Abstract
During postnatal development of the rat the nature of spontaneous degeneration of axons in the pyramidal tract was studied electron microscopically. The degenerating axons were found more frequently in young animals than in old animals, and they were generally found in bundles. On the basis of electron-opacity and visibility of ultrastructural details in the degenerating axons, and frequency of their occurrence at various developmental stages the degenerating axons were classified into four groups representing four stages of severity of spontaneous degeneration of axons in the pyramidal tract. In the context of neuroembryological material, autoradiographic as well as silver-impregnated, it was established that these spontaneously degenerating axons arose from the reticular formation of the brain stem and that they had ascending as well as descending components, the latter descending in the spinal cord. Possible significance of such reticulo-spinal axons in the primitive organization of the embryonic nervous system and its determination of the spontaneous motility in early embryos, and in the apparently directed growth of the cortico-spinal axons during late stages of embryonic development are discussed.
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