Development of pigment cells in the brain of ascidian tadpole larvae: Insights into the origins of vertebrate pigment cells

In vertebrates, melanins produced in specialized pigment cells are required for visual acuity, camouflage, sexual display and protection from ultra violet (UV) radiation. There are three pigment cell types that are classified based on their distinct embryonic origins. Retinal pigment epithelium (RPE) cells originate from the outer layer of the optic cup. Pigment cells of the pineal organ are formed from the developing diencephalon. Melanocytes are derived from the neural crest unique to vertebrate embryos. Some of these pigment cells also play roles that are independent of the activity of tyrosinase, the key melanogenesis enzyme, or melanin: production of substrate(s) for catecholamine synthesis, maintenance of endolymph composition in the cochlea, maintenance of photoreceptor cells in the retina and retinoid metabolism essential for the visual cycle. To deduce the evolutionary origins of vertebrate pigment cells and a possible archetypal genetic circuitry, which may have been modified and utilized to generate multiple pigment cell types, comparison of developmental mechanisms of pigment cells between vertebrates and closely related invertebrate ascidians are proposed to provide useful information. The tadpole-type larva of ascidians possesses two melanin-containing pigment cells, termed the otolith and ocellus pigment cells, in the brain that are believed to be required for photo- and geotactic responses during swimming. In this review, current knowledge on the development of the two ascidian pigment cells is summarized, i.e. complete cell lineage, structure and expression of genes encoding two melanogenesis enzymes, and molecular developmental mechanisms involving BMP-CHORDIN antagonism, and possible evolutionary relationships between ascidian and vertebrate pigment cells are discussed.