Histology and electron microscopy of the ALO. Histological characterization of ALO morphology. (A) Schematic diagram showing the plane of section in panel B. (B) Alcian Blue/PAS-stained (stained acid mucins in purple) transverse paraffin section through the anterior trunk of an adult zebrafish. The green box indicates the region shown in panel C. (C) Magnified image of the boxed region in panel B, showing the right ALO. The blue box indicates the region shown in panel D. (D–F) Magnified images of serial transverse paraffin sections through the anterior trunk of an adult zebrafish stained with Alcian Blue/PAS (D), Movat’s pentachrome (E), and H&E (F, also shown in Fig. 8, G and H) with blue arrows highlighting mucus-producing goblet cells (D and E). Panel D shows the boxed region in panel C. (G) Transmission electron micrograph of a transverse section through an adult zebrafish ALO. The boxed regions show areas with comparable (h) or actual (i) magnified TEM images in subsequent panels. (H) TEM showing the dermal cortex of the ALO with presumptive surface epithelial cells pseudocolored magenta, mid-level epithelial cells yellow, club cell green, and basal epithelial cells blue. (I) Magnified TEM image of box i in panel G, showing the medulla of the ALO with lymphatic vessel pseudocolored green, blood vessels magenta, and FRCs blue (I). Histology, n = 3; TEM, n = 2. Scale bars = 1 mm (B), 50 µm (C), 25 µm (D), 100 µm (G), 10 µm (H and I).