Fig. 5

The ultrastructure of a single APEX2⁺ HSPC (white arrow) and its surrounding niche cells are modeled using 3D SBEM (00:15 from Figure 4—video 1). (A) The APEX2⁺ HSPC is lodged in the perivascular KM niche. (Ai) Surrounding tissues are labeled; the HSPC is anterior to the pneumatic duct, dorsal to intestine and pancreas, and ventral to the somites and pronephric tubule. (Aii) Higher magnification shows the APEX2⁺ HSPC is only two-cell diameters from the vessel lumen (white area). (Aiii) Full resolution detail of the APEX2⁺ HSPC showing high-contrast labeling of the nucleus (APEX2-H2B), mitochondria (mito-APEX2; white arrowhead), and extracellular space dorsal to the cell. (B) (i–iii) SBEM sections at different levels through the APEX2⁺ HSPC (white arrows) as shown in the schematic (iv). The HSPC is simultaneously in contact with multiple niche cells: five endothelial cells (EC1–5), 1 mesenchymal stromal cell (MSC), and a ganglion-like (GL) cell. Two unlabeled APEX2 negative putative HSPCs were lodged in the same niche (HSPC2 and HSPC3). HSPC2 is attached to HSPC3, and the APEX2⁺ HSPC (Biii; asterisk). (C) 3D rendered models of the APEX2⁺ HSPC (solid green) in contact with niche cells. 3D contours are in the same colors as outlines in (B). The APEX2⁺ HSPC is directly contacted by: (i) five ECs; (ii) one MSC; (iii) one HSPC, and a chain of GL-like cells. (iv) The GL-like cell is part of a long continuous chain of similar cells that extends through the niche. Scale bars: 5 µm unless otherwise labeled. Abbreviations: D, dorsal; V, ventral; A, anterior; P, posterior.