Motoneuron death after sciatic nerve section in newborn rats.

When the sciatic nerve was cut in newborn rats almost all of the motoneurons were subsequently lost. The adult sciatic nerve contained 7,640 myelinated fibres of which 1,650 were motor axons. The ventral roots of the sciatic nerve (L4-L6) contained in all 4,080 myelinated fibres; only 2,380 were present 5-30 weeks after the sciatic nerve had been cut at birth. The deficit of 1,700 fibres corresponded to the number of motor axons in the sciatic nerve. The motoneuron pools were depleted and practically no labelled or unlabelled perikarya were found after HRP had been applied to the nerve proximal to the former lesion. The deficit of ventral root fibres was 1,030 when the nerve had been cut at age 1 week; nerve section 4 weeks after birth was not followed by retrograde loss of motor axons. Nerve crush at birth caused a deficit of 1,230 ventral root fibres. The incomplete loss of motor axons after crushing was due to the fact that not all axons had been severed. The continuity of the nerves cut at birth was always restored and myelinated axons which probably were dorsal root ganglion afferents grew into the atrophic muscles. Roughly two-thirds of the myelinated sensory fibres degenerated after axotomy at birth. The denervated muscle fibres were not reinnervated but vanished eventually and were replaced by fat cells.