Microtubule-Associated Proteins in Cerebellar Morphogenesis

Abstract

In his studies of the cerebellum, Ramón y Cajal (1888,1960) noted, with characteristic clarity, the major features of morphogenesis in the various cell types. Many of his observations are now so familiar as to be commonplace, whereas others are less frequently cited, perhaps because they deal with cellular phenomena whose regulation is still not properly understood. For example, Ramón y Cajal was the first to report that the growth of axons and dendrites involves several distinct steps. He observed that the neuroblast first extends short processes of rather irregular outline, as, for example, the granule cell does during its initial horizontal, bipolar phase. Only later do these initial protrusions narrow and lengthen to form a proper “axis cylinder.” Dendrites appear later still. Thus, Purkinje cells have already produced extensive, well formed axons long before dendrites appear. At this early stage the Purkinje cell body is decorated with multiple short protoplasmic extensions that are subsequently resorbed to be replaced by the growth of the mature dendritic tree. Neurogenesis thus involves an initial outgrowth of short, irregular processes followed by the formation of a distinct axon and finally by the gradual development of the dendrites.