Role of chondrogenic tissue in programmed cell death and BMP expression inchick limb buds

In the developing chick leg bud, massive programmed cell death occurs in th e interdigital region. Previously, we reported the inhibition of cell death by separation of the interdigital region from neighboring digit cartilage. In this study, we examined the relationship between cell death and cartila ginous tissue in vitro. First, cell fate was observed with Dil that was use d to examine cell movement in the distal tip of leg bud. Labeled cells in t he prospective digital region were distributed only in the distal region as a narrow band, while cells in the prospective interdigital region expanded widely in the interdigit. In coculture of monolayer cells and a cell pelle t tending to differentiate into cartilage, monolayer cells migrated into th e cell pellet. These results suggested that digit cartilage tends to recrui t neighboring cells into the cartilage during limb development. Next, we ob served the relationship between cell death and chondrogenesis in monolayer culture. Apoptotic cell death that could be detected by TUNEL occurred in r egions between cartilaginous nodules in mesenchymal cell culture. More apop totic cell death was detected in the cell culture of leg bud mesenchyme of stage 25/26 than that of leg bud mesenchyme of stage 22 or that of stage 28 . The most developed cartilaginous nodules were observed in the cell cultur e of stage 25/26. Finally, we observed Smp expression in vitro and in vivo. Bmp-2, Bmp-4 and Bmp-7 were detected around the cartilage nodules. When th e interdigit was separated from neighboring digit cartilage, Bmp-4 expressi on disappeared near the cut region but remained near the digit cartilage. T his correlation between cell death and cartilaginous region suggests that c artilage tissue can induce apoptotic cell death in the developing chick lim b bud due to cell migration accompanying chondrogenesis and Bmp expression.