Mammary Gland Growth in Sheep

Development of mammary glands was assessed in Romney and Romney-cross sheep. Animals were slaughtered at birth + or − 5 days, at 3, 4 and 5 months, at the beginning of pregnancy (2 to 3 days after mating), at the end of pregnancy (140 to 148 days pregnant) and at 5 days lactation. Mammary glands were weighed before and after trimming and the trimmed glands were extracted with lipid solvents. The remaining dried fat-free tissue (DFFT) were weighed and ground. Nucleic acids and nitrogen were measured. Based upon wet weight changes, 25% of growth occurred prior to pregnancy, 55% during pregnancy and 20% during early lactation. When DFFT was evaluated, the increases were 20% from birth to the beginning of pregnancy, 80% during pregnancy and none in early lactation. Percent protein paralleled DFFT changes among groups. However, when DNA, the most realistic measure of mammary growth, was considered, the percentages of development were 20% from birth to the start of pregnancy, 78% during pregnancy and 2% during early lactation. The ratio of RNA to DNA remained near 1 from 3 months of age to the end of pregnancy and increased to 1.7 in early lactation. Unlike many small laboratory species, the sheep does not experience any appreciable post-parturient ‘lactational’ growth of the mammary gland.