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. 2023 Jan 17;12:e85800. doi: 10.7554/eLife.85800

Figure 1. The adult scale epidermis contains atoh1a+ Merkel cells (MCs).

(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’’).

Figure 1.

Figure 1—figure supplement 1. TEM characterization of presumptive MC and adjacent keratinocytes.

Figure 1—figure supplement 1.

(A) Transverse TEM of adult scale epidermis illustrating typical keratinocyte and MC morphologies. Arrowhead indicates a microridge on the surface of a periderm cell. (A’) Pseudocolored overlay showing keratinocytes (blue) and a presumptive MC (green). Scale bar: 2 µm.
Figure 1—figure supplement 2. Characterization of atoh1a reporter transgenes in larvae.

Figure 1—figure supplement 2.

(A-H) Confocal micrographs of 3 dpf larvae expressing the indicated transgenes. Dotted boxes in A and E indicate areas of magnification for panels below. Arrows indicate expression by hair cells of the inner ear. Scale bars: 300 μm (A, E) and 100 μm (B-D, F-H).
Figure 1—figure supplement 3. MCs in the adult epidermis express Sox2.

Figure 1—figure supplement 3.

(A-B’) Lateral confocal micrographs of the scale epidermis showing anti-Sox2 immunostaining of MCs labeled by either Tg(atoh1a:nls-Eos) (A, A’) or Tg(atoh1a:Lifeact-EGFP) (B, B’). Arrows indicate examples of Sox2+/atoh1a- cells. DAPI labels epidermal nuclei. (C) Quantification of the overlap between atoh1a+ MCs and Sox2 immunostaining. 99% of atoh1a:nls-Eos+ MCs expressed Sox2 (769/774 cells from N=5 fish; 25–29 mm SL) and 70% of Sox2+ cells were atoh1a+ (769/1102 cells from N=5 fish; 25–29 mm SL). Scale bars: 5 μm.
Figure 1—figure supplement 3—source data 1. Datasheet for Figure 1—figure supplement 3.