Thyroid hormone-dependent adult pigment cell lineage and pattern in zebrafish

Adult melanophores arise from a transit-amplifying progenitor.

“In contrast to adult melanophores that arise from a transit-amplifying progenitor”

Melanophores of euthyroid fish fail to divide whereas those of hypothyroid fish continue to divide.

“Prior analyses indicated that melanophores of euthyroid fish fail to divide whereas those of hypothyroid fish continue to do so (McMenamin et al., 2014).”

Fish lacking thyroid hormone have about twice the normal number of melanophores and lack visible xanthophores.

“fish lacking TH have about twice the normal number of melanophores and lack visible xanthophores (Figure 1A) (McMenamin et al., 2014).”

Melanoblasts derived from peripheral-nerve associated progenitors are proliferative during adult pigment pattern formation, but this activity largely ceases as the cells differentiate.

“Melanoblasts derived from peripheral-nerve associated progenitors are proliferative during adult pigment pattern formation yet this activity largely ceases as the cells differentiate (Budi et al., 2011; McMenamin et al., 2014).”

Addition of exogenous thyroid hormone to hypothyroid cells ex vivo can promote differentiation of unpigmented melanoblasts to melanophores.

“Indeed, prior analyses showed that addition of exogenous TH to hypothyroid cells ex vivo can promote differentiation of unpigmented melanoblasts to melanophores (McMenamin et al., 2014), contrary to the idea that TH specifically blocks melanophore development.”

Most adult xanthophores develop directly from embryonic-early larval xanthophores that lose their pigment and then reacquire it late in adult pattern formation.

“Most adult xanthophores develop directly from EL xanthophores that lose their pigment and then reacquire it late in adult pattern formation (Figure 3A) (McMenamin et al., 2014).”

Thyroid hormone promotes the melanization of melanoblasts ex vivo.

“TH similarly promotes the melanization of melanoblasts ex vivo and a cessation of proliferative activity in vivo (McMenamin et al., 2014).”

Most adult xanthophores develop directly from embryonic/early larval xanthophores that lose their pigment and then regain color late in adult pattern formation.

“most adult xanthophores develop directly from EL xanthophores that lose their pigment and then regain color late in adult pattern formation”

Fish lacking thyroid hormone have gross defects in pigment cell numbers and pattern, with approximately twice the normal complement of melanophores and a simultaneous absence of visible xanthophores.

“Fish lacking TH have gross defects in pigment cell numbers and pattern with ~two fold the normal complement of melanophores and the simultaneous absence of visible xanthophores (McMenamin et al., 2014).”

Thyroid hormone promotes a cessation of proliferative activity in vivo.

“TH similarly promotes the melanization of melanoblasts ex vivo and a cessation of proliferative activity in vivo (McMenamin et al., 2014).”

Ablated thyroid glands fail to regenerate, and absence of regeneration in this study was confirmed by continued absence of nVenus expression.

“Ablated thyroid glands fail to regenerate (McMenamin et al., 2014) and absence of regeneration in this study was confirmed by continued absence of nVenus expression.”

The authors' data, together with those of others, suggest that some of these signals depend on melanophores, promoting differentiation of iridoblasts toward a state having ordered reflecting crystal platelets that are physiologically responsive (blue ⟷ yellow) in stripes, and away from an alternative state of disordered-crystal platelets lacking physiological responsiveness (silvery-yellow) in the

“Our data, together with those of others, suggest that some of these signals depend on melanophores14,15,25,47, promoting differentiation of iridoblasts toward a state having ordered reflecting crystal platelets that are physiologically responsive (blue ⟷ yellow) in stripes, and away from an alternative state of disordered-crystal platelets lacking physiological responsiveness (silvery-yellow) in”

In zebrafish, thyroid hormone has opposite effects on neural crest-derived pigment cells of the adult stripe pattern: it limits melanophore population expansion yet increases yellow/orange xanthophore numbers.

“In zebrafish, TH has opposite effects on neural crest derived pigment cells of the adult stripe pattern, limiting melanophore population expansion, yet increasing yellow/orange xanthophore numbers.”