Primiparous and multiparous females differ in mammary gland alveolar development: implications for milk production.

Mammary gland capacity is influenced by the number of secretory cells in the gland, the activity of those cells and the size and arrangement of the alveoli that they form. Although reproductive experience has been shown to affect the total number of secretory cells in the gland, its potential effect on the structural development of lobulo-alveolar tissues has not been directly investigated. To examine whether reproductive experience affects lobulo-alveolar development, we took mammary gland biopsies at early and peak lactation from primiparous and multiparous grey seal (Halichoerus grypus) females and used histological techniques to compare cell density, alveolar density and alveolar size within secretory lobules. Primiparous females had a significantly higher cell density compared with multiparous females throughout lactation, suggesting that primiparous females have smaller, less-developed secretory cells. Primiparous females had a significantly smaller average alveolar size compared with multiparous females throughout lactation. Although alveolar density was higher in primiparous females compared with multiparous females at early lactation, there was no significant difference between the groups at peak lactation. These results suggest that the mammary gland of primiparous females may have both a lower secretory capacity and a lower storage capacity on a relative basis than those of multiparous females and demonstrate, for the first time, that reproductive experience has a significant effect on both the rate and pattern of mammary gland alveolar development and, potentially, on a female's capacity for milk production.