Cell death during gangliogenesis in the leech: bipolar cells appear and then degenerate in all ganglia.

The bipolar cells can be recognized very early during gangliogenesis in the leech central nervous system by their expression of antigens that are recognized by the monoclonal antibody Laz1-1. They are the first cells to express these antigens, which are later shared with a distinct set of other cells in the leech nervous system. Their processes extend several segments rostrally and caudally along the forming interganglionic connective nerves; they are first found in anterior segments and gradually appear in more posterior ones over the course of 2-3 d. At about the time bipolar cells appear in the neuromeres of the tail ganglion, those in the most anterior segmental ganglia begin to degenerate. Degeneration proceeds caudally over the next 3 d, until all bipolar cells have disappeared. Bipolar cells are, thus, members of that class of cells that exists for only a short period in neurogenesis and then presumably disappears once its functions are no longer required. These cells' morphology and appearance at the earliest stages in the formation of the interganglionic connective nerves are suggestive of a role in the establishment of these longitudinal pathways in the leech CNS.