Foliation of the cerebellar vermis in trisomy 19 mice

Spontaneous malformations of the cerebellar vermis: Prevalence, inheritance, and relationship to lobule/fissure organization in the C57BL/6 lineage

The complex neuronal circuitry of the cerebellum is embedded within its lamina, folia, and lobules, which together play an important role in sensory and motor function. Studies in mouse models have demonstrated that both cerebellar lamination and lobule/fissure development are under genetic control. The cerebellar vermis of C57BL/6 mice exhibits spontaneous malformations of neuronal migration of posterior lobules (VIII-IX; molecular layer heterotopia); however, the extent to which other inbred mice also exhibit these malformations is unknown. Using seven different inbred mouse strains and two first filial generation (F1) hybrids, we show that only the C57BL/6 strain exhibits heterotopia. Furthermore, we observed heterotopia in consomic and recombinant inbred strains. These data indicate that heterotopia formation is a weakly penetrant trait requiring homozygosity of one or more C57BL/6 alleles outside of chromosome 1 and the sex chromosomes. Additional morphological analyses showed no relationship between heterotopia formation and other features of lobule/fissure organization. These data are relevant toward understanding normal cerebellar development and disorders affecting cerebellar foliation and lamination.

Dysplasie du cortex cérébelleux : aspects en IRM et signification

Because it is now possible to obtain high-resolution multiplanar MR imaging of the cerebellum and because of the developing interest on the role of the cerebellum on higher brain functions, we have decided to study the process of cerebellar fissuration. All brain MRI examinations performed in children for varied neurological and neurosurgical indications, especially children with non-specific mental retardation and patients with cerebral malformation detected at initial imaging work-up, were reviewed. Fissuration and lobulation anomalies (abnormal orientation of fissures, pseudopolymicrogyria, cortical thickening, subcortical cysts and heterotopia) were identified that we called cerebellar cortical dysplasia (CCD). In order to better understand the origin of this malformation, current data on cerebellar embryogenesis and histogenesis will be reviewed, and the pathological and radiological features will be illustrated. Milder forms of CCD represent a distinct group of anomalies that should be distinguished from other types of cerebellar dysplasia (agenesis, hypoplasia or complex dysplasia with involvement of the cerebellar vermis (rhombencephalosynapsis)) or combined cerebellar and cerebral dysplasia (muscular dystrophies and lissencephaly). Recognition of cerebellar cortical dysplasia could be a first step towards a broader understanding of its pathogenesis and significance.

Developmental instability of the cerebellum and its relevance to Down syndrome

It has been recognized for many years that cerebellar abnormalities are frequently observed in association with Down syndrome (DS). An important question to be asked about these and other findings in DS is whether their occurrence (i) is attributable to specific loci on the triplicated chromosome or chromosomal segment or (ii) derives from exaggerated responses secondary to the genetic imbalance resulting from trisomy (Ts). Recently, similar cerebellar alterations were observed in subjects with DS and in Ts65Dn mice (Baxter et al., 2000), mice segmentally trisomic for a portion of chromosome 16, which is homologous for loci on the long arm of human chromosome 21. It was concluded by these authors that the occurrence of similar cerebellar changes in DS and in the DS mouse model resulted from triplication of these homologous loci in the two trisomic organisms, i.e. cerebellar development is affected similarly by homologous loci in each species. They wrote that their study of Ts65Dn mice "correctly predicts an analagous pathology in humans". . . and that. . . "The candidate region of genes on chromosome 21 affecting cerebellar development in DS is therefore delimited to the subset of genes whose orthologs are at dosage imbalance in Ts65Dn mice, providing the first localization of genes affecting a neuroanatomical phenotype in DS." Findings described in this review suggest otherwise--that cerebellar findings in DS and in the Ts65Dn mouse are a result of exaggerated vulnerability in general of the cerebellum to disturbing events and that liability to expression of response(s) is exacerbated by trisomy. This conclusion is based on the following: (i) the cerebellum has an extended postnatal development; (ii) numerous genetic, environmental, epigenetic and metabolic conditions express cerebellar changes similar to those observed in Down syndrome; (iii) most if not all chromosomal imbalance syndromes express similar cerebellar abnormalities; (iv) the cerebellum is particularly sensitive to diverse toxic agents which may act prenatally, postnatally and/or in the mature organism; and (v) cerebellar abnormalities similar to those found in Ts65Dn mice have been described in Ts19 mice which have no segments homologous to any segment of human chromosome 21. An unavoidable conclusion from the review is that triplication of specific loci on 21q is an unlikely explanation for the cerebellar findings in DS. A simple positive control, in which the effect of triplication of loci other than those in question on a specific phenotype, should be used in experiments comparing human and experimental trisomies. As pointed out many years ago by Lorke and his coworkers (Lorke et al., 1989; Lorke, 1994; Lorke and Albrecht, 1994) similar phenotypic findings in the presence of different trisomies in the same species would suggest that the trisomic state itself rather than the gene content of a particular trisomy is responsible for the genesis of traits at issue.

