The authors review ten essential amino acids with regard to their metabolic, physiologic and therapeutic effects throughout the human body. Physical properties of these biologically active compounds are discussed as a foundation for their diverse roles in special nitrogen containing products, neurotransmitters, and as alternative energy sources. Both normal and abnormal amino acid metabolism are considered in the areas of digestion, elimination of metabolic products, metabolic intermediates, and defects in these systems. Recent developments in therapeutic applications are further examined for clinical utility and as an economical alternative to traditional clinical treatment modalities.

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Reece and Warbritton ( l ) , using the colchicine technique, reported that mammary glands of rats pregnant during the first lactation exhibited very little mitotic activity between the fifth day of lactation and the second parturition. This indirect evidence indicated that some of the secretory cells present in the first lactation were probably present in the second lactation in rats pregnant during the first lactation. Hence, it was desirable to obtain direct evidence on this problem.The use of tritiated thymidine to label alveolar cells has the advantage that labeled cells could be traced through successive lactations through autoradiography. Although cell numbers can be estimated by DNA content, the DNA procedure can not be used to determine whether or not cells are involuting in instances where hyperplasia is also occurring. However, by using tritiated thymidine to label cells and the DNA procedure to estimate cell numbers, the percentage of "carryover" of alveolar cells from the first lactation to the second lactation can be determined in animals pregnant during the first lactation.Materials and Methods. Virgin female rats of the hooded Norway strain, weighing 160 to 170 g, were mated. They were maintained at a temperature of 72 t 2°F and under 10 hr of artificial light daily. The animals received Purina Laboratory Chow and water ad libitum. Prior to parturition, female rats were either isolated or cohabitated with a sexually mature male rat in a lactating cage. In cohabitated female rats, vaginal smears were examined each morning following parturition for the presence of sperm (day 1 of pregnan-lPaper of the Journal Series, N.J. Agr. Exp. Sta., Rutgers University, The State University, New Brunswick, N J. cy) until day 3 of lactation at which time the male rat was removed and the litter was standardized at 6 pups. T o aid implantation, litters were removed from mother rats at 4:OO p.m. on day 5 of pregnancy and returned at 8: 00 a.m. on day 6 (2).Rats that received tritiated thymidine were injected subcutaneously on the third day of lactation with 0.3 pCi/g of body weight (sp act = 6.7 Ci/mmole; New England Nuclear, Boston, MA), since La Ganga (3) found that mammary proliferation was greatest at this time in lactating hooded Norway rats. The effect of tritiated thymidine on mammary function (litter growth index and nucleic acid content) was determined in rats sacrificed on day 30 of the first lactation and in rats pregnant during the first lactation and sacrificed on day 4 of the second lactation. The percentage of alveolar cells present in the first lactation that was present in the second lactation (carry-over percentage) of rats pregnant during the first lactation was based on two criteria: the mean labeled daughter cell index and the mean total mammary gland DNA content found after injecting tritiated thymidine on day 3 of the first lactation. Rats were sacrificed on day 3 of the first lactation (1 hr after injection), on day 4 of the second lactation (with and without a nonlactating period prior to the...

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