Molecular cell identities in the mediodorsal thalamus of infant mice and marmoset

The mediodorsal thalamus (MD) is a higher‐order nucleus located within the central thalamus in many mammalian species. Emerging evidence from MD lesions and tracer injections suggests that the MD is reciprocally connected to the prefrontal cortex (PFC) and plays an essential role in specific cognitive processes and tasks. MD subdivisions (medial, central, and lateral) are poorly segregated at the molecular level in rodents, leading to a lack of MD subdivision‐specific Cre driver mice. Moreover, this lack of molecular identifiers hinders MD subdivision‐ and cell‐type‐specific circuit formation and function analysis. Therefore, using publicly available databases, we explored molecules separately expressed in MD subdivisions. In addition to MD subdivision markers, we identified several genes expressed in a subdivision‐specific combination and classified them. Furthermore, after developing medial MD (MDm) or central MD (MDc) region‐specific Cre mouse lines, we identified diverse region‐ and layer‐specific PFC projection patterns. Comparison between classified MD marker genes in mice and common marmosets, a nonhuman primate model, revealed diverging gene expression patterns. These results highlight the species‐specific organization of cell types and their projections in the MD thalamus. The mediodorsal thalamus (MD) can be divided into three distinctive regions, which project to the different cortical areas. Here we searched for subdivision‐specific molecular markers in developing mouse MD thalamus. We made an MD region‐specific Cre mouse line based on this search, which clearly shows specific projections to PrL/IL and AID in PFC. Further, we showed molecular organization in developing common marmoset, which showed dynamic expression differences compared to mouse MD, suggesting diverse MD function and connectivity in different species.