Regenerative healing of incisional wounds in midgestational murine hearts in organ culture

Objective: Using an organ-culture fetal heart repair model, we explored fetal repair in tissues other than dermis. Methods: Wounded fetal mouse hearts of 14 and 18 days' gestation (term = 20 days), as well as hearts of 22 days' gestation (newborn), were maintained in serum-free medium. Specimens were fixed at 2, 7, and 11 days and then processed for histologic examination. Small fragments of fetal hearts from all time points were cultured as explants. The migration of cells from the periphery of the explants was compared at day 4, and the pattern of microfilaments in these cells was assessed. Results: In 14-day hearts (n = 18), tissue architecture was rapidly reestablished without an inflammatory response or scarring, constituting regenerative repair. In 18-day hearts (n = 18), no reestablishment of muscle fibers or wound closure occurred. In the 22-day explants (n = 12) the wounds closed by scarring. Cell migration from 14-day explants was 4.7 ± 2.3 ocular units; from 18-day explants, it was 2.6 ± 1.1 ocular units; and from 22-day explants, it was 0.9 ± 0.4 ocular units. Microfilaments of 14-day cells were arranged at the periphery of the cell consistent with cardiomyocytes. Microfilaments of 18- and 22-day cells were arranged in parallel arrays (stress fibers) that were consistent with fibroblasts. Conclusions: We propose that regenerative healing of 14-day fetal hearts is by the migration of cardiomyocytes. At 18 and 22 days, cardiomyocytes are too differentiated and unable to migrate; hence cell migration is limited to resident fibroblasts, which are deficient at 18 days but sufficient at 21 days to be repaired by the scarring process.