Can We Explain Thousands of Molecularly Identified Mouse Neuronal Types? From Knowing to Understanding

At the end of 2023 the Whole Mouse Brain Atlas was announced, revealing that there are about 5,300 molecularly defined neuronal types in the mouse brain. We ask whether brain models exist that contemplate how this is possible. The conventional columnar model, implicitly used by the authors of the Atlas, is incapable of doing so with only 20 brain columns (5 brain vesicles with 4 columns each). We argue that definition of some 1,250 distinct progenitor microzones, each producing over time minimally 4-5 neuronal types, may be sufficient. This is nearly achieved presently by the prosomeric model amplified by secondary dorsoventral and anteroposterior microzonation of progenitor areas, plus clonal variation of cell types produced on average by each of them.