Reeler Mutant Mice Research Articles

Distribution and morphology of callosal commissural neurons within the motor cortex of normal and reeler mice.

Retrograde transport of horseradish peroxidase was used to examine the cells of origin of the callosal commissural fibers (CC neurons) in the primary motor cortex of normal and reeler mice. Quantitative analysis of the intracortical, laminar distribution, and dendritic orientation of CC neurons was performed in conjunction with qualitative observation of their morphology. For comparison, similar quantitative data were obtained for the cells of origin of the corticospinal tract (CST) of normal and reeler mice from materials described previously by Terashima et al. ('83). In the normal mouse, CC neurons are distributed in a bilaminar pattern such that the largest number of cells are located in supragranular layers II and III and in infragranular layer V. The majority of CC neurons are normal (upright) pyramids, although a few in the upper zone of layer VI are inverted pyramidal cells. In the reeler mutant, CC neurons are found in all cortical layers, but two-thirds are situated in the lower half of the cortex. On the basis of the celL shape and orientation of the apical dendrite, CC neurons of the reeler were classified into six morphological types: (1) typical pyramidal, (2) inverted pyramidal, (3) tumbled, (4) hook-shaped, (5) polymorphic, and (6) simple. The apical dendrites of the CC neurons in all layers of the cortex of the reeler mouse are randomly oriented; no direct relationship between the intracortical position of the soma and orientation of the apical dendrite was found. In contrast, CST neurons in the reeler mutant are concentrated in the outer third of the cortex, and there is a relationship between the laminar distribution of these cells and the alignment of their dendrites with respect to the pial surface: the apical dendrites of CST neurons lie in superficial layers tend to be oriented obliquely, whereas those of CST neurons in the deeper of cortex most often are oriented vertically, i.e, toward the pial surface. Quantitative analysis revealed that the relative intracortical positions of CC and CST neurons are reversed in the reeler mutant although both populations exhibited greater laminar disposition, and as a consequence, there is more intermingling of the two cell groups in the reeler than in the normal mouse. Thus, the present study suggests that the normal cytoarchitectonics of the primary motor cortex are inverted in the reeler mutant mouse.

Observations on golgi epithelial cells and granule cells in the cerebellum of the reeler mutant mouse

Cerebella of reeler mutant mice at two months of age were studied by the Golgi impregnation method to elucidate the detailed configurations of the Golgi epithelial cells and granule cells. On the basis of configuration of their Bergmann fibers, the Golgi epithelial cells of the reeler were relatively normal, or assumed inverted, swan neck, stellate, bipolar, small, or fan shapes, although their morphological changes were continuous rather than abrupt. Radially ascending Bergmann fibers, as well as disoriented ones, arose from the soma of these varieties of Golgi epithelial cells. In many areas, the granule cells were normally aggregated under the molecular layer and constituted the granular layer, although several granule cells were regionally arrested in migration within the molecular layer in some small areas. The axon of the granule cells ascended sinuously or obliquely, extended horizontally, looped around, or descended. In some cases, the granule cells had no T-like bifurcation of the axon, and assumed a bipolar form.

Comparative localization of Cajal-Retzius cells in the neocortex of normal and reeler mutant mice fetuses

The Cajal-Retzius cells (CRc) appear as an histochemically advanced subpopulation, since they are the only neocortical neurons able to express an acetylcholinesterase activity at an early stage of corticogenesis. Thus a comparative study of the CRc population was carried out on normal and reeler mice fetuses. The use of an appropriate cytochemical technique to reveal this endogenous cell marker, in conjunction with coronal and tangential sections show that: the Cajal-Retzius cells form a sheet in the more superficial layer of the embryonic cortex, with a relative cell density giving the same value in both phenotypes; a difference is observed in the relative depth position of these cells with regard to the pial basal lamina and in the pattern of cell grouping between the heterozygous and homozygous mutant littermates. This technique appears of great interest for a further analysis of the CRc population during cortical ontogenesis.

Analysis of abnormality of corticohistogenesis in the reeler mutant mice by producing chimera mice: Absence of abnormality in neurons in the reeler.