Cerebellar microfolia and other abnormalities of neuronal growth, migration, and lamination in the Pit1dw-J homozygote mutant mouse

The Snell dwarf mouse (Pit1dw-J homozygote) has a mutation in the Pit1 gene that prevents the normal formation of the anterior pituitary. In neonates and adults there is almost complete absence of growth hormone (GH), prolactin (PRL), thyroxin (T4), and thyroid-stimulating hormone (TSH). Since these hormones have been suggested to play a role in normal development of the central nervous system (CNS), we have investigated the effects of the Pit1dw-J mutation on the cerebellum and hippocampal formation. In the cerebellum, there were abnormalities of both foliation and lamination. The major foliation anomalies were 1) changes in the relative size of specific folia and also the proportional sizes of the anterior vs posterior cerebellum; and 2) the presence of between one and three microfolia per half cerebellum. The microfolia were all in the medial portion of the hemisphere in the caudal part of the cerebellum. Each microfolium was just rostral to a normal fissure and interposed between the fissure and a normal gyrus. Lamination abnormalities included an increase in the number of single ectopic granule cells in the molecular layer in both cerebellar vermis (86%) and hemisphere (40%) in comparison with the wild-type mouse. In the hippocampus of the Pit1dw-J homozygote mouse, the number of pyramidal cells was decreased, although the width of the pyramidal cell layer throughout areas CA1-CA3 appeared to be normal, but less densely populated than in the wild-type mouse. Moreover, the number of granule cells that form the granule cell layer was decreased from the wild-type mouse and some ectopic granule cells (occurring both as single cells and as small clusters) were observed in the innermost portion of the molecular layer. The abnormalities observed in the Pit1dw-J homozygote mouse seem to be caused by both direct and indirect effects of the deficiency of TSH (or T4), PRL, or GH rather than by a direct effect of the deletion of Pit1.

Altered postnatal development of the visual cortex in trisomy 19 mice

The development of the visual cortex was studied in 30 Trisomy 19 (Ts19) mice aged 1-16 days postpartum and their euploid littermates. Morphogenesis of the Ts19 visual cortex, though delayed in development, followed the regular sequence observed in control littermates. Early morphogenetic events, such as obliteration of the ventricular lumen, disappearance of the ventricular zone, formation of a visible apical dendrite, as well as disappearance of both migrating neurons and the columnar organization of bipolar preneurons were delayed by 1 day; maturation steps occurring later such as appearance and disappearance of perikaryal basophilia were delayed by 2 days. Myelination of the white matter was similarly retarded by 2 days. The fronto-occipital length of the cerebral hemispheres and the thickness of the visual cortex were decreased by about 20%, consistent with a hypoplasia of the Ts19 neocortex. Unlike in the cerebral cortex of human Ts21, morphometric analysis of the visual cortex of Ts19 mice did not give any indication of a selective deficit in a particular neuron population; the increased cell density and the reduced nuclear volume observed during early postnatal development are attributable to a maturational delay. The relevance of these results with respect to the mechanisms underlying neuropathological alterations in human Ts21 is discussed.