FGF-2-induced imbalance in early embryonic heart cell proliferation: A potential cause of late cardiovascular anomalies

Background: This laboratory previously demonstrated that placement of fibro blast growth factor-2 (FGF-2)-soaked beads adjacent to the developing ventr icle at stage 24 caused cardiovascular anomalies by embryonic day 15. We so ught to characterize early cellular changes that may suggest mechanisms for the abnormalities observed at day 15. Because levels of both myocyte proli feration and immunohistochemically detectable endogenous FGF-2 begin to dec line before stage 24 in untreated embryos, it was of interest to determine whether exogenous FGF-2 might maintain cardiac myocyte proliferation at or near peak levels. Methods: Chick embryos were incubated to stage 18 (2.8 days), at which time beads soaked in phosphatebuffered saline (PBS) or 100 mu g/ml FGF-2 were p laced adjacent to the developing ventricle and development was allowed to c ontinue. After 3 days (stage 29), bromodeoxyuridine (BrdU) was applied to m ark dividing cells, followed by double fluorescent assessments to detect re lative numbers of dividing and nondividing cells. Results: Quantitative image analysis, using Metamorph software, showed that exogenous FGF-2 caused a 62% increase in the overall number of dividing ce lls (P < 0.01), concomitant with a 25% increase in total cell number (cell density: P < 0.05). Expressed in relative terms, these changes corresponded to a 25% increase in the proliferation labeling index: 30% of all cells we re proliferating in FGF-treated hearts, in contrast with only 24% in contro l hearts. Conclusions: Taken together, these data suggest that an FGF-induced imbalan ce in myocardial cell proliferation at early developmental stages of heart development causes cardiovascular anomalies during late embryogenesis. (C) 2000 Wiley-Liss, Inc.