MIGRATION OF MYOGENIC CELLS FROM THE SOMITES TO THE FORE-LIMB BUDS OFDEVELOPING MOUSE EMBRYOS

In this study, we have isolated newly formed somites from the caudal r egions of 8.5 day mouse embryos and transplanted them orthotopically i nto correspondingly staged hosts at the level of the prospective limb- forming region. The experimental embryos were then cultured intact for 32-36 hr. The donor somites used were pre-labelled with DiI, a fluore scent lipophilic dye, or were obtained from transgenic embryos that ca rried a 1 kb 5' regulatory sequence of the desmin gene linked to the g ene encoding Escherichia coli beta-galactosidase. The transgene is spe cifically expressed in skeletal muscles (Li et al. [1993] Development 117:947-959). The aim of these experiments was to show definitively th at the musculature of the mammalian limb is derived from the somites. The results demonstrated that DiI-labelled cells from the implanted so mites were able to invade the proximal region of the fore-limb bud dur ing the course of development, The use of transgenic somites as grafts confirmed that some of the semitic cells found in the limbs were myog enic cells. To determine whether the displacement of semitic cells is an active or passive process, somatopleure obtained from the prospecti ve limb-forming regions of day 8.5 day embryos was implanted into 8.5 day hosts. We did not detect the presence of DiI-labelled somatopleura l cells in the fore-limb after 32-36 hr of culture. This suggests that semitic cells reached the limb bud via active locomotion rather than as a result of being passively dragged there, as the limb elongates du ring development, In addition, we injected latex beads into the somite s, as probes, to determine whether extracellular matrix-driven translo cation plays a role in driving the semitic cells to the limb bud. In a majority of the specimens examined, we could not detect the presence of these beads in the limb bud. However, in the trunk of these embryos , the beads were found dispersed throughout the ventral neural crest p athway. (C) 1995 Wiley-Liss, Inc.