中枢神経系の出現：ウニ実験が示唆する二段階進化モデル

The centralized nervous system (CNS) is a key innovation of bilaterian animals, enabling complex behaviors and body plans. However, how a CNS evolved from ancestral nerve nets remains unresolved. Here, drawing on experimental manipulations in sea urchin embryos, I propose a two-step model for the evolutionary emergence of a CNS, focusing on how signaling pathways partition the embryonic ectoderm. First, an anterior neuroectodermal domain-analogous to a proto-brain field-was specified independently of TGF-β signaling via Wnt-mediated anterior restriction, a mechanism observed even in cnidarians. Second, BMP and Nodal signaling were co-opted to define non-neural ectoderm, restricting neurogenesis in the posterior and creating a condensed, cord-like domain. The integration of these two patterning systems enabled neurons to accumulate along specific axial regions rather than being dispersed across the body. Sea urchin embryos offer a powerful deuterostome model to test this hypothesis. Their ectoderm is sharply divided into anterior and posterior neurogenic territories, separated by TGFβ-signaling-specified non-neural ectoderm. Blocking Wnt signaling leads to ectoderm-wide anterior neurogenesis, while inhibiting TGFβ-signaling causes neurogenesis to expand across the entire ectoderm. These findings strongly support the "neural default model" and underscore the importance of combining Wntbased anterior patterning with TGFβ-signaling-mediated neural restriction in CNS centralization. Although the idea is not entirely new, the data from sea urchins provide rare experimental evidence for how ancient axial patterning systems could have cooperatively shaped the emergence of a centralized nervous system in early bilaterians.