Reeler mutant mouse is characterized by the derangement of neuron positions in the cerebral cortex. In order to analyse the pathogenesis of the reeler abnormality, we have here analysed the chimera composed of reeler and normal on a half-and half basis judged from the coat color and GPI isozyme pattern. By hematoxylin-eosin staining and Golgi staining, the cerebral cortex was found to be normalized. The cortico-spinal tract neurons retrogradely labelled after injection of horseradish peroxidase into the lumbar cord were situated in layer V (the inner pyramidal layer) in the chimera, as in the normal mouse, whereas in the reeler such neurons were scattered diffusely throughout all levels of the corresponding cortical area.These observations suggest that reeler mutation does not directly affect the neurons and that abnormalities in process formation of neurons and fiber trajectory are consequential upon malposition of the cell.

Cytoarchitecture of Cerebral and Cerebellar Cortices of the Reeler Mouse

Homozygous reeler mice have action tremor, dystonic posture, and reeling atxic gait. The mutation at the reeler genetic locus (gene symbol: rl) causes cytoarchitectonic abnormalities to the cerebral and cerebellar cortices. Retrograde transport of horseradish peroxidase (HRP) was used to examine the cells of origin of the corticospinal tract (CST) and the callosal commissural fibers (CC) in the primary motor cortex of normal and reeler mice. In normal controls, CST neurons retrogradely labelled after. HRP injection into the lumbar cord were situated only in layer V. In the reeler, by contrast, the labelled CST neurons were scattered difusely all levels of the corresponding cortical area. In addition to the malpositioning of the somata, the labelled CST neurons in the cortex of the reeler could be divided into two major classes according to their dendritic pattern; typical pyramidal neurons and atypical ones. The typical pyramidal neuron had an atypical dendrite projecting from the superior pole of the soma and ascending toward the pia mater. The atypical pyramidal neurons consisted of six types: inverted, tumbled, bipolar, V-shaped, hook-shaped, and superficial polymorphic. The typical pyramidal neurons in the reeler tend to be situated relatively deep in the cortex and the atypical neurons tend to lie relatively superficially in the cortex.In the normal mouse, CC neurons retrogradely labelled after the injection of HRP into the primary motor cortex are distributed in a bilaminar pattern such that the largest number of cells are located in supragranular layers II and III and infragranular layer V. In the reeler mutant, CC neurons are found in all cortical layers, but two-thirds are situated in the lower half of the cortex. On the basis of the cell shape and orientation of the apical dendrite, CC neurons of the reeler are classified into six morphological types: typical pyramidal, inverted pyramidal, tumbled, hook-shaped, polymorphic, and simple. The apical dendrites of the CC neurons in all layers of the cortex of the reeler mouse are randomly oriented: no direct relationship between the intracortical position of the soma and orientation of the apical dendrite was found. In contrast, CST neurons in the reeler mutant are concentrated in the outer third of the cortex, and there is a relationship between the laminar distribution of these cells and the alignment of their dendrites with respect to the pial surface : the apical dendrites of CST neurons lie in superficial layers tend to be oriented obliquely, whereas those of CST neurons lie in the deeper cortex most often are oriented vertically, i.e., toward the pial surface. Quantitative analysis revealed that the relative intracortical positions of the CST and CC neurons are reversed in the reeler mutant although both populations exhibited greater laminar disposition, and as a consequence, there is more intermingling of the two cell groups in the reeler than in the normal mouse. Thus, the present study suggests that the normal cytoarchitectonics of the primary motor cortex are inverted in the reeler mutant mouse.Cerebella of the normal and reeler mutant mice at two months of age were studied by the Golgi impregnation method to elucidate the detailed configurations of the Golgi epithelial cells. On the basis of configuration of their Bergmann fibers, the Golgi epithelial cells of the reeler were relatively normal, or assumed inverted, swan neck, stellate, bipolar, small, or fan shapes, although their morphological changes were continuous rather than abrupt. A relatively normal type of Golgi epithelial cells which somewhat resemble those of normal controls was present in the reeler, although some differences between them in detailed appearance existed. It is important to note that the radially organized Bergmann fibers ascending from the somata of this type of Golgi epi-thelial cells gave rise to a glial architecture of normal appearance in small areas of

Mosaic Expression of the Reeler and Normal Phenotypes in the Cerebral Cortex in Reeler-Normal Chimeras at a Late Embryonic Stage: (reeler mutant mouse/mouse chimera/cerebral cortex/radial bundle/matrix cell).

