Axon composition of the proprioceptive PD nerve during growth and regeneration of lobster claws

A prominent chordotonal organ spanning the propus and dactyl (PD) joint in the chelipeds of the lobster Homarus americanus has the somata of its sensory cells embedded in an elastic strand while their axons collectively travel in a nerve to the ganglion. Electron microscopic examination of the PD nerve revealed several large and many small axon profiles denoting few movement‐sensitive and many position‐sensitive cells. This axon composition typified the nerve in the normally occurring, paired asymmetric crusher and cutter claws, as well as in the experimentally induced, paired symmetric cutter claws. In the asymmetric condition, the crusher nerve had many more axons than the cutter while in the symmetric condition, the numbers were equivalent between the paired cutter claws. In a size‐graded series of lobsters with paired asymmetric claws there was an increase in the number and size of axons in keeping with an increase in claw size. In a newly regenerated claw, the PD nerve was a miniature version of its pristine counterpart with smaller and fewer axons. Moreover, in newly regenerated paired asymmetric claws, the crusher nerve did not have a larger number of axons than the cutter, suggesting that PD nerve has not fully differentiated. The distribution of axons according to size was, however, similar to the pristine nerve in that there were few large axons and many small ones, suggesting that the ratio of large, movement‐sensitive cells to small, position‐sensitive ones in the PD organ is rigidly specified during regeneration and growth.