Synaptic transmission and cell death during normal ganglionic development.

1. During normal embryonic development of the chick ciliary ganglion, cell death over a 4-day period (Stages 35-39) reduces the number of ganglion cells by half, from 6500 to 3200. Both ciliary and choroid populations are affected by approximately the same amount.2. Previous to cell death, preganglionic fibres form functional synapses on all ganglion cells, indicating that synapses form on cells which are destined to die.3. Shortly before the period of cell death, there is a failure of transmission in approximately half the cells. Some evidence suggests that transmission failure in at least some of the cells is of preganglionic origin.4. Cell death is nearly synchronous with the establishment of peripheral connexions by ganglion cells, at least with respect to the ciliary population which forms functional synapses with iris muscle. This implies that those cells which die do so because they have failed to form adequate peripheral connexions.5. It is suggested that many of the cells in which transmission has failed die, bringing transmission through the ganglion back to 100%. However, transmission failure appears to be a transitory phenomenon in other cells which survive and probably results from death of their preganglionic elements. Restoration of transmission would then be brought about by the formation of new or more effective synapses by surviving preganglionic fibres.