Defining the number, proportion, or lineage of distinct cell types in the developing human brain is an important goal of modern brain research. We defined single cell transcriptomic profiles for 40,000 cells at mid-gestation to identify cell types in the developing human neocortex. We define expression profiles corresponding to all known major cell types at this developmental period and identify multiple transcription factors and co-factors expressed in specific cell types, providing an unprecedented resource for understanding human neocortical development including the first single-cell characterization of human subplate neurons. We characterize major developmental trajectories during early neurogenesis, showing that cell type differentiation occurs on a continuum that involves transitions that tie cell cycle progression with early cell fate decisions. We use these data to deconvolute regulatory networks and map neuropsychiatric disease genes to specific cell types, implicating dysregulation of specific cell types, as the mechanistic underpinnings of several neurodevelopmental disorders. Together these results provide an extensive catalog of cell types in human neocortex and extend our understanding of early cortical development, human brain evolution and the cellular basis of neuropsychiatric disease.One Sentence Summary: Comprehensive single cell transcriptomes in developing human cortex inform models of cell diversity, differentiation and disease risk.
Main text:The human cortex is composed of billions of cells estimated to encompass hundreds or thousands of distinct cell types each with unique functions (1, 2). It is reasonable to assume that to understand a system as complex as the human cortex, it is necessary to understand its components (3,4). Ground breaking work in mouse revealed the power of single-cell transcriptomics for providing a framework for understanding the complexity and heterogeneity of cell types in the brain (5-10). The availability of high quality tissue and advances in single-cell transcriptomic technologies now permit us to catalog the cell type diversity of the human cortex in a comprehensive and unbiased manner (11). Despite the enormous progress that has been made in characterizing early cortical development (1,(12)(13)(14)(15)(16), many of the molecular mechanisms underpinning the generation, differentiation, and development of the diverse types of cells remain largely unknown (17). Molecular taxonomies of cortical cell types from developing human brains enable us to understand the mechanisms of neurogenesis and how the remarkable cellular diversity found in the human cortex is achieved (18-22). Developing knowledge of neocortical cell types and their transcriptional programs during this epoch is a key step in understanding mechanistic dysregulation in neurodevelopmental disorders with cell type resolution. Several recent studies have taken a first step in this direction, analyzing several hundred, or a few thousand cells from developing human brain (18-22). However, advances in te...