Figure 2. A model of clonal origins of neocortical interneuron diversity and distribution.

(A) RGPs in the VZ of the MGE are likely heterogeneous in proliferative behavior and neurogenesis potential. A majority of RGPs (red colored) actively divide starting at the beginning of neurogenesis. They undergo asymmetric divisions to give rise to an interneuron or an IP, which divides symmetrically one or more rounds to produce a number of interneurons concurrently. As time proceeds, these RGPs may progressively be depleted or be maintained to generate more interneurons (broken lines and symbols), resulting in an interneuron lineage that form a vertical or horizontal cluster (in broken parentheses) with distinct subtypes (represented by shapes) in the mature neocortex, likely via coordinated migration. In comparison, some RGPs (blue colored) only become mitotically active at the late stage of neurogenesis. They may generate a lineage of a few interneurons that are restricted in defined layer(s). Cell death may further sculpt individual lineages in composition and distribution. (B) MGE progenitors produce neurons (including GABAergic interneurons and projection neurons, as well as cholinergic interneurons and projection neurons) and oligodendrocytes that are distributed in the neocortex, as well as the hippocampus and subcortical regions, e.g. the striatum, globus pallidus, and basal forebrain. It remains unclear whether individual progenitors produce the same or distinct cell types that are distributed in one or different brain regions.