A comparison of the early development of axodendritic and axosomatic synapses upon embryonic mouse spinal motor neurons

The early embryonic development of axodendritic and axosomatic synapses upon motor neurons has been investigated in mouse spinal cord. This investigation had two main objectives: (a) to determine the earliest embryonic day on which recognizable synaptic contacts occur upon motor neurons, and (b) to compare the development of synaptic contacts upon the dendrites and somata of motor neurons during the early synaptogenic period. The broad goal of this investigation was to determine whether the location of early‐forming synapses upon motor neurons is consistent with the possibility that axosomatic synapses might be involved in the primary induction of motor neuronal dendrogenesis. Embryonic day 11 (E11) was the earliest developmental time at which synaptic contacts were observed in developing mouse spinal cord. All of these synaptic contacts appeared to be located upon motor neuronal dendrites within the lateral and ventral marginal zones. The number of synaptic contacts observed on E11 was too small to be detected by the quantitative sampling procedure used in this investigation, but the procedure was sufficiently sensitive to detect synaptic contacts on embryonic day 12. Both axodendritic and axosomatic synapses were found upon E12 motor neurons, but there were about four times as many synaptic contacts per unit length of dendritic membrane as there were per equivalent length of somal membrane. Furthermore, dendritic membranes continued to exhibit a higher density of synaptic contacts on all of the remaining embryonic days (i.e., 13–16) examined in this investigation. These quantitative data and the E11 observations indicate that axosomatic synaptic contacts are not a necessary prelude to the formation of motor neuronal dendrites. Therefore, it is suggested that axosomatic synapses do not play an obligatory role in the primary induction of motor neuronal dendrogenesis. The experimental findings of other investigators, however, have provided reasons to suspect that early‐forming axosomatic synapses may somehow facilitate dendritic development once it has been induced. This possibility is discussed in terms of our observation that early‐forming axosomatic synapses rather commonly occur at sites which may represent somal growth regions. This relationship leads us to suggest that early axosomatic synapses may facilitate dendritic development by signalling the motor somata that the formation of a synaptogenic axonal field is underway. Furthermore, we speculate that the positioning of early axosomatic contacts might be providing directive cues as to the location of the developing synaptogenic field. Thus a directive facilitation of dendritic growth is suggested as a function of early axosomatic synapses rather than one involved with the primary induction of dendrogenesis.