Ultrastructure of the zebrafish AGM. Electron microscopy of longitudinal sections of the DP joint at 48 hpf (A-D), 26 hpf (E-H,K,L), and 36 hpf (I,J). (A,B) An hematopoietic early precursor (arrows) visible in the DP joint (A) is shown at higher magnifications in (B). (C) A similar precursor, at higher magnification, surrounded by processes of mesenchymal cells (arrows). (D) Structure of the DP joint mesenchyme, with irregular mesenchymal cells (arrows), lipid-laden cells (arrowheads), and melanin-laden pigment cells (thin arrows). (E) Typical overlap (dotted ellipse) between adjacent endothelial cells of the PCV dorsal wall, in contrast with the continuous appearance of the DA endothelial wall. (F,G) Higher magnification of the thin overlapping processes of adjacent PCV endothelial cells, showing cell junctions between them (arrows), though shorter and less dense than those between endothelial cells of the DA (H, arrows). (I) Adjacent DA endothelial cells also show interdigitations penetrating each other (arrowhead), in addition to cell junctions (arrows). (J) Intimate apposition between a stromal reticular cell (arrow) and PCV endothelium (arrowheads). (K) A stromal reticular cell (arrow) bridging a short gap between 2 adjacent PCV endothelial cells (arrowheads). (L) Two endothelial cells (arrowheads) keeping a gap, underlaid by 2 stromal reticular cells (arrows), one of which covers the gap. Ntc indicates notochord; and yt, yolk tube.