SERIAL CHANGES IN GRANULOPRIVAL CEREBELLAR CULTURES AFTER TRANSPLANTATION WITH GRANULE CELLS AND GLIA - A TIMED ULTRASTRUCTURAL-STUDY

Granuloprival cerebellar cultures derived from neonatal mice were tran splanted at nine days in vitro with granule cells and glia, and the ch anges induced in the host explants were examined daily with the electr on microscope From one to nine days post-transplantation. Granule cell s and astrocytes had migrated into the host cultures within 24 h, and astrocytic processes began to ensheath Purkinje cells and to interpose themselves between axon terminals and Purkinje cell somata, reducing the number of axosomatic synapses. Occasional degenerating Purkinje ce lls were present. At two days posttransplantation, synapse formation b etween parallel fibre terminals and Purkinje cell dendritic spines was initially evident, and Purkinje cells began to proliferate dendritic spines near astrocytic processes. Degenerating Purkinje cells were mor e frequently encountered. Myelin was first observed in host cultures a t three days after transplantation, and astrocytes continued to enshea th Purkinje cells and reduce the population of axosomatic synapses, a process that began to stabilize at four days post-transplantation. At this time astrocytic ensheathment had extended ro Purkinje cell dendri tes and dendritic spine synapses. proliferation of Purkinje cell dendr itic spines accelerated, and occasional synapses with presumptive para llel fibre terminals were present among clusters of proliferated spine s. At five days after transplantation, contours of Purkinje cells were rounded, and there was a decrease of somatic spines and of synapses w ith somatic spines. Purkinje cells were Fully ensheathed by astrocytic processes by six days post transplantation and had assumed a mature a ppearance. Homotypical parallel fibre-Purkinje cell dendritic spine sy napses were predominant in more developed areas of cortical neuropil a s heterotypical recurrent axon collateral-Purkinje cell dendritic spin e synapses were reduced. Increasing synapse Formation was evident amon g clusters of proliferated spines, which continued at seven days past- transplantation, as the spine clusters became less Frequent. At eight days after transplantation, space between Purkinje cells had increased and the cortical neuropil resembled that of comparably aged control c ultures. Occasional degenerating Purkinje cells were still evident at nine days post-transplantation, at which time residual clusters of pro liferated unattached dendritic spines were scarce. The sequence of cha nges after transplantation was consistent with the specific roles of t he transplanted elements. Astrocytes were involved with the regulation of synapse density, including reduction of some heterotypical synapse s, and induced proliferation of Purkinje cell dendritic spines. Granul e cell axons synapsed with Purkinje cell dendritic spines, further red ucing heterotypical synapses and restoring cortical circuitry to a nea r control state. The loss of heterotypical synapses was associated wit h programmed cell death of excess Purkinje cells, reducing the Purkinj e cell population to control levels.