UC San Diego

The limb is one of the most evolutionarily diverse structures in vertebrate morphology. Our lab focuses on the lesser Egyptian jerboa, Jaculus jaculus, as an opportunity to identify the mechanisms that reshape the limb as a consequence of natural selection. The jerboa is a bipedal rodent with elongated hindlimbs and three toes on feet that lack intrinsic foot muscles. During my thesis work, I sought to understand the progression and mechanism of muscle loss in the context of its relationship with the motor nerve. We initially tested two alternate hypotheses: either jerboas never develop foot muscles or muscle fibers form and are lost over time. RNA in situ hybridization to MyoD and immunofluorescence detection of myosin heavy chain determined that jerboas are born with muscles in their feet, which are virtually nonexistent by post-natal day 7. I examined the structure of the nerve and its associated neuromuscular junctions to evaluate if proper innervation occurs at early critical stages and if that innervation is maintained during the earliest stages of myofiber degeneration. I utilized immunohistochemistry to examine the integrity of the nerve structure and to identify structural synapses. I also injected neonatal jerboas with CTB-555 labeling dye to retrograde trace motor neurons of the foot back to their origin in the spinal cord. I discovered that the muscles in the feet of the jerboa receive proper innervation and that this innervation is maintained for several days after birth, even after the myofibers begin to disorganize and degenerate. These results indicate it is unlikely a developmental innervation failure is responsible for muscle loss in the jerboa but rather itself degenerates concurrent with loss of its innervation target.