The cerebral cortex of reeler-normal chimera embryos was studied by hematoxylin-eosin staining and fractographic scanning electron microscopy in comparison with the cortices of normal and reeler mutant mice. The cerebral cortex of normal mice had a plexiform layer, which was composed of a fine meshwork of matrix cell processes. Spindle shaped neuroblasts formed a radial lining columnar structure, which was formed by attachment of migrating neuroblasts to the radial bundles. The cerebral cortex of reeler mutants did not show a plexiform layer and the cells were round with no radially columnar structures, and no radial bundles. In reeler-normal chimera embryos, the thickness of the plexiform layer varied in different parts of the cerebral cortex. In parts where the plexiform layer was present, neuroblasts were spindle-shaped and had a radially oriented columnar structure (normal type). But where the plexiform layer was absent, the neuroblasts were round with a radial architecture (reeler type). Intermediates between the reeler and control types were also observed. Since mosaic expression of the two phenotypes, was observed in chimeras, the reeler abnormality is apparently not caused by humoral factors. The possible mechanism of cell migration is discussed.

Immunohistochemical studies and 2-dimensional gel electrophoretic analysis on the cerebellum from the staggerer mutant mouse

Immunohistochemical studies and 2-dimensional gel electrophoretic analysis on the cerebellum from the staggerer mutant mouse

A qualitative and quantitative light microscopic study of the inferior olivary complex in the adult staggerer mutant mouse.

In the homozygous staggerer (sg/sg) mutant mouse, most of the Purkinje cells (the primary targets for olivocerebellar climbing fibers) are missing or ectopic. In this study, the organization and cell number of the inferior olivary complex in sg/sg were determined and compared to the inferior olive of the wildtype (+/+) mouse. Our results indicate that there is a marked disorganization and loss of almost 60% of the cells within the inferior olivary complex. This suggests that the inferior olive may be secondarily affected by the staggerer gene due to the defects in its main target cell, the Purkinje cell.

Immunohistochemical studies and 2-dimensional gel electrophoretic analysis on the cerebellum from the staggerer mutant mouse

Immunohistochemical studies and 2-dimensional gel electrophoretic analysis on the cerebellum from the staggerer mutant mouse

3H]GABA binding in the cerebellum of the reeler murine mutant

In the cerebellum of the reeler mutant mouse, characterized morphologically by depletion of the granule cell population and abnormal synapse formation, increased GABA concentration and alterations in [ 3H]GABA binding have been observed. This study shows decreased affinity of the Na +-independent, high affinity GABA binding component of synaptosomal membranes and an increased affinity of the Na +-dependent, high affinity GABA binding component in reeler cerebellar homogenate and synaptic membranes. In contrast to the changes in affinity, the number of both Na +-dependent and Na +-independent binding sites was not significantly altered. The decreased affinity of the Na +-independent GABA binding and the increased affinity of the Na +-dependent binding, evidenced only in cerebellar tissue, were interpreted to indicate, respectively, hypo- and hypersensitivity of the postsynaptic and presynaptic elements of cerebellar GABAergic synapses, induced by the depressed excitatory granule cell input and/or the increased mossy fiber contact with the ectopic Purkinje cells.

Altered Myelinated Fiber Trajectory at Various Postnatal Days in the Cerebral Cortex of Reeler Mice by Immunohistochemical Stain with MBP Antiserum

The cerebral cortex of reeler mutant mice characterized by the derangement of cell position was analyzed by an immunohistochemical staining technique using antiserum against myelin basic protein. All the myelins of the myelinated fibers were stained, so that the fiber trajectories could be clearly traced. In the present study the changes in the fiber trajectories were accompanied by changes in myelination sites. Myelinated fibers were localized in the central white matter and gradually became sparser toward the surface of the normal cerebral cortex, while in the reeler myelinated fibers were observed throughout the cortical plane. From the immunohistochemical analysis of the changes occurring in the myelinated fiber pathways in developmental sequence, it was also shown that the fiber bundle accumulation was thin at the earlier stages (e.g. the 15th postnatal day) and became thick as the animal grew, and the timing of myelination in the reeler was similar to that of the control.

Architectonic and hodological organization of the cerebellum in reeler mutant mice

