ALO contains two different types of chemosensory cells and has two external microvilli. (A and B) Confocal micrograph maximum intensity projections of an ALO from a Tg(gng13a:eGFP)^y709 transgenic zebrafish treated with BODIPY 633, showing uptake of the fluorescent dye by chemosensory cells. Panel B shows a higher magnification image of the orange boxed area in panel A, showing only a subset of chemosensory cells are gng13a-positive. (C and D) Single-plane side-view confocal/DIC micrographs of the cortical surface of wild-type adult zebrafish ALOs either (C) probed for HCR with alox12 showing expression in a chemosensory cell located just below two surface epithelial cells, or (D) treated with BODIPY 633 showing uptake by chemosensory cells, as well as chemosensory cell microvilli extending into the environment (black arrows). (E–G) Confocal micrograph extended depth-of-focus projections of ALOs from a Tg(gng13a:eGFP)^y709 transgenic adult zebrafish, showing (E and F) overview images of EGFP-positive chemosensory cells with long projections just underneath the surface of the ALO cortex, and (G, also shown in Fig. 4 P) a side view of the ALO cortex of a fish treated with BODIPY 633, showing uptake by the chemosensory cell microvillus (white arrow) and cell body. (H) Single section from an array tomography image stack of an adult zebrafish ALO cortex, with two separate chemosensory cells pseudocolored blue and yellow. Single sections such as these show individual chemosensory cell microvilli extending from these cells into the environment (green arrows). (I–K) Volume reconstructions of the same array tomography data set of an adult zebrafish ALO cortex from which the image in panel A was taken, with the same two chemosensory cells pseudocolored blue and yellow. Images show volume reconstructions with (I) or without (J) semitransparent fill of non-chemosensory cell portions of the cortex, or with only the segmented chemosensory cells shown (K). The volume reconstructions show that each of the chemosensory cells has two microvilli projecting into the environment (green arrows in panel I). See Videos 2 and 3 for additional array tomography images of chemosensory cells and their paired microvilli. Scale bars = 50 µm (A), 25 µm (B), 10 µm (C–G), 2 µm (H).