AbstractDevelopment of the mammalian embryonic palate depends on the precise temporal and spatial regulation of growth. The factors and mechanisms underlying differential growth patterns in the palate remain elusive. Utilizing quiescent populations of murine embryonic palate mesenchymal (MEPM) cells in vitro, we have begun to investigate hormonal regulation of palatal cell proliferation. MEPM cells in culture were rendered quiescent by 48 hr serum deprivation and were subsequently released from growth arrest by readdition of medium containing 10 (v/v) serum. The progression of cells into S‐phase of the cell cycle was monitored by autoradiographic analysis of tritiated thymidine incorporation. Palate mesenchymal cell entry into S‐phase was preceded by a 6‐ to 8‐hr prereplicative lag period, after which time DNA synthesis increased and cells reached a maximum labeling index by 22 hr. Addition of 10 μM isoproterenol to cell cultures at the time of release from growth arrest lengthened the prereplicative lag period and delayed cellular entry into S‐phase by an additional 2 to 4 hr. The rate of cellular progression through S‐phase remained unaltered. The inhibitory effect of isoproterenol on the initiation of MEPM cell DNA synthesis was abolished by pretreatment of cells with propranolol at a concentration (100 μM) that prevented isoproterenol‐induced elevations of cAMP. Addition of PGE2to cell cultures, at a concentration that markedly stimulates cAMP formation, mimicked the inhibitory effect of isoproterenol on cellular progression into S‐phase. These findings demonstrate the ability of the β‐adrenergic catecholamine isoproterenol to modulate MEPM cell proliferation in vitro via a receptor‐mediated mechanism and raise the possibility that the delayed initiation of DNA synthesis in these cells is a cAM
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