Genetic Mechanism for the Cyclostome Cerebellar Neurons Reveals Early Evolution of the Vertebrate Cerebellum.

The vertebrate cerebellum arises at the dorsal part of rhombomere 1, induced by signals from the isthmic organizer. Two major cerebellar neuronal subtypes, granule cells (excitatory) and Purkinje cells (inhibitory), are generated from the anterior rhombic lip and the ventricular zone, respectively. This regionalization and the way it develops are shared in all extant jawed vertebrates (gnathostomes). However, very little is known about early evolution of the cerebellum. The lamprey, an extant jawless vertebrate lineage or cyclostome, possesses an undifferentiated, plate-like cerebellum, whereas the hagfish, another cyclostome lineage, is thought to lack a cerebellum proper. In this study, we found that hagfish Atoh1 and Wnt1 genes are co-expressed in the rhombic lip, and Ptf1a is expressed ventrally to them, confirming the existence of r1's rhombic lip and the ventricular zone in cyclostomes. In later stages, lamprey Atoh1 is downregulated in the posterior r1, in which the NeuroD increases, similar to the differentiation process of cerebellar granule cells in gnathostomes. Also, a continuous Atoh1-positive domain in the rostral r1 is reminiscent of the primordium of valvula cerebelli of ray-finned fishes. Lastly, we detected a GAD-positive domain adjacent to the Ptf1a-positive ventricular zone in lampreys, suggesting that the Ptf1a-positive cells differentiate into some GABAergic inhibitory neurons such as Purkinje and other inhibitory neurons like in gnathostomes. Altogether, we conclude that the ancestral genetic programs for the formation of a distinct cerebellum were established in the last common ancestor of vertebrates.

Differential evolution of cerebral and cerebellar fossae in recent Homo: A new methodological approach.

The endocranium shows the influence of the shape and development of brain tissues and overall brain modifications. During the late Upper Pleistocene and Holocene smaller brains appeared and the higher position of endinion relative to inion might indicate changes in cerebellar and occipital lobes. In previous studies, the depths of the cerebral and cerebellar fossae were not specifically considered; new tools for quantitatively measuring these irregular, problematic curved areas need to be developed. This paper's main objective is to investigate to what degree changes in the fossae's depths of extant humans have occurred with respect to fossil anatomically modern humans (AMH) and older Homo species. The proportions of the occipital and nuchal planes are compared measuring the inner and outer surfaces of the bone. Additionally, this paper proposes a quantitative geometric methodology based on endocranial landmarks that create a plane with which to measure the position of the deepest part of the fossa: it represents a curvature maxima - concavity - associated with local structures. The four points thus obtained could be framed in Bookstein's Type II landmarks but without biomechanical implication. Through univariate, bivariate and multivariate analyses (principal components analysis) of raw and size-corrected data we study the differential evolution in recent Homo species, which presents a more vertical occipital area than ancient fossils. Our results corroborate this derived trait; additionally, we have observed a tendency towards a relative decrease in the depth of the cerebral fossae and maintenance of the cerebellar fossae.