Regenerative healing of incisional wounds in murine fetal lungs maintained in organ culture

Although fetal dermal repair is known to be fundamentally different from adult healing, the response to wounding in other organs is less well characterized. Scarless repair in mid-gestation dermis with a transition to adult-type healing at term has been shown in fetal organ culture. A lung explant culture system was used to investigate whether wound repair in the fetal lung shows characteristics similar to those found in fetal dermis. Lungs from 14-day and 18-day CD-1 murine fetuses and 2-day-old newborns, (term = 20 days, n = 24) were wounded by linear incision and incubated at 37°C, in a 21% O₂, 5% CO₂ environment, in BGJb supplemented with vitamin C and antibiotics. Medium was changed daily. Samples were fixed at 7 days and embedded in paraffin. Sections were stained with hematoxalyn-eosin and Masson Trichrome. Additional 14-day and 18-day samples were frozen in freon and immunohistochemical staining for TGF-β performed. Other frozen tissues from each time point were homogenized and used to assay for endogenous TGF-β levels by Western blot analysis. Histology showed reconstitution of tissue architecture across the wound in 14-day and 18-day specimens. In representative histological sections, intact bronchial architecture developed across the previous wound site. No cellular inflammatory response was observed, and collagen deposition was undetectable at the site of the wound by Trichrome staining. By 22 days the lung explants showed a much less ordered repair, including disorganized collagen deposition. Immunohistochemistry showed TGF-β throughout the lung parenchyma of 14-day and 18-day lungs without recruitment at the wound site. Western blotting showed a marked increase in signal for TGF-β in 18-day compared to 14-day explants. A smaller increase was seen from 18 days to 22 days. The authors conclude that the murine fetal lung shows scarless repair even late into gestation. This implies a plasticity in lung not seen in the integument, as well as an ability of tissues to exert a strong degree of local control over the healing response.