Figure 2.

Early colonization of the human embryo

(A) Representative sagittal cross-section through a CS10 (late 3^rd pcw) human embryo. Immunolabeling was done on consecutive 8-μm sections identifying the presence of PU.1 cells in organs apart from the brain and the absence of IBA1⁺ cells overall. Scale bars: 500 μm (left); 40 μm (right).

(B) Representative sagittal sections through whole human embryos from CS12, the age of appearance of IBA1⁺ cells across the entire embryo, through to CS21. Insets show the location of proliferating IBA1⁺ cells in both the liver and the dorsal telencephalon. Scale bars: 1 mm (top); 100 μm (bottom).

(C) IBA1 proliferative dynamics and cell densities in the developing brain (n = 14) and the liver (n = 11). Black arrows represent the two most relevant time points for microglial densities: CS12 (4^th pcw) for the first colonization of the brain rudiment and CS23 (9^th pcw), the transition from embryonic to early fetal life.

(D) Mean IBA1 proliferation in the liver (n = 11) and the developing brain (n = 11) across the embryonic period. All data are shown as mean ± SEM.

(E) Cumulative frequency distribution plots of densities and proliferative indices by sex in the developing brain (7F:7M) (top panel); mean differences between sexes in microglial densities and proliferative indices across the embryonic age in the developing brain (7F:7M) (bottom panel, data are shown as mean ± SEM).

(F) Representative migratory profiles of IBA1⁺ cells in the bilaminar telencephalon at 5 pcw (n = 2). Data are shown as mean ± SEM.

(G) IBA1⁺ cell distributions in the CS12 embryo showing very few cells in the forebrain and a much higher density of cells in the hindbrain and midbrain. Scale bars: 500 μm.

(H) Entry routes of IBA1⁺ cells into the brain rudiment in the forebrain and the hindbrain. Scale bars: 100 μm. C, cardiac muscle; CP, cortical plate; GE, ganglionic eminence; H, hindbrain; L, liver; M, meninges; MB, midbrain; Me, mesenchyme; ML, mantle layer; SC, spinal cord; T, telencephalon; VZ, ventricular zone. For all panels, asterisks represent adjusted p value significance as follows: ^∗p < 0.05, ^∗∗p < 0.001, ^∗∗∗p < 0.0001, and ns p > 0.05.