The architectonic and hodologic organization of the reeler cerebellum has been studied by means of immunohistochemistry, general cell and fiber stains and by horseradish peroxidase and autoradiographic tracing methods. Malposition of Purkinje cells, which varies in degree, is the most salient architectonic anomaly of the mutant cerebellum. Mapping the distribution of Purkinje cells is facilitated by a monoclonal antibody which selectively stains neurons of this class in the cerebellum. Although some Purkinje cells form a normal monolayer, most lie in heterotopic positions within or below the granule cell layer. The major contingent is segregated in subcortical masses in the depths of the cerebellum. Fiber bundles continuous with the cerebellar peduncles run in septa between the subcortical Purkinje cell masses. The distribution of Purkinje cell masses as well as the roof nuclei and areas of normal cortex and fiber bundles are identical from animal to animal. These consistent architectonic variations serve to partition the reeler cerebellum into 7 sagittally oriented compartments: one medial, two intermediate, two lateral and two additional lateral lobular appendages which may correspond to paraflocculus and/or flocculus of the normal cerebellum. The topography of the reeler olivocerebellar, or climbing fiber, system is normal in that the caudal-to-rostral axis of the olivary complex maps onto the medial-to-lateral axis of the contralateral hemicerebellum. The climbing fiber projection in reeler, like that of the normal animal, appears to be organized in parasagittal strips. In the mutant, mossy fibers from the pons and spinal cord project respectively to the lateral and medial cerebellar fields, and overlap in the intermediate compartment. They thus invest different and to a large extent complementary cerebellar territories, which approximate the architectonic divisions. This segregation of the two principal mossy fiber systems is not so marked in the normal cerebellum. In terms of laminar distribution, the pontine projection is distributed principally to the granule cell stratum in the mutant. The reeler spinocerebellar afferents, by contrast, project not only to the granule cell layer but also to the heterotopic Purkinje cells. The present observations suggest that the primary defect in the reeler cerebellum is malposition of Purkinje cells. As appears to be the case during development of the forebrain in reeler, the mutation may affect the terminal phase of migration of Purkinje cells in the cerebellum. However, this mechanism alone is insufficient to account for the observation that the anomaly of Purkinje cell position varies in degree, particularly at the boundaries between architectonic compartments. Since these boundaries are also zones of overlap in the distribution of mossy fiber terminations from the brainstem and the spinal cord, abnormal interaction of young postmigratory Purkinje cells with these afferent systems may contribute to the architectonic anomaly of the reeler cerebellum.

Electrophysiological properties of in vitro hippocampal pyramidal cells from normal and staggerer mutant mice

Electrophysiological properties of intracellularly recorded CA1 pyramidal cells from normal and staggerer mice were compared by using hippocampal slices maintained in vitro. In staggerer mice, the passive membrane properties of these neurons as well as their synaptic potentials elicited by stratum radiatum stimulation were very similar to those observed in normal mice. In control and mutant mice and in standard Krebs solution, CA1 pyramidal cells mainly fired tetrodoxin (TTX)-sensitive fast spikes but could also generate slow spikes. In both groups, replacement of calcium (Ca) by barium (Ba) or introduction of TEA in the bathing medium prolonged the repolarization of the fast spikes and suppressed the brief spike afterhyperpolarization which normally followed them, thus suggesting that both events involve fast potassium conductances. Furthermore, in both groups of animals, TEA and Ba enhanced the slow spikes and induced the appearance of prolonged depolarizations. These slow events were TTX-resistant and were abolished by the Ca channel blockers cadmium or cobalt, thus suggesting that they are Ca-dependent. On the whole, the present results indicate that the staggerer mutation which yields marked abnormalities in the bioelectrical properties of cerebral Purkinje cells has no such effect on CA1 pyramidal cells.

Cellular distribution of gangliosides in the developing mouse cerebellum: analysis using the staggerer mutant.

The distribution of cerebellar gangliosides was studied in staggerer (sg/sg) mutant mice, where the majority of granule cells die after completing their migration across the molecular layer. In addition, the external granule cell layer in sg/sg mice persists longer than in normal mice. Moreover, in the sg/sg cerebellum, Purkinje cells are significantly reduced in number, and almost none have tertiary branchlet spines. The loss of Purkinje cells and granule cells in sg/sg mice is accompanied by an early-onset reactive gliosis that continues through adulthood. By correlating changes in ganglioside composition with the well-documented histological events of cerebellar development in normal and sg/sg mice, we obtained strong evidence for a nonrandom cellular distribution of gangliosides. The sharpest reduction in the GD1a content of sg/sg cerebellum occurred after 15 days of age, coincident with granule cell loss. GT1a, on the other hand, was significantly reduced from 15 through 150 days in the sg/sg mice. GD3 is a major ganglioside of the undifferentiated granule cell, but it becomes rapidly displaced by the more complex gangliosides with the onset of granule cell maturation. In the sg/sg mice, GD3 persisted at abnormally high levels from 15 to 28 days and then accumulated through adulthood. These findings, and those from other cerebellar mouse mutants, suggest that GD1a is enriched in granule cells and that GT1a is enriched in Purkinje cells. Our findings also suggest that GT1a is more concentrated in branchlet spines than in other regions of the Purkinje cell membrane. GT1b appears to be enriched in both granule cells and Purkinje cells, whereas GM1 appears to be enriched in myelin. Furthermore, the apparent persistence of the embryonic ganglioside GD3 in sg/sg mice results from an early-onset reactive gliosis, together with a partial retardation in granule cell maturation. The accumulation of GD3 beyond 28 days reflects the continued accretion of GD3 in reactive glia.

