Ectopic Purkinje cells in the adult rat: olivary innervation and different capabilities of migration and development after grafting.

The abnormal location of large numbers of neurones is characteristic of genetic mutations which impair the migratory processes of developing nerve cells. Nevertheless, the presence of small amounts of ectopic neurones is a fairly common finding even in normal adult animals. The first aim of this study was to investigate a series of features of ectopic Purkinje cells in normal adult rats and particularly to assess whether these cells are still capable of interacting with their normal afferents. Several displaced Purkinje cells, identified by anti-D28k calbindin immunolabelling as well as by typical morphological features, were present in the brainstem and cerebellum of all the examined animals. Two distinct morphological types of such cells could be recognized: 1) noncortical Purkinje cells, located in several areas of the dorsal brainstem and cerebellum, characterised by poorly developed and randomly oriented dendrites; and 2) cortical Purkinje cells, exclusively located in the dorsal cochlear nucleus, characterised by large dendritic trees oriented along parasagittal planes. Tracing experiments, in which Phaseolus vulgaris leucoagglutinin (PHA-L) was injected into the inferior olive, revealed that several ectopic Purkinje cells, belonging to both types, were contacted by the terminal arbours of olivary axons, structurally similar to cerebellar climbing fibres. On the basis of the observation that ectopic Purkinje cells were more frequent in the dorsal cochlear nucleus than in any other of the examined regions, we tested the hypothesis that this nucleus might represent a particularly favourable environment for the survival and development of Purkinje cells. By grafting embryonic cerebellar tissue in the fourth ventricle, only minimal migration of Purkinje cells into the recipient parenchyma was observed when the transplant was placed on the brainstem surface caudal to the cochlear nucleus. By contrast, when the graft was apposed to the latter nucleus, large numbers of Purkinje cells migrated and developed in its superficial layers. These Purkinje cells passed through all the different phases which characterise the normal ontogenesis of this neuronal population and finally developed mature structural features similar to those displayed by cortical ectopic Purkinje cells. This study demonstrates that at least some of the ectopic Purkinje cells receive their physiological olivary input. This fact indicates that Purkinje cells are able to attract olivary axons and establish specific connections, even if they are displaced in an abnormal environment. In addition, we show that the dorsal cochlear nucleus represents a particularly favourable environment for the survival and the development of Purkinje cells.