Isolated smooth muscle cells and fibroblasts from the newborn guinea-pig vas deferens were grown in culture. In the first 2 days, all cells characterized as smooth muscle by phase-contrast microscopy reacted intensely with fluoresceinated antibodies against smooth muscle myosin. The fluorescence was in the form of particles (termed here "myosin aggregates"), which were often aligned to give the cell a striated appearance. After 3-5 days, coarse fluorescent fibrils were also visible. These were termed "attachment fibrils" ("A-fibrils") since they were thought to represent myosin in microfilament bundles. Between 6 and days in culture, the smooth muscle cells began to dedifferentiate morphologically. At this time, the "myosin aggregates" became clumped and less intensely fluorescent. "A-fibrils" also decreased in fluorescence intensity. By 8 days in culture, the dedifferentiated cells had undergone intense proliferation and gave only a minimal reaction with myosin antibodies. However, when a confluent monolayer of cells formed on day 9 or 10, they immediately began to redifferentiate ultrastructurally and to regain immunofluorescence in both "myosin aggregates" and "A-fibrils". Throughout the entire culture period, cells characterized as fibroblasts by phase contrast microscopy gave only a weak reaction with fluoresceinated antibodies to myosin showing "A-fibrils" but no "myosin aggregates".
Explants and enzyme-dispersed cells of adrenal medulla from 10-12 day old rats were studied in culture for up to 3 weeks. Adrenomedullary chromaffin cells, nerve cells and satellite cells were clearly discernible. The nerve cells were few in number and did not show catecholaminespecific fluorescence. Chromaffin cells stored catecholamines, as judged by the Falck and Hillarp method, in varying amounts decreasing with age of the cultures and the distance from the explants. Exocytosis profiles observed with the electron microscope suggested that cultured chromaffin cells also released catecholamines. Moreover, the cells formed processes and frequently migrated into the outgrowth. After 6 days in culture, the great majority of chromaffin cells stored noradrenaline as revealed by electron microscopy with few adrenaline-storing cells being visible. Granular vesicles (approximately 80-240 nm in diameter) with cores of different electron densities were occasionally present in the same cell suggesting the occurrence of mixtures of primary and secondary amines. Apart from "chromaffin" granules, small clear and dense-cored vesicles (approximately 40-60 nm) were found both in the somata and cell processes. Chromaffin cells and their processes were often closely apposed and occasionally formed specialized attachment zones. As a whole, chromaffin cells in culture resembled small granule-containing cells in sympathetic ganglia. 0.5 mM dbcAMP prevented dedifferentiation of chromaffin cells as judged by the lack of processes, the size and amount of "chromaffin" granules and the high number of adrenaline-storing cells present after 6 days in culture. NGF caused a striking increase in the number of axons growing out from explants.
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