Melanophore multinucleation pathways in zebrafish

Abstract

In zebrafish, apart from mononuclear melanophores, bi- and trinuclear melanophores are frequently observed; however, the manner in which multinucleation of these cells occurs during fish development remains unknown. Here, we analyzed the processes underlying multinucleation of zebrafish melanophores. Transgenic zebrafish in which melanophore nuclei were labeled with a histone H2B-red fluorescent reporter protein were used to evaluate the distribution of mono-, bi-, and trinuclear melanophores in both the trunk and fin. Half of the melanophores examined were binuclear and approximately 1% were trinuclear. We compared cell size, cell motility, and survival rate between mono- and binuclear melanophores grown in a culture dish, and we found that cell size and survival rate were significantly larger in binuclear melanophores. We then analyzed the behavior of melanoblasts and melanophores from transgenic zebrafish using in vivo and in vitro live-cell imaging. We detected division and differentiation of melanoblasts, as well as melanoblast nuclear division without subsequent cellular division. In addition, we observed cellular and nuclear division in melanophores, although these events were very infrequent in vitro. On the basis of our findings, we present a scheme for melanophore multinucleation in zebrafish.

Thyroid hormone regulates distinct paths to maturation in pigment cell lineages

“…If planar areas of concentrated melanin granules are assumed spherical, then euthyroid melanophores had on average ~1.7 x the total melanin content of hypothyroid melanophores. (Data in supplementary file Figure 5—source data 1) ( D ) Fully differentiated melanophores of zebrafish are often binucleate (Usui et al, 2018). Left panel shows a binucleate stripe melanophore in a euthyroid fish (12 SSL).…”

“…Finally we examined multinucleation, a condition linked to increased cell survival and size (Orr-Weaver, 2015; Usui et al, 2018). In euthyroid fish, ~20% of melanophores were binucleate near the onset of adult melanophore differentiation but >50% were binucleate by juvenile stages, confirming an overall increase in binucleation with somatic stage and melanophore age (Figure 5—figure supplement 1E).…”

Thyroid hormone regulates distinct paths to maturation in pigment cell lineages

“…Genes expressed during terminal differentiation of melanophores from euthyroid fish were expressed at lower levels in melanophores of hypothyroid fish, suggesting an impediment to maturation in the absence of TH (Figure 5A). Cytologically, mature melanophores of euthyroid fish are frequently binucleate-a condition associated with increased survival and larger size (Orr-Weaver, 2015;Usui et al, 2018)-whereas we found that melanophores of hypothyroid fish were frequently mononucleate, suggesting they had failed to complete differentiation (Figure 5B and C). Given these findings and an earlier observation that melanophores of euthyroid fish fail to divide whereas those of hypothyroid fish continue to do so (McMenamin et al, 2014), we hypothesized that TH promotes a terminally differentiated state of cellular senescence.…”

“…The largest cluster (#3) contains 41% of the genes and represents loci expressed late in pseudotime of euthyroid melanophores but downregulated in hypothyroid melanophores, identifying novel candidate genes for roles in melanophore maturation (see Table 6, in which published melanophore-related genes are highlighted ) (Baxter et al, 2018). (B) Fully differentiated melanophores of zebrafish are often binucleate (Usui et al, 2018) as, shown in vivo for a stripe melanophore isolated from a euthyroid fish (12 SSL). By contrast, melanophores of hypothyroid fish at equivalent stages were often mononucleate.…”

Whole-organism 3D Quantitative Characterization of Zebrafish Melanin by Silver Deposition Micro-CT

“…This observation is consistent with high resolution photographs of black background-adapted dorsal melanophores reported previously (Logan et al, 2006). Several of these transparencies were in close proximity ( Figure 2D ); zebrafish melanophores have been frequently shown to be multinucleated (Usui et al, 2018). In contrast to the flattened, diffuse melanin in the stripes, pigment units not associated with the four larval stripes appeared to have a globular, condensed morphology.…”