Requirements for Growth of Single Human Cells
The experiments reported in this paper may foreshadow a change in the traditional concept of so-called essential amino acids in human nutrition. It has been shown that a number of strains of single types of human cells may be propagated serially in a medium containing 13 "essential" amino acids along with other ingredients when a relatively large inoculum of cells is employed. In the present experiments it was learned that this medium did not permit successful propagation if relatively small inocula of cells were used, unless additional amino acids not included in the original medium, were added. The efficiency of the medium in promoting growth in the case of lighter inocula was restored when seven amino acids, not required for optimal growth with heavy inocula, were added to the basal medium. Serine was the most important of the seven non-essential amino acids that were not required in the medium to support optimal growth with heavy inocula. Thus the essentiality or non-essentiality of an amino acid may depend upon the capacity of the number of cells involved to meet the requirements for amino acids by biosynthesis within the cells, as may be determined by the population of cells and their rate of growth.
