Pathfinding by growth cones of commissural interneurons in the chick embryo spinal cord: A light and electron microscopic study

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Abstract
To investigate putative axonal guidance mechanisms used by commissural interneurons in the chick embryo spinal cord, we have examined growth cone morphology, the microenvironment through which the growth cones advance, and interactions Between growth cones and their surroundings. Growth cones of both early and late developing commissural interneurons were examined. The growth cones were visualized by injection of either horseradish peroxidase (HRP) or the fluorescent dye Di‐I. Unlabelled growth cones as well as HRP‐labelled growth cones were also examined by electron microscopy. The early developing growth cones project circumferentially without fasciculation until they reach the region of the longitudinal pathway in the contralateral ventral funiculus (CVF). In their trajectory towards the floor plate, axons exhibited elaborate growth cones with filopodia and lamellipodia. They projected between processes of neuroepithelial cells within abundant extracellular spaces. Upon arrival at the ipsilateral ventral funiculus, growth cones did not appear to contact preexisting longitudinal axons. Within the floor plate, the growth cones were less complex and lacked long filopodia and exhibited bulbous or varicose shapes with short processes. Electron microscopic observations of the floor plate at this stage revealed that there was only a small amount of extracellular space and that the basal portion of the floor plate cells were directionally oriented (polarized) in the transverse plane. It is of particular interest that contacts between growth cones and the basement membrane in the floor plate were often observed. When the growth cones reached the contralateral ventrolateral region, they again exhibited an elaborate morphology. Close contacts between growth cones and the preexisting contralateral longitudinal axons were observed. Growth cones advancing in the contralateral longitudinal pathway exhibited various shapes and were observed to contact other axons and processes of neuroepithelial cells. Most of the later developing growth cones of commissural cells exhibited lamellipodial shapes irrespective of their location along the circumferential trajectory. Electron microscopic observations revealed that these late developing growth cones always contacted or fasciculated with preexisting axons and that the cellular environment through which they grow is oriented in such a way that the growth cones appear to be guided in specific directions. Growth cones entering the CVF exhibited more elaborated shapes with ramified lamellipodia that made multiple contacts with preexisting longitudinal axons. The present results indicate that differential axonal guidance mechanisms may be employed along the pathway followed by spinal commissural interneurons and that axons and growth cones projecting along this pathway at different developmental stages employ different mechanisms for pathfinding and guidance. These observations suggest that there are developmentally regulated transitions in the molecular cues used by growth cones at different times and in different regions along their pathway.