Development of the marginal zone in the rhombenecephalon of Xenopus laevis.

The marginal zone is the most superficial component of the developing central nervous system. In this study we describe the pattern of formation of the marginal zone in Xenopus laevis as seen with the light microscope and correlate its appearance with developing axon bundles seen with the electron microscope. The marginal zone was first identified at stage 26 on the ventrolateral aspects of the neural tube in restrictive foci throughout the medulla. By stage 28 this presumptive fiber area had expanded to occupy a longitudinal zone along the ventrolateral aspect of the rhombencephalon. The marginal zone continued expanding rostrocaudally, mediolaterally, and dorsoventrally coming to occupy a substantial area along the periphery of the central nervous system by the time of hatching (stage 36). Electron microscopic (EM) observations allowed us to identify axons and growth cones on the lateral borders of the neural tube at stage 22. Foci of marginal zones identified in the caudal medulla at stage 24 consisted of several fascicles grouped together on the ventrolateral surface. These fascicles, separated and surrounded by ependymal processes contained axons and growth cones. The pattern of marginal zone development reflected the addition of axons within fascicles and also the addition of new fascicles of axons.