Figure 4.

Ultrastructure of zebrafish skin and trigeminal sensory axons at several development stages. TEM images of the wild-type zebrafish epidermis at 18, 30, and 54 hpf. P, peridermal cell; B, basal cell; BM, basement membrane; m, mitochondrion. A,B: The skin consisted of two layers at 18 hpf. Although tight junctions joined the apical ends of peridermal cells (arrowhead), no adherens junctions or desmosomes appeared to hold skin cells together. Asterisks indicate potential neurite profiles located below the immature basal cell layer. C,D: At 30 hpf, adherens junctions or desmosomes appeared at relatively low frequency between skin cells (red arrowhead); an immature basement membrane (BM) was present. The arrow in C indicates the axon shown at higher magnification in D. E: At 54 hpf, many more cell junctions were present (red arrowheads), and an electron-dense, continuous BM was apparent. Yellow arrowhead indicates an apical tight junction. Asterisks indicate the many axons observed in the skin by this time. Arrow indicates the axon shown at higher magnification in F. Double arrowhead indicates an axon enveloped within a basal cell. F: Two axons are visible (asterisks), one containing a large mitochondrion and clearly defined axon membrane (arrow) distinct from the two basal cell membranes that surround it. Scale bars = 1 μm in A–C,E; 0.2 μm in D,F.