The long-standing model of pigment cell origins is implicitly based upon a progressive fate restriction model of neural crest development and postulates the existence of a partially restricted progenitor common to all pigment cell types (i.e., a chromatoblast) [38] .
Sodium is the fourth most abundant element on earth, comprising about 2.6% of the earth's crust; it is the most abundant of the alkali group of metals. It is now obtained commercially by the electrolysis of absolutely dry fused sodium chloride.
Medical Definition of SODIUM. : a silver white soft waxy ductile element of the alkali metal group that occurs abundantly in nature in combined form and is very active chemicallyâsymbol Na; see element table. a silver-white soft waxy ductile element of the alkali metal group that occurs abundantly in nature in combined form and is very active chemically â see element table. See sodium defined for English-language learners. See sodium defined for kids.