BACKGROUND:Nerve root avulsion is a frequent finding in patients with brachial plexus injury following road traffic accidents or as a result of severe arm traction during complicated deliveries.This injury constitutes a challenging clinical and surgical problem.The orphological characteristics of motoneurons after nerve root avulsion deserve further analysis.OBJECTIVE:To study the different morphological changes of u -motoneurons under light and electron microscopy after C8 spinal ventral rootlets avulsion and transection at various stages.DESIGN:Controlled animal study.SETTING:Department of Anatomy,King Faisal University.MATERIALS:The experiment was carried out at the Department of Anatomy,College of Medicine,King Faisal University between January 2005 and March 2006.Six adult Sprague Dawley rats weighing 200-350 g, irrespective of gender,were used for this study.The animals were bred at the animal house,College of Medicine,King Faisal University,and fed on rat maintenance diet.Water and standard diet were supplied ad libitum.Animal interventions were carried out according to animal ethical standards.METHODS:Three animals were randomly chosen for avulsion of the right ventral rootlets of C8 spinal nerves.The other three received transection of the right ventral rootlets of C8 spinal nerves.①Avulsion experiment:After rats were anesthetized,the right ventral rootlets of C8 spinal nerves were identified.The ventral rootlets were avulsed from the spinal cord by traction with a fine hook(Fine Science Tools Inc.,No. 10031-13,Germany).Traction was exerted in a direction parallel to the course of the spinal root.Under the operating microscope,the Cs segment was exactly located.After checking the successfulness of the surgical procedure,the Ca segment was separated from the spinal cord.The outcome of the avulsion procedure was as follows:two animals had true avulsion,i.e.,no remaining stump was attached to the spinal cord surface.One rat had a stump still attached.The animal was sacrificed 1 week later and was included in the transaction experiment for analysis.②Transection experiment:Using fine scissors,the ventral rootlets were transected close to the area of the right ventral rootlets junction with the dorsal rootlets.MAIN OUTCOME MEASURES:After the rats were anesthetized,Ca spinal cord segments were collected for the avulsion experiment from 2 rats,l rat,and 1 rat at weeks 1,4,and 8,respectively,and from 1 rat each after 4 and 8 weeks in the transection experiment.The morphology of a-motoneurons was observed under light microscopic and electron microscopic examination.RESULTS:All six rats were included in the final analysis.①Avulsion experiment:a-motoneurons of the Ca spinal segment were found to survive avulsion injury up to 4 weeks.Thereafter,signs of degeneration occurred in a gradual process,involving first the most anterior motoneurons and probably proceeding posterior in a zonal pattern.A clear reduction in motoneuron size was noted.The largest cell body detected was smaller than the control.Nuclei were rounded and central.Nissl substance appeared granular and was dispersed over the cytoplasm,and the a-motoneurons were of normal electron density.At 8 weeks,three zones were observed in the ventral horn.The anterior zone was the most affected and showed intensely basophilic shrunken motoneurons.Nearly all nuclei were centrally located,but were irregular in outline. ②Transection experiment:one week after ventral root transection,nuclei were slightly eccentric and irregular in outline,but of normal size.Nucleolar vacuolation was observed in several neurons,and the ultrastructure of the chromatolytic Nissl bodies(NBs)showed disorganized arrangement of rough endoplasmic reticulum(rER)cisternae.The motoneuron cell bodies were of regular shape and size at week 4.Nuclei returned to their normal central location and contour.Many lipofuscin granules and lysosomes were observed in the motoneuron cytoplasm,a-motoneurons appeared to have normal cell bodies; the nuclei were perfectly ound,but slightly enlarged with enlarged nucleoli at week 8.There was almost complete recovery of the a-motoneurons.CONCLUSION:a-motoneurons of the C8 spinal segment can survive in the right ventral root in rats after transection at the right ventral rootlets junction to the dorsal rootlets.α-motoneurons of the C8 spinal segment in rats can survive for 4 weeks after avulsion injury.Thereafter,signs of degeneration occur,and the Oegeneratlve process proceeds in a zonal pattern.