Selective loss of neurites during differentiation of cells in the leech central nervous system.

The arborizations of annulus erector (AE) motoneurons in the central nervous system of the leech, Hirudo medicinalis, have been examined during embryogenesis to determine how segmental differences in their branching patterns arise. Early in development AE motoneurons all along the ganglionic chain had a similar central arborization, with major branches extending both rostrally and caudally along the connectives that link adjacent ganglia. As the embryo grew, the processes in the connectives elongated but failed to increase significantly in caliber and eventually atrophied and were lost. This sequence of events did not occur uniformly along the cord, however. AE motoneurons in midbody ganglia lost both anterior and posterior branches, cells near the head lost only their posterior branch, while cells near the tail lost only their anterior branch. In this way, the selective atrophy of neurites during development produced a systematic segmental difference in the morphology of homologous cells.