Studies on ectopic granule cells in the cerebellar cortex. III. An investigation into the restoration of the external granular layer after partial destruction.

In order to strengthen a hypothesis concerning the occurrence of ectopic granule cells, one of the assumptions made was tested systematically. The reaction of the EGL to partial destruction by various single doses by hydroxyurea at various ages was followed. Under all conditions examined, re-population of the EGL takes place--rapidly after lower doses, slowly after high doses of HU. The phenomena observed are discussed with a view on the hypothesis mentioned. Re-population is beneficial, but may itself be a major pathogenetic factor in certain developmental malformations by upsetting the "time-schedule". The results are also of potential interest for an analysis of the regulation of normal cerebellar morphogenesis.