In 1950, Morgan, Morton and Parker described one of the first totally defined media that did not depend largely on animal products or extracts as nutritive sources. This medium supported the growth of primary chick embryo heart cells. It has since become known as Medium 199. Its current use is with the addition of serum in virus and vaccine production, and in the culture of many non-transformed cells. It is also used in combination with less complex media. Medium 199 modified with Earle's Balanced Salts is designed for use with cells maintained in ambient (non-CO2) atmospheric conditions. M2852 differs from the original formulation by containing all L-amino acids. The original formulation contained 10 DL-amino acids, which are less biologically active on a per gram basis than the corresponding L-amino acids.
The search for a synthetic medium to replace serum for maintaining cells in vitro began in the late nineteenth century and continues to this day. Ringer, Locks and Tyrode substituted physiological salt solutions augmented with glucose for serum and thereby laid the foundation for the development of defined media. As biochemical and analytical techniques have improved, more of the components in serum such as vitamins, hormones, and amino acids have been identified and incorporated into physiological salt solutions, reducing, and in some cases eliminating the concentrations of animal sera required as a medium supplement. Although there have been many modifications to the original formulas in efforts to produce fully defined media, salt solutions still play an important role in tissue culture. A salt solution's basic function, to maintain the pH and osmotic balance in the medium and to provide the cells with water and essential inorganic ions, is as valuable today as when it was first developed a century ago. Earle
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