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Fig. 2 Pronephric nephron development. (A) Cross section of wild-type pronephroi at 24 hpf shows the forming nephron primordium (np) as an invagination of the coelomic lining. The lumen of the nephron precursor, or nephrocoel, is still connected to the coelom (c) at this stage by a transient nephrostome (nst; arrow). (B) Sagittal section at 24 h shows the anterior pronephric duct (pd) abutting on the lateral aspect of the nephron primordium (np). C) At 32 h, the nephron primordium is closed off from the coelom and appears as a double-layered sac of cells with a central lumen. (D) A sagittal view at 32 h reveals early stages of nephron morphogenesis with lateralmost cells in the position of future tubules (tubule primordium; tp) becoming distinct from medial cells in the position of the future glomerulus (glomerular primordium; gp). (E) By 40 h, the nephron primordium has become partitioned into a glomerular domain medially directly under the dorsal aorta (da) and tubule domains laterally with a distinct basement membrane surrounding the developing glomerulus (arrow). Pigment cells envelope the pronephros at this stage. (F) A sagittal view of the 40 h pronephric nephron shows the medial paired glomeruli (gl) with the pronephric tubules (pt) fused to their posterior side. da, dorsal aorta. (G) Longitudinal sections through the forming glomerulus (gl) at 40 h show its position below the notochord (nc) and its intimate association with the dorsal aorta (da) and cells (arrow), which surround the forming glomerular basement membrane. (H) Sagittal view of the pronephros at 50 h with the glomeruli (gl) fused at the midline and the pronephric tubule (pt) arcing over to connect to the pronephric duct (pd). (I) The pronephros at 2.5 days postfertilization shows paired glomeruli (gl) ventral to the aorta and pronephric tubules (pt) connecting laterally to the pronephric ducts. (J) By 3 days, nephron formation is essentially complete and the direct connection between Bowman’s space and the lumen of the pronephric tubules is evident (arrows). On the right are schematized versions of the micrographs in B, D, F and H, derived from serial sections, illustrating the morphological transitions that occur during pronephric nephron morphogenesis. In all cross sections, the top of the figure is dorsal and in all sagittal sections the top of the figure is anterior.