Dermal appendage-dependent patterning of zebrafish atoh1a+ Merkel cells

Abstract

Touch system function requires precise interactions between specialized skin cells and somatosensory axons, as exemplified by the vertebrate mechanosensory Merkel cell-neurite complex. Development and patterning of Merkel cells and associated neurites during skin organogenesis remains poorly understood, partly due to the in utero development of mammalian embryos. Here, we discover Merkel cells in the zebrafish epidermis and identify Atonal homolog 1a (Atoh1a) as a marker of zebrafish Merkel cells. We show that zebrafish Merkel cells derive from basal keratinocytes, express neurosecretory and mechanosensory machinery, extend actin-rich microvilli, and complex with somatosensory axons, all hallmarks of mammalian Merkel cells. Merkel cells populate all major adult skin compartments, with region-specific densities and distribution patterns. In vivo photoconversion reveals that Merkel cells undergo steady loss and replenishment during skin homeostasis. Merkel cells develop concomitant with dermal appendages along the trunk and loss of Ectodysplasin signaling, which prevents dermal appendage formation, reduces Merkel cell density by affecting cell differentiation. By contrast, altering dermal appendage morphology changes the distribution, but not density, of Merkel cells. Overall, our studies provide insights into touch system maturation during skin organogenesis and establish zebrafish as an experimentally accessible in vivo model for the study of Merkel cell biology.

Dendriticatoh1a+cells serve as transient intermediates during zebrafish Merkel cell development and regeneration

“…Staging of zebrafish post-embryonic development relies on standard length (SL) in millimeters, with sexual maturity attained ∼18 mm SL (Parichy et al, 2009). Cells with the prototypical MC morphology—spherical with actin-rich, microvillar protrusions of ∼1-2 microns in length— populate several adult zebrafish skin compartments, including trunk skin ( Figure 1A,B ; (Brown et al, 2023)). Atonal homolog 1 ( Atoh1 ) encodes a transcription factor necessary and sufficient for murine MC development (Morrison et al, 2009; Ostrowski et al, 2015; Van Keymeulen et al, 2009).…”

“…Atonal homolog 1 ( Atoh1 ) encodes a transcription factor necessary and sufficient for murine MC development (Morrison et al, 2009; Ostrowski et al, 2015; Van Keymeulen et al, 2009). Zebrafish atoh1a is an ortholog of murine Atoh1 , and zebrafish MCs express reporters inserted into the upstream region of the atoh1a locus (Brown et al, 2023). Using a nuclear localized atoh1a reporter to label MCs, we previously found that trunk MCs begin to appear during the onset of squamation (scale formation) at approximately 9 mm SL and rapidly increase in density from 10-15 mm SL (Brown et al, 2023).…”

“…Zebrafish atoh1a is an ortholog of murine Atoh1 , and zebrafish MCs express reporters inserted into the upstream region of the atoh1a locus (Brown et al, 2023). Using a nuclear localized atoh1a reporter to label MCs, we previously found that trunk MCs begin to appear during the onset of squamation (scale formation) at approximately 9 mm SL and rapidly increase in density from 10-15 mm SL (Brown et al, 2023). As this nuclear reporter did not allow for visualization of cell shapes, it is unclear whether morphological heterogeneity exists within developing zebrafish MCs, including the possible presence of dMCs.…”

“…To assess MC morphology during squamation, we conducted a similar developmental staging time course using Tg(atoh1a:lifeact-egfp) , which expresses a filamentous actin (F-actin) reporter in MCs, allowing detailed visualization of the MC actin cytoskeleton in vivo ( Figure 1C ; (Brown et al, 2023)). To simultaneously visualize scales, we stained animals with Alizarin Red S, which labels the calcified scale matrix.…”

“…The precise and reproducible cell shape of MCs has been proposed to impact their sensory function (Garant et al, 1980; Takahashi-Iwanaga, 2003; Takahashi-Iwanaga and Abe, 2001; Toyoshima et al, 1998; Yamashita et al, 1993); however, it is unknown how MCs acquire this stereotyped morphology. Cre-based lineage tracing experiments indicate that MCs derive from basal keratinocyte precursors in both mammalian and zebrafish skin (Brown et al, 2023; Morrison et al, 2009; Van Keymeulen et al, 2009). While these experiments begin to address MC progenitor identity, they fail to shed light on how cells within this lineage lose basal keratinocyte characteristics and adopt the hallmark MC morphology.…”