Tendon morphogenesis in the developing avian limb: plasticity of fetal tendon fibroblasts.

The differentiation and patterning of tendon fibroblasts is at present a poorly understood aspect of musculoskeletal development in the vertebrate limb. Precursors of tendon fibroblasts originate in the somatic mesoderm adjacent to the early limb bud and gradually become incorporated into the limb mesenchyme as development proceeds. It is unclear whether these progenitor cells are committed to the tendon lineage at this early stage, or whether cells become committed only as they are incorporated into a developing tendon. Following a review of our current knowledge of early tendon development, we present recent results from our preliminary studies looking at tendon cell commitment. Using a lacZ encoding replication-deficient retrovirus, we have mapped regions of the early limb bud that contain presumptive tendon progenitor cells, and later used these sites for implanting labelled fetal tendon fibroblasts into developing limbs. Following implantation, we found that these cells successfully re-incorporated into developing proximal and distal tendons, but also surprisingly contributed to other tissue lineages within the limb. Our results suggest that fetal tendon fibroblasts may not be irreversibly committed to a tendon cell fate in the limb and may be somewhat plastic in their ability to integrate into other tissue lineages during development.