An anterograde tracer study of the developing corticospinal tract in the rat: three components.

Light microscopic analysis of anterogradely transported wheat germ agglutinin-conjugated horseradish peroxidase (WGA-HRP) has been used to study the developing corticospinal tract (CST) in the rat. This study was carried out to examine the relationship between the site of injection within the cortex and the pattern of labelling of the developing CST in the spinal cord from postnatal day 1 (P1) through postnatal day 10 (P10). For this purpose the cortex was subdivided into 3 equal areas along the rostrocaudal axis: anterior, intermediate and posterior. After the operation the animals were allowed to survive for 24 h. The caudal extension of labelled CST axons originating in the anterior cortical area was restricted (L1 at P7 or P10) as compared with that of the CST fibres originating in the intermediate cortical area (S3 at P10). The axons of the posterior corticospinal (CS) neurones reach their most caudal extension in the spinal cord (T5) at P7 but then gradually disappear up till P14. Quantitative analysis of the amount of label along the length of the outgrowing CST fibres revealed the formation of a large stable peak at the level of the cervical enlargement after labelling of either the anterior or the intermediate cortical area. The formation of a second 'running' peak which moves caudally from mid-thoracic levels at P5 to mid-lumbar levels at P10 was only accomplished by labelling the intermediate cortical area and is probably caused by the accumulation of label in the growth cones at the distal ends of the outgrowing CST fibres. After labelling the posterior cortical area, no peaks could be detected, neither at the cervical nor at the lumbar intumescence. The major spinal grey termination field of the anterior CS neurones appeared to be the cervical intumescence, whereas the major spinal grey termination field of the intermediate CS neurones is the lumbar enlargement. By contrast, axons of posterior CS neurones never showed any outgrowth into the spinal grey matter at any level. Concluding, the developing CST in the rat consists of 3 components: the first having its originating neurones in the anterior part of the cortex and its termination field in the cervical intumescence; the second with its originating neurones in the intermediate part of the cortex and its termination field predominantly in the lumbar enlargement, and a third transient one, originating in the posterior cortex and gradually disappearing from spinal cord levels. Research using anterograde tracing techniques in combination with electron microscopy is necessary to further analyse these 3 different components.