Motoneuron death and motor unit size during embryonic development of the rat.

Chronic paralysis of rat embryos during the last 4 to 6 prenatal days causes a diminution in skeletal muscle fiber numbers but inhibits motoneuron death. Consequently, as paralyzed muscles develop, an increased number of motoneurons attempts to form synapses at a reduced number of synaptic sites. Paralyzed muscle fibers receive their synapses at a single endplate, as in control muscles, but these endplates are hyperinnervated, with about twice the normal number of inputs. Counts of axons, synaptic inputs, and muscle units showed that motoneurons normally contact a maximum number of muscle fibers shortly before birth, and this number remains stable for several days postnatal until it finally is reduced to the adult number. The average motor unit size in paralyzed embryos at the time of birth was the same as in controls. We suggest that it is not necessary to postulate the existence of competition between embryonic nerve terminals in order to explain regulation of the number of muscle fibers initially contacted by a motoneuron. Motoneuron death was not immediately affected by paralysis, but paralysis "rescued" all motoneurons whose death normally would have occurred 24 hr or more after the time when paralysis was initiated, regardless of when this was. This implies that the peak period for determination to die is during embryonic day 14, when myotube formation is just beginning and no recognizable endplate structures are present in muscles. When paralyzed, motoneurons normally destined to die are capable of forming a normal number of functional nerve-muscle contacts.