Growth of Guinea Pig Mammary Glands Through Their First Six Lactations

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Abstract
Guinea pigs were killed on day 5 of lactation. Each of lactations 1 through 6 was represented by 6 animals. Body wt was measured and mammary glands were removed. Wet wt of mammary gland, dry wt of fat-free tissue, and weight and percent (relative to dry fat-free tissue) of DNA acid, RNA and hydroxyproline were measured. Collagen and noncollagenous dry fat-free tissues were estimated. Body wt increased from 800 g in the 1st lactation to 1035 g in 5th and 975 g in 6th lactation. Mammary wet wt increased quadratically from 20 g for 1st lactation to 31 g for 5th and 26 g for 6th lactation. Dry fat-free tissue followed a similar pattern, increasing from 2330 mg at 1st lactation to 3647 mg at 5th and 3242 mg at 6th lactation. DNA rose linearly from 70 mg at 1st lactation to 121 mg at the 5th and 110 at 6th lactation. RNA followed a similar pattern, increasing from 182 mg at 1st lactation to 289 mg at 6th lactation. Ratio of RNA to DNA was constant at 2.2. Amount of collagen was also constant at 257 mg. The percent collagen decreased linearly from 12% at 1st lactation to 7.1% at 6th lactation. Mammary gland increases in size over subsequent lactations to 5 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.