Histogenesis of cortical layers in human cerebellum, particularly the lamina dissecans

Abstract Patterns of lamination during development of the fetal human cerebellar cortex were analyzed in Nissl‐ and H the external granular layer attained maximum cell number at some stage after birth. At the 5‐layer stage from about 21 to 32 weeks, the interrelationships between various classes of young neurons in the cerebellar cortex became very complex. The Purkinje cells developed ascending branched dendritic processes with growth cones and displayed transient short cytoplasmic processes that extended from the soma in all derections. Basket cell neurons had formed but their axons appeared not to envelop the Purkinje somas as yet. Less mature, smaller cells were beginning to migrate from the external granular layer inward past the Purkinje somas. Their cell bodies in the newly forming granular were separated from the Purkinje cell bodies by a dense tangle of axons in the lamina dissecans. Many of these axons terminated in swellings interpreted tentatively as immature mossy endings, while others passed outward to enclose the cell bodies and proximal dendrites of the Purkinje cells. Some general points emerged from a comparison of cerebellar development in man and animals. The time of cell origin cannot be inferred necessarily from the time of overt differentiation; deep cerebellar neurons and Purkinje neurons arise in the first trimester, but the former cells differentiate much earlier. Purkinje cells acquired characteristic shapes by the middle of gestation, when very few granule cell neurons had yet formed, and thus appear to develop relatively independently of the granule cells. Although the adult cerebellum appears to be organized similarly among mammals, a developmental component, the lamina dissecans, has been illustrated only in man and whale; its appearance may reflect the combination of early Purkinje cell and late granule cell differentiation in species with a prolonged period of development. One of the most intriguing features of the lamina dissecans is that it appears to contain axon terminals at a time prior to the arrival of the postsynaptic cells.