MAMMARY CELL CHANGES DURING PREGNANCY AND LACTATION

This review describes the changes in mammary secretory cell number and activity that occur during the lactation cycle of cows and goats. Magnetic resonance imaging has been used to measure mammary growth in goats in vivo and, in combination with determinations of cellular differentiation made in biopsy samples, this has allowed a complete picture of mammary development to be built up. Secretory tissue volume increases exponentially during gestation and early lactation through the combination of cellular hyperplasia (initially) and hypertrophy (later). Involution occurs gradually during declining lactation, due entirely to reduced cell number, and more rapidly after milking ceases. Secretory tissue volume is greater at the start of the second gestation than the first, and the rate of tissue growth is also greater during the later stages of this second gestation. Milk yield and secretory tissue volume are both greater in the second lactation than the first. The effects of new milk production strategies are described. Increased milking frequency enhances milk yield and also stimulates mammary cell activity and then number. Treatment with growth hormone (bovine somatotrophin or BST) also increases milk yield, and when the two treatments are applied together the yield effects are additive. BST accelerates the differentiative response to frequent milking and also increases secretory tissue mass.