ALO cortical goblet cells and Merkel cells. (A) A single section from an array tomography image stack of an adult zebrafish ALO, with goblet cells pseudocolored green. (B–E) Transmission electron micrographs of goblet cells with intact mucous granules (B and C) or with mucous granules breaking down and mucus being extruded (D and E). Panels C and E show higher magnification images of the green boxed areas in panels B and D, respectively. (F) Confocal micrograph maximum intensity projection showing in situ HCR of an adult ALO probed for mucin 5.1, showing specific labeling of goblet cells. (G and H) Confocal micrographs of ALOs treated in situ with LPS-Cy5 and then excised and imaged ex vivo, showing specific uptake of LPS-Cy5 by goblet cells. Images include a maximum intensity projection (G; comparable view to the image shown in panel F) and a single confocal section with DIC showing a side view of the dermal cortex with a single LPS-Cy5–labeled goblet cell (H). See Videos 2 and 3 for additional array tomography images of goblet cells. (I) Single section from an array tomography image stack of an adult ALO, with a Merkel cell pseudocolored brown. (J and K) Confocal/DIC extended depth-of-focus images of an adult Tg(atoh1a:nls-Eos)^w214 transgenic zebrafish ALO showing Merkel cell nuclei in green. Images include (J) an ALO overview (also with BODIPY 633 in magenta) and (K) a side view of the dermal cortex. (L–O) Individual sections from transmission electron microscopic array tomography of an adult zebrafish ALO, showing Merkel cells with large nuclei and small amounts of cytoplasm. Panels M and O show higher magnification images of the yellow and orange boxed areas in panels L and N, respectively. The single inwardly projecting microvillus characteristic of Merkel cells is shown in panel M (yellow arrows). The magnified image in panel O shows that the Merkel cell outer membrane is only 60 nm from the outside surface of the ALO. See Video 2 for additional array tomography images of Merkel cells. Scale bars = 5 µm (A and I), 2 µm (B and D), 500 nm (C and E), 10 µm (F–H), 100 µm (J), 20 µm (K), 1 µm (L–N), 60 nm (O). LPS-Cy5, Cy5-labeled lipopolysaccharide.