Effect of excitatory amino acids on purkinje cell dendrites in cerebellar slices from normal and staggerer mice

The sensitivity of Purkinje cells to short pulse applications of l-aspartate, l-glutamate and related derivatives in their dendritic fields was tested in normal and staggerer mutant mice using cerebellar slices maintained in vitro. In normal mice, the response of Purkinje cells to l-aspartate and l-glutamate consisted of a transient and dose-dependent increase of their firing of simple spikes. The potency of l-aspartate in exciting Purkinje cells was lower than that of l-glutamate when the two drugs were released from adjacent barrels of the same iontophoretic electrode. Quisqualate was an even more potent excitant of these cells than l-aspartate and l-glutamate, whereas N-methyl- dl-aspartate had little or no effect. In staggerer mutant mice, the sensitivity of Purkinje cells to l-aspartate, l-glutamate and quisqualate was not significantly altered. On the contrary, N-methyl- dl-aspartate had a much stronger potency than normal in exciting Purkinje cells although this was still smaller than that of the other agonists tested. These results suggest that the sensitivity of Purkinje cells to l-aspartate and l-glutamate, i.e. the putative neurotransmitters of the climbing and parallel fibers respectively, remains largely normal in staggerer mice. In contrast, in the mutant, N-methyl- d-aspartate receptors are likely to be much more developed than normal.

Observations on the brainstem-spinal descending systems of normal and reeler mutant mice by the retrograde HRP method.

Brainstem neurons which project to the lumbar spinal level were identified in both reeler mutant mice and normal controls (Balb/c mice) by the retrograde horseradish peroxidase (HRP) technique. In normal controls after HRP injection into the lumbar cord, retrogradely labelled neurons were observed in (1) the lateral vestibular nucleus, (2) the pontine and medullary reticular formations including the nucleus centralis caudalis pontis, nucleus gigantocellularis, nucleus paragigantocellularis, nucleus raphe magnus et pallidus, and nucleus centralis medullae oblongatae pars ventralis et dorsalis, and (3) the dorsal column nuclei, i.e., the nucleus gracilis and nucleus cuneatus medialis. In reeler mutant mice, labelled neurons were again seen in the nuclei referred to above, and their cellular type and distribution patterns within the corresponding nuclei were similar to those of the normal controls. These observations suggest that (1) the brainstem nuclei of reeler mutant mice which project to the lumbar spinal cord are cytoarchitecturally normal, (2) the reeler genetic locus (rl) does not affect the nonlaminated structures in the brainstem, at least those referred to above, and (3) the motor dysfunctions observed in the reeler, such as action tremor, dystonic posture, and reeling ataxic gait, are not attributable to the brainstem-spinal descending systems.

Selective absence of calcium spikes in Purkinje cells of staggerer mutant mice in cerebellar slices maintained in vitro.

The bioelectrical properties of Purkinje cells (intrasomatic recordings) were studied in sagittal cerebellar slices of both adult staggerer and control mice. Mean input resistances of Purkinje cells were 25 +/- 4 M omega and 48 +/- 7 M omega in normal and staggerer mice respectively. In both groups, time-dependent inward rectifications were apparent in the hyperpolarizing voltage-responses. In normal mice, tetrodotoxin (TTX)-sensitive simple spikes and slower-rising multiphasic spikes, abolished when Ca was replaced by Cd in the bath, spontaneously occurred in Purkinje cells. These Na- and Ca-dependent spikes were also elicited by depolarizing current pulses. In the mutant, Ca spikes were never observed, even in strongly depolarized cells. On the contrary, TTX-sensitive simple spikes occurred spontaneously or were elicited by depolarizing current pulses. When Ca was replaced by Ba in the bath, the Ca spikes evoked in normal Purkinje cells by direct stimulation were first enhanced and then replaced by prolonged action potentials (1-6 s in duration) which were TTX-resistant and Cd-sensitive. These (Ba) action potentials were also triggered by climbing fibre activation of the cells. In staggerer mice, Ca spikes were never elicited by direct stimulation in Ba-containing medium, although in a few cells prolonged action potentials were occasionally elicited by depolarizing current pulses. However, this latter type of response was never evoked by climbing fibre activation of Purkinje cells. In the mutant, extracellular application of tetraethylammonium (TEA) generated prolonged action potentials, the plateaux of depolarization of which were less positive than those elicited by Ba in control mice. These plateaux were abolished by TTX and left unaffected by the substitution of Ca by Cd in the bath, suggesting that they were due to a non-inactivating Na conductance. On the whole, the present study strongly suggests that voltage-dependent Ca channels are missing in most staggerer Purkinje cells or at least that their characteristics and/or distribution are such that they cannot be activated. Na channels appear unaffected.

