Diffusion-mediated control of myoblast fusion

Abstract

Previous observations suggest that myoblast fusion in cultures of embryonic avian muscle cells occurs when a threshold level of cell density is attained. Our results implicate some diffusion-mediated, cell density-dependent control over the time of initiation of fusion. Microcultures of uniform diameter, prepared from embryonic avian breast muscle cells were positioned on collagen-treated petri plates. Varying inoculum sizes as well as the volume of medium demonstrates that the onset of fusion is delayed by either decreasing the inoculum size or by increasing the volume of medium. Furthermore, initiation of fusion is also delayed by continuous circulation of the medium. This delay can be circumvented by increasing the initial cell density or by using media withdrawn from cultures of fusing myoblasts. These results suggest a transmission of the effect of cell density via the medium. Whenever fusion is experimentally delayed proliferation continues, at an undiminished rate, for a longer period of time generating a larger cell population. Thus, although close proximity is a necessary condition for fusion it is not a sufficient condition. It is not presently known whether fusion is promoted by the accumulation of cell products or by the depletion of media constituents. Activity is associated, however, with the high molecular weight fraction (> 300,000) of the medium and is not generated by any of three other cell types assayed.