Hormones have essential roles in regulating the development of adult traits. For example, thyroid hormone is essential for metamorphosis of an amphibian tadpole to an adult frog; it drives limb development while simultaneously causing the tail to be resorbed. How can one hormone have such different effects? The Parichy Lab looked at this using zebrafish pigment cells.
In zebrafish, thyroid hormone limits the population of black melanophores but expands the population of orange xanthophores. Using single cell RNA-sequencing, the lab first documented the normal states of stem cells and their progeny in pigment cell lineages. Then, by comparing cells from euthyroid and hypothyroid fish, and using developmental genetic experiments, they showed that thyroid hormone drives maturation of both melanophores and xanthophores, but in very different ways. For melanophores, thyroid hormone drives the cells into a terminally differentiated state, in which they stop dividing (limiting population increase). For xanthophores, thyroid hormone promotes the acquisition of orange carotenoids in cells already poised to differentiate (making more cells visible).
The results are likely relevant to other derivatives of these stem cells (like glia and bone/cartilage) and they provide a nice example of how hormone regulate different cell types in coordinating the development of adult phenotypes.
The paper is available at open-access.