Proliferation and differentiation of smooth muscle cell precursors occurs simultaneously during the development of the vessel wall.

Formation of the blood vessel wall depends on the recruitment, proliferation, and differentiation of smooth muscle cell (SMC) precursors. The temporal events associated with the onset of expression of several SMC proteins have been well characterized in mouse and avian species. However, the timing of cell proliferation during this process has not been explored. More importantly, it has not been clear whether commitment to the smooth muscle pathway precludes proliferation during development. In the present study, we have determined the kinetics of replication in developing chick aortae between days 2.5 and 19 and have correlated these data with the expression of various SMC differentiation markers. We found that proliferation of aortic SMC precursors occurs in two waves; an early phase of rapid proliferation (15-17%; between days 4 and 12), and a second phase, when replication was reduced to less than 5% (days 16 to hatching). Proliferation of SMC during the first wave occurred concomitantly with the progressive accumulation of SMC contractile proteins, such as SM alpha-actin, calponin, myosin heavy chain, and the 1E12 antigen. We also found that the relative proliferation capacity within each compartment of the vessel wall, ie., intima, media, and adventitia varies throughout development. Approximately, 55-63% of all replicating cells were found in the tunica adventitia from days 6 to 12, whereas 35% were found in the tunica media (tunica media:adventitia = 1:2). This ratio was inverted after day 12, when most of the replicating cells were located in the tunica media (tunica media:adventitia = 2:1). In addition, we observed a ventral-to-dorsal gradient in the proliferation of SMC precursors between days 2.5 and 5. The ventral-to-dorsal proliferation gradient was similar to the previously described differential expression of two early SMC markers: alpha-actin and the 1E12 antigen. These data support the concept that a polarity exists either in the pool of SMC precursors or, in expression of factors that regulate recruitment of presumptive SMC.