The establishment of essential and metabolically derived amino acid profiles in developing chick embryo organs

The accumulation of certain essential and metabolically derived amino acids in the free amino acid pools of three excitable tissues has been studied in the chick embryo. Valine together with leucine are at the onset present in the yolk at higher concentrations than any of the other essential amino acids. By 15 days all the amino acids studied have accumulated in the embryonic pools at a higher rate than valine, although certain amino acids, such as phenylalanine or methionine, always remain at lower relative concentrations than valine. This reflects their low supply in the yolk, rather than a more rapid rate of disappearance (utilization). During early embryogenesis (E2-E4), tissues preferentially concentrate glutamic acid, besides taurine and phosphoethanolamine (6). The next distinct stage of development (E4-E7) is marked in the brain by a gradual rise in glutamic acid, glutamine and aspartic acid; the same three amino acids do not demonstrate a further increase in the pool of the heart, while in the whole eye the amino acid profile begins to resemble the blood. Leucine in all three tissues declines rapidly, to reach isoleucine levels by day 7 of development; tyrosine increases slowly in apparent reciprocity to an equally gradual phenylalanine decrease. Into the second week of embryo growth (E7-E15), GABA appears in the mesencephalon (E7) and the eye (E9-E10). In the mesencephalon, the free amino acid pool composition exhibits a rather sudden increase of most metabolically derived amino acids.