Cellular and Axonal Diversity in Molecular Layer Heterotopia of the Rat Cerebellar Vermis

Abstract

Molecular layer heterotopia of the cerebellar primary fissure are a characteristic of many rat strains and are hypothesized to result from defect of granule cells exiting the external granule cell layer during cerebellar development. However, the cellular and axonal constituents of these malformations remain poorly understood. In the present report, we use histochemistry and immunocytochemistry to identify neuronal, glial, and axonal classes in molecular layer heterotopia. In particular, we identify parvalbumin-expressing molecular layer interneurons in heterotopia as well as three glial cell types including Bergmann glia, Olig2-expressing oligodendrocytes, and Iba1-expressing microglia. In addition, we document the presence of myelinated, serotonergic, catecholaminergic, and cholinergic axons in heterotopia indicating possible spinal and brainstem afferent projections to heterotopic cells. These findings are relevant toward understanding the mechanisms of normal and abnormal cerebellar development.

Neurodevelopmental Malformations of the Cerebellar Vermis in Genetically Engineered Rats

“…Consistent with a model of migration defect of granule cells as a primary cause of these malformations which we have termed, molecular layer heterotopia (MLH), our group and others have shown that Bergman glial cells expressing nestin [23] or glial fibrillary acidic protein [22] have disorganized radial fibers. Moreover, migrating granule cells expressing doublecortin were found to have abnormal migratory morphologies and polarity in areas with developing heterotopia [23]. Why heterotopia are only found along the primary fissure remains unknown.…”

“…Remarkably, these malformations are almost exclusively found along the primary fissure in regions lacking pia and are characterized by granule cells present in the molecular layer that fail to migrate to the internal granule cell layer. Although other neuron and glial cell types are present in malformations [22], the overwhelming majority of cells present are granule cells expressing doublecortin [23] or alpha-6 ionotropic gammaaminobutyric acid receptors [22]. Consistent with a model of migration defect of granule cells as a primary cause of these malformations which we have termed, molecular layer heterotopia (MLH), our group and others have shown that Bergman glial cells expressing nestin [23] or glial fibrillary acidic protein [22] have disorganized radial fibers.…”

“…Although other neuron and glial cell types are present in malformations [22], the overwhelming majority of cells present are granule cells expressing doublecortin [23] or alpha-6 ionotropic gammaaminobutyric acid receptors [22]. Consistent with a model of migration defect of granule cells as a primary cause of these malformations which we have termed, molecular layer heterotopia (MLH), our group and others have shown that Bergman glial cells expressing nestin [23] or glial fibrillary acidic protein [22] have disorganized radial fibers. Moreover, migrating granule cells expressing doublecortin were found to have abnormal migratory morphologies and polarity in areas with developing heterotopia [23].…”

“…For example, we showed that the distribution of myelinated axons associated with heterotopia was altered which we infer to include spinocerebellar afferents [23]. Moreover, we demonstrated axonal changes in MLH from brainstem afferents including cholinergic, catecholaminergic, and serotonergic axonal subtypes [23]. Together, these data raise the possibility that functional/behavioral changes may accompany MLH.…”

“…While physiological evidence of altered cerebellar function in brains with heterotopia is also lacking, we recently demonstrated axonal remodeling associated with MLH. For example, we showed that the distribution of myelinated axons associated with heterotopia was altered which we infer to include spinocerebellar afferents [23]. Moreover, we demonstrated axonal changes in MLH from brainstem afferents including cholinergic, catecholaminergic, and serotonergic axonal subtypes [23].…”