Isolated fetal mouse limbs: Gestational effects on tissue repair in an unperfused system

While it is known that adult tissue repair is tightly regulated through local effects in the wound environment mediated by circulating blood elements such as platelets, white cells, cytokines, and hormones, the degree to which each is required in fetal tissue repair is uncertain. This raises the following questions regarding regulation of fetal tissue repair: (1) Is the local wound matrix alone sufficient to sustain fetal tissue repair in the same regenerative manner seen in previous in vivo whole animal studies? (2) Will it occur only during the period of rapid fetal growth and development in early and mid gestation? To address these fundamental questions, an organ culture system has been designed to grow isolated, unperfused, developing fetal mouse limbs in a chemically defined, serum-free media. Amputated fetal mouse forelimbs (n≥10) were wounded with linear incisions at gestational days 14, 16, and 18 (term=19); the wounds were closed primarily. These amputated and wounded limbs were placed on steel grids in organ culture petri dishes, then partially submerged in a chemically defined, serum-free media. The limbs were grown at 37°C in humidified 95% air/5% CO₂ for 1 week. These wounded limbs were examined histologically at days 0, 3, and 7 postwounding to determine their viability and whether or not tissue repair occurred. In the 14-day group, limb growth and differentiation was evident during the incubation period. Normal dermal and epidermal architecture was restored at the wound site without abundant collagen deposition by day 7 postwounding. In contrast, the 16- and 18-day gestation limbs showed abundant, disorganized collagen deposition in the wounded dermis by day 7 postwounding. These observations indicate: (1) The local wound environment with its cell to cell interactions, autocrine and paracrine functions is sufficient for fetal tissue repair to occur at gestational day 14. (2) Differences in the repair between the regenerative type in the 14-day limbs and the adult-like repair seen in 16- or 18-day limbs reflect the changes in matrix composition as fetal development nears completion in late gestation. This corroborates the gestational effects on tissue repair previously seen in whole animal sheep models.