Day length affects simultaneously mammary epithelium integrity and mammary epithelial cell exfoliation in milk

Abstract Text:

Some of the mammary epithelial cells (MEC) responsible for milk synthesis are exfoliated into milk during the lactation process. MEC exfoliation into milk could play a role in mammary epithelium integrity. A trial was carried out to study the effects of day length and type of diet on milk calcium content. Calcium is known to play a role of cement for tight junction closure between epithelial cells. The aim of this study was to identify a potential effect of day length and dietary anion-cation differences on mammary epithelium integrity and MEC exfoliation in milk. A trial was carried out according to a Latin square design using 8 dairy cows averaging 103 ± 44 DIM with two treatments in a factorial arrangement with 4 periods of 14 d. The cows received 2 levels of dietary anion-cation differences (DCAD; 0 mEq/kg DM for D0 and 400 mEq/kg for D400) and 2 d lengths (8 h of light/d for short days and 16 h/d for long days). The cows were only exposed to solarium lights providing UVA and UVB. Once per period, milk was collected to purify MEC from milk after centrifugation and immunocytochemical sorting. MEC exfoliation was evaluated using the determination of MEC concentration in milk. The percentage of apoptotic MEC was determined by flow cytometry after TUNEL labeling. Epithelium integrity was monitored using the determination of blood lactose sampled 1 hour before morning milking, and the ratio Na:K in milk. Blood prolactin concentrations from samples collected at 7 am and 2 pm were determined by RIA. Data were analyzed using Mixed procedure. There was no significant interaction between day length and DCAD level. Milk yield did not vary with any treatments averaging 32.7 kg. day^-1. DCAD treatment did not affect any of the parameters. Blood lactose and Na:K ratio were higher with short compared with long days (P<0.05) indicating that mammary epithelium integrity was more disrupted with short days. More MEC and more apoptotic MEC were exfoliated in milk with short days compared to long days (338 vs 227. 10⁶ exfoliated MEC per day, for respectively short and long days; P < 0.05). As expected blood prolactin concentration was lower with short days (P < 0.05). Taken together these results suggest that MEC exfoliation could be induced by low prolactin concentration during short days.

Keywords: mammary epithelial cell, mammary epithelium integrity, photoperiod, feeding