Longitudinal changes of stem-like cells in colostrum and milk of dairy cows

This study aimed to identify and quantify the various stem-like cell types in dairy cows' colostrum and milk at the onset of lactation. Five second parity Holstein cows were monitored from calving until the seventh-day postpartum. Mammary secretions were collected immediately after calving, then every 3 h until 12 h during day (d) 0, and during morning milking on d 1, d 2, d 4 and d 7. Cells were prepared from mammary secretions and analysed by flow cytometry using relevant cellular markers. The highest total and viable cell concentrations were observed in colostrum collected at calving and up to 6 h, with these concentrations decreasing substantially in samples collected later at d 0. Then, the concentrations of both total and viable cell populations continued to slowly decrease until d 7, the kinetic curves reaching a baseline plateau. Flow cytometry showed that the CD49f^posCD24^pos population, which identifies mammary epithelial stem cells, represented about 0.9% of viable cells at calving and about 0.1% 12 h later, the mammary epithelial stem cell concentration therefore being at its highest level in the very first colostrum. In contrast, the percentage of mesenchymal stem-like cells, defined as the population of CD34^negCD105^posCD90^posCD29^pos cells, was roughly constant (≈0.3%) during the first two milkings and decreased mainly during the first day to a basal level close to 0. Concerning haematopoietic stem-like cells, defined as the CD45^negCD34^posCD117^posCD90^pos cell population, they were only observed in the colostrum collected at calving. All the types of stem cells studied here were therefore only present in substantial quantities in the colostrum of the very first hours after calving, a period during which the calf's intestine is permeable, possibly allowing the transfer and integration of these cells in the tissues of the newborn calf.