Fate of grafted embryonic purkinje cells in the cerebellum of the adult “purkinje cell degeneration” mutant mouse. I. Development of reciprocal graft‐host interactions

Purkinje cell replacement, with subsequent synaptic integration into the cortex of host deficient “Purkinje cell degeneration” mutant cerebella, can be obtained by cerebellar grafting (Sotelo and Alvarado‐Mallart: Proc. Natl. Acad. Sci. USA 83:1135–1139, 1986; Neuroscience 20:1–22, 1987). In this paper, we have morphologically studied the developmental events underlying the neuronal replacement, 3–21 days after grafting. Despite their abnormal environment, Purkinje cell progenitors proceed with their proliferation in the grafted neuroepithelium, with a time window similar to that characterizing proliferation of this neuronal class in control mouse embryos. Only postmitotic Purkinje cells leave the grafts and migrate to the host molecular layer following stereotyped pathways. These neurons invade the host molecular layer, either through a tangential migration under the pial basal lamina from the graft/host interface or breaking locally the latter, and passing directly from the lateral swellings of the graft lying on the surface of the host folia. Whatever the pathway for host invasion, the migrating Purkinje cells penetrate radially and/or obliquely into the host molecular layer until their inward‐oriented processes attain the molecular/granular layer interface, which occurs about 7 days after grafting. At the end of their migration, the grafted Purkinje cells with bipolar shapes and long and smooth processes begin to build up their ultimate dendritic trees. This dendritogenesis proceeds with constructive and regressive processes, passing through the same three developmental phases described by Ramón y Cajal ( Trab. Lab. Invest. Biol. Univ. Madrid 24:215–251, 1926) for control Purkinje cells (phase of the fusiform cell, phase of the stellate cell with disoriented dendrons, and phase of orientation and flattening of the dendrites). In the grafted cerebella, the duration of the second and third phases is somewhat shorter than during normal cerebellar ontogenesis. Synaptogenesis between adult host axons and grafted Purkinje cells starts when the latter attain their second phase of dendritic development. Somatic filopodia emerging from grafted Purkinje cells begin, 10–11 days after grafting, to be synaptically contacted by axonal sprouts of the host climbing fibers resulting, 2 days later, in the formation of pericellular nests. Synaptogenesis between slender dendritic spines and host parallel fibers, together with that of axon terminals from host molecular layer interneurons and the smooth surface of the grafted Purkinje cells somata, begin earlier than in control mouse development, being almost simultaneous with climbing fiber/Purkinje cell synaptogenesis. Fourteen days after grafting, the climbing fibers have begun the translocation from their somatic to their dendritic location. By 21 days after grafting, the synaptic investment of the grafted Purkinje cells is qualitatively similar to that observed after long‐term survivals. All these morphological observations, together with the physiological ones of the companion paper, permit the conclusion that embryonic and adult neurons interact according to a tempo imposed by the immature grafted Purkinje cells, which seem to follow a predetermined pattern of maturation almost independent of environmental signals, as if they were regulated by an internal clock. Furthermore, the cellular mechanisms underlying the sequential critical steps, from neuronal proliferation to selective elimination of synapses, taht are needed for the formation of specific circuitry of the cerebellar cortex seem to be operative for the Purkinje cell replacement, leading to the restoration of the deficient mutant cerebellar cortex.