Postnatal developments of AChE activity in the hippocampus of the reeler mutant mouse

The acetylcholinesterase (AChE) activity of the hippocampus and the dentate gyrus in the reeler mutant mouse was studied histochemically by the method of Karnovsky and Roots 13. Developmental studies were done on days 0,3,6,9,12,15,18 and 21 postnatally, and in adults. The adult reeler mouse differs from the normal mouse in that there is no accumulation of the activity at the junction between the stratum lacunosum-moleculare and the stratum radiatum, weaker and divided activity in CA1, and the translocation of the activity to the granule cell zone of the dentate gyrus. The results obtained are considered to be due to the cells' ectopia. But also, other factors such as genesis of the cells in regard to constructing the cytoarchitecture, may influence them. According to the postnatal observations, the developmental pattern of AChE activity in the reeler mouse is about the same as in the normal mouse. AChE-rich cells in the hippocampus showed maximum activity and number from days 12 to 15. The neuropile reactions increased after day 9 in both kinds of mice. The relationship between the movement of the AChE positive neuropiles and cells and their functions are also discussed.

Role of staggerer gene in determining cell number in cerebellar cortex. I. Granule cell death is an indirect consequence of staggerer gene action

Mouse chimeras were made by aggregating two embryos at the 8-cell stage of development. These experimental mosaic animals are useful in assessing whether the effect of a mutant gene in a given cell type is direct (intrinsic) or indirect (extrinsic). The results reported are an analysis of the involvement of the staggerer gene in the phenotype of granule cell death observed in staggerer mutant mice. The ichthyosis mutation has been used as an independent cell marker. This mutation produces no known neurological defects but does cause a characteristic clumping of heterochromatin, especially in small cells. Analysis of two staggerer ↔ ichthyosis chimeras leads to the conclusion that staggerer granule cells, that would have died in the mutant, are rescued in the mosaic cerebella. The death of granule cells in the homozygous staggerer mutant is thus an indirect, epigenetic consequence of direct gene action in other cells. The implications and limitations of this conclusion are discussed.

Modified composition of major ontogenetically regulated mRNAs and proteins in the cerebellum of old and of staggerer mice

The macromolecular composition of the cerebellum was examined in young and old mice, and in staggerer mutant mice, as compared with their background control strain. We examined the in vitro translation products of cerebellum mRNA, which reflect the biosynthetic potential of cell bodies endogenous to the cerebellum. Simultaneously, we examined the composition of the major cerebellum proteins, which includes the contribution of incoming fibers, in addition to the proteins composing cerebellar cells. Changes in the concentrations of various major proteins and a significant reduction in the translational efficiency of RNA were observed in the cerebellum of old BALB/c mice. This reduction probably does not reflect a specific damage to interneurons, since RNA from the cerebellum of 5-month-old staggerer mice was as efficient in translation in vitro as RNA from the cerebellum of mice from C57B6J normal background strain. Several of the major cerebellar proteins were identified by 2-dimensional gel electrophoresis. Changes were observed at the level of and the microheterogeneity of tubulin from the cerebellum of old, as compared with young mice. The aging-related modifications in cerebellar tubulin may be regulated at the level of mRNA, since mRNA from the cerebellum of old mice appeared to produce lower amounts of a polypeptide band co-migrating with tubulin. When compared with translation products directed by mRNA from normal cerebellum, most of the major identified polypeptides produced by mRNA from staggerer cerebellum showed marked differences in their relative inensity. Thus, this mutation appears to change the composition of cerebellar mRNA. These differences were analyzed together with previously obtained data on the composition of translation products during development of normal and of irradiation-agranulated cerebellum. The combined analysis of cerebellar mRNA products permits us to tentatively assign defined protein markers to specific cerebellar cell types and periods in development.