The cerebellum at birth in therian mammals, with special reference to rodents

The stage of cerebellar development at birth was assessed in 23 species of placental mammals. Serial histological sections were examined and five stages in the differentiation of the cerebellar cortical layers were defined. A wide diversity of conditions at birth was found. The available evidence (after parsimony reconstruction) suggests that the last common ancestor of placentals was born with an altricial cerebellum in which the molecular layer was just present between the external granular layer and the prospective Purkinje cell layer. Some placental species have an even more altricial cerebellum at birth (e.g., Muscardinus avellanarius, Sorex araneus), with Mesocricetus auratus as the most altricial species among the taxa studied. In the newborn M. auratus a cerebellar anlage was present with only a loose accumulation of cells located at the dorsal cerebellar anlage above the ventricular neuroepithelial layer. The five species of caviomorph rodents examined here are relatively precocial as far as the cerebellum is concerned. The only other rodent species that has a similarly advanced state was the murid Acomys sp. Most of the life history variables examined were not strongly correlated with the cerebellar stage at birth if at all. However, a significant positive correlation (r(2) = 0.67) was observed between the cerebellar stage at birth and the gestation length and a significant negative correlation (r(2) = 0.31) was observed between cerebellar stage and the average litter size. The weak correlation may be due to sampling among different distantly related clades. The most mature cerebella at birth still had an external granular layer, indicating that the mossy fiber-granule cell connectivity is not yet fully developed and further indicating that this connectivity may depend on external experience to fully mature. All species that have their eyes open at birth also have the most mature cerebelli. The growth of the cortical layers was also studied in a postnatal ontogenetic series of the marsupial Monodelphis domestica. As is the case with placentals, the most advanced stage of cerebellar development coincides with the opening of the eyes.