The cerebellar nuclei are the primary output regions of the cerebellum and vary in number across species. The authors set out to determine developmental mechanisms that could explain how new cerebellar nuclei arise during evolution. Specifically, they investigate whether diversification of progenitor populations in the rhombic lip (RL) drives the emergence of additional nuclei and how excitatory and inhibitory neuronal lineages contribute to nuclear identity and organization.
To address these questions, the study uses developmental time courses combining single-cell transcriptomics and spatial transcriptomics in two vertebrate models: mouse and chicken. These approaches were applied across developmental stages to resolve progenitor populations, their lineage relationships, and spatial patterning within the RL and emerging cerebellar nuclei. The comparative design between mouse and chicken is used to infer conserved versus species-specific developmental programs that could underlie evolutionary diversification.
The authors report that evolutionarily newer, canonical cerebellar nuclei are produced through diversification of progenitors in the early rhombic lip. In this model, the early RL contains progenitor pools that become nucleus-specific and generate excitatory neurons that define new nuclear identities. Diversification within the early RL therefore provides a progenitor-level mechanism yielding additional cerebellar nuclei with progressively higher-order functional specializations.
In contrast to the early RL, the late rhombic lip contributes specifically to higher-order subnuclei of the medial nucleus. The late RL-derived populations assemble into structures that the authors describe as forming a non-canonical olivocerebellar circuit. This distinction between early and late RL contributions suggests temporal layering of progenitor output that assigns different structural and circuit-level roles to excitatory neurons born at distinct developmental windows.
The study finds that inhibitory neurons are not drivers of new nucleus formation. Rather, inhibitory neurons are incorporated into nuclear territories after excitatory nuclei are established by RL-derived progenitors. This sequence indicates that excitatory lineage diversification sets nuclear boundaries and identity first, with inhibitory populations subsequently integrating into those preformed territories.
Analyses indicate that nuclear identity and spatial organization are established in part by co-option of programs involved in border formation within conserved progenitor cell types. In other words, conserved progenitors adopt local border-forming molecular programs that help segregate sister cell types into discrete nuclear territories. This co-option of patterning programs is proposed as a developmental mechanism enabling the emergence of distinct nuclei from a conserved progenitor repertoire.
Based on these developmental observations in mouse and chicken, the authors propose a model in which new brain regions, exemplified by additional cerebellar nuclei, can evolve through diversification of excitatory progenitors and spatial segregation of their sister cell types. In this model, the early diversification of excitatory progenitors yields new output nuclei, while inhibitory neurons—described as generic relative to the emergent excitatory identities—fill in the territories after they have been specified. The authors suggest this sequence could underlie stepwise increases in cerebellar complexity across evolution.
This work is presented as a preprint and has not been certified by peer review. The summary above reflects results and interpretations reported by the authors using comparative single-cell and spatial transcriptomic datasets in mouse and chicken. Specific experimental details, quantitative results, raw data, and extended analyses are reported in the preprint and supplementary material; those items should be consulted directly for in-depth evaluation. The authors declared no competing interests.