Purkinje Cell Ontogeny: Formation and Maintenance of Spines

Abstract

This chapter discusses the morphological analysis of microenvironmental factors influencing the molding of the Purkinje cell dendritic tree. Emphasis is given on the problem of the formation and maintenance of dendritic spines, because they are, by far, the most numerous receptor sites in these neurons. During normal development, Purkinje cell differentiation occurs in a concomitant manner with the development of its synaptic inputs. Climbing fibers reach Purkinje cells, establishing early axo-somatic synapses that are already functional postnatally by day three. This pericellular climbing plexus originates from 3–4 different climbing fibers. The further development of the Purkinje cell dendritic tree parallels the growth of climbing fibers and the maturation of their synapses. By day 12–15, the climbing fiber varicosities make synapses on thorns emerging from the thick Purkinje cell dendritic branches. At this stage, the immature multiple innervation is transformed into the definitive one-to-one relationship, which characterizes the climbing fiber-Purkinje cell synapses in adult animals. During the 2nd and 3rd postnatal weeks, migration of the bulk of granule cells takes place, giving rise to the millions of parallel fibers found in the molecular layer. The development of parallel fibers is accompanied by the formation of thin terminal branches of the Purkinje cell dendritic arborization—the spiny brundzlets—and by the achievement of the espalier arrangement of these dendritic trees that give the Purkinje cells their unique tridimensional shape.