SUMMARY In vertebrates and invertebrates, neurons and glia are generated in a stereotyped manner from neural stem cells, but the purpose of invariant lineages is not understood. We show that two stem cells that produce leg motor neurons in Drosophila also generate neuropil glia, which wrap and send processes into the neuropil where motor neuron dendrites arborize. The development of the neuropil glia and leg motor neurons is highly coordinated. However, although motor neurons have a stereotyped birth order and transcription factor code, the number and individual morphologies of the glia born from these lineages are highly plastic, yet the final structure they contribute to is highly stereotyped. We suggest that the shared lineages of these two cell types facilitate the assembly of complex neural circuits and that the two birth order strategies—hardwired for motor neurons and flexible for glia—are important for robust nervous system development, homeostasis, and evolution.

Fgf10 regulates the specification and early outgrowth of vertebrate limbs. However, the arrested growth of limb buds and neonatal death in Fgf10‐deficient embryos has precluded an understanding of its putative roles in later stages of development. To obtain clues about these roles, we have studied the localization and immediate fate of Fgf10‐expressing cells from Embryonic days 9.0 to 18.5, by subjecting Fgf10‐lacZ reporter embryos to immunocytochemistry with cell type‐specific markers, optical‐tomography scanning and in situ hybridization analysis. These studies have revealed an asymmetric expression of Fgf10 along the anterior‐posterior axes suggestive of critical roles in anterior‐posterior patterning of the limb. Moreover, we have discovered a discreet population of Fgf10‐expressing muscle progenitors that invade the limbs from the body trunk and a specific expression of Fgf10 in select tendon progenitors. Together, these novel findings implicate Fgf10 in multiple and critical roles that underlie the proper formation and sculpturing of vertebrate limb. Conditional loss and gain‐of‐function experiments are in progress to investigate the regulatory roles of Fgf10 in distal mouse limb development.