During early embryogenesis of the chick, the yolk represents the only source of free amino acids. Leucine and glutamic acid, together with valine, predominate in this nonrenewable nutrient supply; taurine and phosphoethanolamine are present in the lowest concentration. The tyrosine level is already twice as high as phenylalanine.
At embryo day 2 (Ez), the not-yet fully functioning vitelline blood plexus demonstrates an amino acid profile that rather closely resembles that of the yolk, except for three amino acids. During the transfer process from yolk to plexus, a certain amount of glutamic acid appears to be metabolized to glutamine and aspartic acid. Taurine and phosphoetanolamine are highly concentrated in the circulation, with a tenfold increase of taurine and a 50-fold rise in phosphoethanolamine. The tyrosine-to-phenylalanine ratio remains 2:1, but their levels relative to valine and other essential amino acids fall by roughly 50%.
In the already contracting heart tube and the developing neural tissue on Ez, taurine and phosphoethanolamine also accumulate preferentially; the levels in neural tissue are ten-and fivefold higher than in the circulation for the respective amino acids, whereas the heart has three time the plexus content of taurine and two times the amount of phosphoethanolamine. Tyrosine and phenylalanine, still in the same proportion, have risen to approximately twice the circulation values. The heart tube, unlike the primitive brain, concentrates leucine, whereas isoleucine accumulates in both organs.
Injections of valine (50 pmol) and taurine (200 pmol) into the yolk on El demonstrate that a higher yolk content of an amino acid can result in a two-to threefold tissue increase. This implies that the comparative amino acid profdes of early embryonic organs are to a large extent determined by the relative abundance of amino acids in the yolk. However, within confines of this limited supply, the organs are already able to extract preferentially certain amino acids.