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identical in the 2 specimens indicating the absence of a selective proliferation of nonlabeled cells.In conclusion it appears that liver regeneration in contradistinction to red cell regeneration is the result of multiplication of the general liver cell population rather than multiple divisions of a small segment of that population. Summary. A study of hepatic regenerationin rats was carried out to see whether the stimulus of partial hepatectomy promotes proliferation of parenchymal liver cells or, in analogy with red cell regeneration, promotes the differentiation of stem cells to new hepatic parenchymal cells. By labeling a large proportion of hepatic cells with tritiated thymidine it was shown that the labeled cells rather than the presumably unlabeled stem cells were responsible for hepatic regeneration following partial hepatectomy.The growth of tissue cells (L cells) in suspension in a chemically defined medium, NCTC 109 with methylcellulose, has been reported by Bryant et aZ.(l). Bakken et aZ. ( 2 ) reported that this medium also supported the growth of suspension cultures of a line of human skin cells. Merchant and Hellman ( 3 ) have recently described the growth of L-M strain mouse cells in suspension culture using chemically defined 2X Eagle's medium. A serum-free medium containing lactalbumin hydrolyzate described by Higuchi (4) that supports the growth of several cell lines in monolayer cultures was modified for the growth of tissue cells in suspension culture by Tribble and Higuchi( 5 ) .This report presents a chemically defined medium formulated as a result of experiments with a cat kidney cell line, and its successful application for the growth of 5 other cell lines. Materials and methods. Preparation of the medium.Salts (except sodium bicarbonate), glucose, amino acids, and glutamine were dissolved together in hot distilled water. Vitamins, stored at -20°C as a lOOX stock solution, were added to the cooled mixture. Sodium bicarbonate and phenol red dissolved together in a small amount of water were added and the resulting solution was sterilized by filtration through a 0.2 p membrane filter and stored at 5°C as a 5X solution. Prior to inoculation, insulin, methylcellulose, streptomycin and penicillin were added aseptically and the final volume adjusted by addition of sterile distilled water. Methylcellulose was stored as a sterile 2% solution; antibiotics were stored at -20°C at lOOX concentrations. The medium used for determining amino acid requirements was prepared by adding aseptically to the filtered salts-vita,min solution individual amino acids ( 1 W X by guest on July 27, 2015 ebm.sagepub.com Downloaded from

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