Cell Stem Cell Previews Postmitotic Fate Refinement in the Subplate

The cerebral cortex is a six-layered structure that expanded dramatically during primate evolution. Although cortical layers contain distinct classes of neurons, the stages, cell types, and anatomical compartments that specify this diversity are still being unraveled. Prior to birth, radial glia in the germinal zone generate neurons that migrate outward to form cortical layers in an ‘‘inside-out’’ pattern. Molecular programs in radial glia contribute to projection neuron diversity as the fate of daughter neurons becomes progressively restricted over time. However, the majority of subtype-specific decisions appear to occur postmitotically (McKenna et al., 2015). Notably, overexpression of a single transcription factor can alter fate in postmitotic projection neurons during embryonic development (Ye et al., 2015), leaving open thepossibility that cues receivedduring neuronal migration and maturation influence cell fate. In this issue of Cell Stem Cell, Ozair et al. (2018) discover an unexpected link between human cortical neuron fate specification and the subplate. The subplate is a transient structure beneath the cortex composed of some of the earliest-born cortical neurons. These neurons coordinate the first connections between thalamus and cortex and are required for forming normal sensory maps. Although the subplate expanded during primate brain evolution, these enigmatic neurons are thought to largely disappear shortly after birth through programmed cell death, leaving behind only a remnant population of corticothalamic projection neurons in layer 6b and interstitial white matter neurons (Hoerder-Suabedissen andMolnár, 2015). Interestingly, some transient structures, such as the hippocampal intermediate zone, may serve as a ‘‘waiting compartment’’ where newborn neurons receive synaptic innervation before migrating to their final locations (Altman and Bayer, 1990). However, subplate neurons have been difficult to isolate due to the mixing of resident subplate cells with later-born migratory neurons and their transcriptional similarities to deep-layer neocortical projection neurons. To better characterize the function of human subplate during cortical fate specification, Ozair et al. performed immunostaining for transcription factors that define distinct classes of cortical neurons. Surprisingly, subplate neurons in postconception week (PCW) 15 frontal lobe co-expressed the conserved subplate marker NURR1 with markers of callosal (SATB2), subcerebral (CTIP2), and corticothalamic (TLE4) projection neuron identity and with markers of neuronal maturation (MEF2C). The observation that competing molecular programs for distinct neuronal subtypes co-exist in individual cells suggests that maturing subplate neurons retain the potential to form multiple classes of projection neurons. Indeed, the early co-expression of these class-specific transcription factors is consistent with the complex molecular circuits that guide development of projection neuron connectivity patterns (McKenna et al., 2015). To directly study the fate of human subplate neurons, Ozair et al. use human pluripotent stem cell models of neuronal development. After patterning the stem cells to forebrain lineage progenitors, the authors use a Notch signaling inhibitor to generate a synchronized cohort of postmitotic neurons