Alterations in Actin‐Binding β‐Thymosin Expression Accompany Neuronal Differentiation and Migration in Rat Cerebellum

The β4‐and β10‐thymosins, recently identified as actin monomer‐sequestering proteins, are developmentally regulated in brain. Using specific mRNA and protein probes, we have used in situ hybridization and immunohis‐tochemical techniques to investigate the distribution of the β‐thymosin mRNAs and their proteins in developing rat cerebellum. Early in postnatal development, both β‐thymosin mRNAs were expressed at highest levels in the postmitotic, premigratory granule cells of the external granular layer; expression diminished as granule cells migrated to and differentiated within the developing internal granular layer. In addition, both β‐thymosin proteins were present in bundles of cerebellar afferent fibers in the white matter at this time. Throughout the maturation period, both proteins were present in elongating parallel fibers in the upper portion of the molecular layer. Later in cerebellar development, thymosin β4, but not thymosin β10, was expressed in Golgi epithelial cells and Bergmann processes. Thymosin β4 was expressed in a small population of cells with microglial morphology scattered throughout the gray and white matter. Thymosin β10 was detected in an even smaller population of glia. Expression of thymosin β4 and thymosin β10 in premigratory granule cells and in growing neuronal processes is consistent with the possibility that both β‐thymosins are involved in the dynamics of actin polymerization during migration and process extension of neurons.