells.
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