In a small sample of 36 non-psychiatric control subjects (including 32 in this study), we recently uncovered evidence of significant differences in the neuronal composition across the lifespan, including a loss of a progenitor-like epigenetic signature after birth, and a rising non-neuronal signature in postnatal life22. We sought to more fully characterize these composition changes in this much larger sample, and again found strong evidence of age-dependent changes in cell composition based on cell type epigenetic signature analysis (Figure 2). We identified significant linear changes in measures of composition within the second trimester of fetal life, reflecting the large gene expression changes present in this developmentally important time period1, including a decrease in progenitor-like cells based on DNAm signatures found in embryonic (Figure 2A, p= 1.21×10−24) and neural progenitor (Figure 2B, 5.74×10−25) cells, and a rise of mature adult neurons (Figure 2C, p=3.57×10−24) and non-neuronal cells (Figure 2D, p=8.09×10−86). The composition values of the fifth cell type in the dataset (ES-derived dopamine neurons) did not differ between pre- and post-natal samples (p=0.99). These composition profiles explained much of the variability
