Growth of guinea pig mammary glands through their first six lactations.

Guinea pigs were killed on day 5 of lactation. Each of lactations one through six was represented by six animals. Body weight was measured, and mammary glands were removed. Wet weight of mammary gland, dry weight of fat-free tissue, and weight and percent (relative to dry fat-free tissue) of deoxyribonucleic acid, ribonucleic acid, and hydroxyproline were measured. Collagen and noncollagenous dry fat-free tissues were estimated. Body weight increased from 800 g in first lactation to 1035 g in fifth and 975 g in sixth lactation. Mammary wet weight increased quadratically from 20 g for first lactation to 31 g for fifth and 26 g for sixth lactation. Dry fat-free tissue followed a similar pattern, increasing from 2330 mg first lactation to 3647 mg fifth and 3242 mg sixth lactation. Deoxyribonucleic acid rose linearly from 70 mg first lactation to 121 mg fifth and 110 sixth lactation. Ribonucleic acid followed a similar pattern, increasing from 182 mg first lactation to 289 mg sixth lactation. Ratio of ribonucleic acid to deoxyribonucleic acid was constant at 2.2. Amount of collagen was also constant at 257 mg. However, percent collagen decreased linearly from 12% first lactation to 7.1% sixth lactation. Mammary gland increases in size over subsequent lactations to five and then decreases slightly. The increase is not of connective tissue or stroma, as measured by hydroxyproline, but is an increase in parenchyma or cells involved with milk secretion and harvest.