Figure 1
(A) Illustration of the adult zebrafish trunk anatomy showing the organization of epidermis, scales, and dermis. Scales are flat bony discs arranged in an overlapping, imbricated pattern and coated on their external surface by epidermis. (B) Transmission electron microscopy (TEM) of a presumptive MC from the scale epidermis. Dotted boxes indicate regions of magnification in B’–B’’’. (B’) Magnification of B showing cytoplasmic granules (g, brackets) juxtaposed to a putative axon (a) contact containing a mitochondrion (m). (B’’ and B’’’) Magnifications of B showing desmosomal-like (d, arrows) attachments between keratinocytes (B’’) and between a presumptive MC and keratinocyte (B’’’). (C and C’) TEM of a presumptive MC from the scale epidermis showing a microvillar process (p, arrowhead). (D) Illustration of a cross section of the scale epidermis based on TEM observations. Periderm cells (superficial epidermis; dark blue) are located in the uppermost epidermal stratum, and basal keratinocytes (light blue) are located in the lowermost epidermal stratum. MCs containing cytoplasmic granules, extending microvillar processes, and contacting axons localize between keratinocytes. (E) Lateral confocal micrograph of the trunk epidermis in an adult expressing reporters for keratinocytes (Tg(actb2:LOXP-BFP-LOXP-DsRed)) and atoh1a-expressing cells (Tg(atoh1a:nls-Eos)). Dotted boxes indicate areas of magnification in E’ and E’’. (E’) Magnification of E showing atoh1a+ hair cells (HCs) and progenitors within neuromasts (nm) of the posterior lateral line. (E’’) Magnification of E showing atoh1a+ MCs scattered throughout the scale epidermis. (F) Lateral and reconstructed cross sectional confocal micrographs of the trunk in an adult expressing reporters for keratinocytes (Tg(actb2:LOXP-BFP-LOXP-DsRed)) and atoh1a-expressing cells (Tg(atoh1a:nls-Eos)) and stained with Alizarin Red S (ARS) to label the mineralized scale matrix. Note that atoh1a+ MCs localize to the epidermis above scales (arrowhead). (G) Lateral confocal micrograph of the scale epidermis in an adult expressing reporters for keratinocytes (Tg(krt4:DsRed)) and F-actin within atoh1a+ MCs (Tg(atoh1a:Lifeact-EGFP)). Note that all atoh1a+ MCs extend multiple microvilli. (G’) Magnification of G with arrowheads indicating individual microvillar processes on the surface of MCs. (G’’) Reconstructed cross section along the yellow line in G. MCs localize to the upper epidermal strata as diagrammed in D. Note that Tg(krt4:DsRed) (blue) preferentially labels keratinocytes in the upper epidermal strata, but not in the basal cell layer. Scale bars: 1 µm (B and C), 0.1 µm (B’–B’’’), 0.5 µm (C’), 50 µm (E–E’’ and F), 10 µm (G), and 5 µm (G’ and G’’).
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