Effects of phenol red on CFU-f differentiation and formation.

Osteoblastic induction is commonly studied using the colony-forming unit-fibroblastic (CFU-f) assay, in which bone marrow stromal cells (BMC) are grown in a tissue culture environment permissive for osteoblastic differentiation (DMEM containing dexamethasone, ascorbic acid and beta-glycerophosphate). These cells form colonies, which express alkaline phosphatase, and form a collagenous matrix that becomes calcified. However, these same cells originate in the bone marrow where under normal circumstances they do not proliferate or differentiate despite being subjected to many of the same growth factors and hormones present within the tissue culture environment. We show here that phenol red, present within tissue culture medium as a pH indicator, may itself be a factor that permits osteoblastic recruitment. BMC cultured in the presence of the bone anabolic agents PGE2, PGA2, or bFGF, but in the absence of phenol red, failed to respond to these agents in terms of total or osteoblastic colony number. This effect was dose dependent, with low (2.5 mg/l) and high (15-20 mg/l) doses of phenol red being nonpermissive for the stimulatory effects of PGE2 whereas doses of 5-10 mg/l were permissive. Furthermore, the effects observed in the absence of phenol red could not be abrogated by the addition of 17beta-estradiol indicating that these effects cannot be attributed to estrogenic impurities within the phenol red preparation. This indicates that phenol red itself can affect the differentiation of BMC by a mechanism not previously described.