Coronary vascularization during development in the rat and its relationship to basic fibroblast growth factor.

OBJECTIVE Our overall aims were to elucidate the temporal and spatial sequence of coronary vascularization during development in the rat, and to determine whether basic fibroblast growth factor expression corresponds to any phase of the vascularization process. METHODS Immunohistochemical, histochemical, morphometric and in situ hybridization analyses were performed on prenatal and postnatal hearts of various ages. RESULTS Coronary vascularization, which begins at embryonic day 13 (E13) with blood island-like structures in the epicardium, progresses from this layer toward the endocardium as indicated by a transmural gradient of vascular volume throughout the ventricles. Vascular smooth muscle first appears in E17 hearts at the time a capillary-like plexus coalesces and penetrates the aorta to form the main coronary arteries. These vessels maintain an anastomatic morphology and must undergo subsequent remodeling in order to assume adult branching characteristics. The early postnatal period is characterized by development of the arterial tree and the enzymatic differentiation of the arteriolar and venular ends of the capillary bed. Although bFGF is expressed both prenatally and postnatally, the highest mRNA expression was noted during the early period of vascularization (E14 and E15), and the early neonatal period (1-6 days) which corresponds to a period of substantial microvascular growth. CONCLUSIONS Coronary vascularization follows a temporal sequence which includes transmural expansion of the capillary bed, arteriolar formation subsequent to vascular penetration of the aorta, and postnatal growth, differentiation, and remodeling. Since high levels of bFGF expression are correlated with key time points in coronary vascular growth, bFGF may play an important role in this process.