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Disruption of circadian clocks in the prepartum dry period negatively affect mammary development.

K. Teeple




Disruption of circadian clocks in the prepartum dry period negatively affect mammary development.
K. Teeple*1, A. Suarez-Trujillo1, C. McCabe1, J. Townsend2, S. Donkin1, K. Plaut1, J. Boerman1, T. Casey1. 1Department of Animal Sciences, Purdue University West Lafayette, IN, 2College of Veterinary Medicine, Purdue University West Lafayette, IN.

Circadian clocks generate 24-h rhythms of physiology and behavior to coordinate internal metabolism and synchronize it to the environment. Studies of rodents indicate that disruption of circadian clocks negatively impacts mammary development. Our objective was to disrupt circadian clocks of cattle by exposing them to light (L)-dark (D) cycle phase shifts during the prepartum nonlactating dry period, and determine the effect on mammary development. Multiparous cattle were dried off at 60 d before expected calving (BEC) and assigned to control (n = 6) or phase shifted (PS; n = 6) treatments. Control were exposed to 16 h of L: 8 h D. PS group light exposure shifted forward 6 h every 3 d. Both groups began treatments 35 d BEC and stayed on respective treatments until calving. Following parturition all cows were exposed to 16:8 h L:D cycle. Mammary biopsies were taken at 21 d BEC and 21 d postpartum (PP). Biopsies were fixed in 10% formalin, embedded in paraffin, and sectioned for hematoxylin and eosin staining and immunostained for proliferating cell marker KI67. Images (n = 5/biopsy/cow) of tissue were captured at 40X to measure alveolar, lumen, and intralobular stromal area using ImagePro Plus 5.1. To determine proliferation index of mammary epithelial cells (MEC) and stromal cells, the total number of KI67 stained and unstained cells were counted (5 images/biopsy/cow) at 200X. The lumen to epithelial ratio was calculated by dividing the luminal area by the alveolar area. Milk yield was measured daily through 60 d PP. SAS (V 9.4) PROC MIXED was used for statistical analysis. Proliferation index of MEC at 21 BEC was lower (P < 0.05) in PS (5.2% � 0.8) vs control (12.4% � 0.8) cows; there was no difference in proliferation rates at 21 PP (C = 0.77% � 0.75; PS = 0.78% � 0.70). Alveolar area was different between time points (P = 0.002). Treatment and time point affected lumen to epithelial ratio (P < 0.05). Milk production was lower (P < 0.05) in PS (40.3kg � 0.803) vs control (42.5kg � 0.803). Findings support that phase shifting the L:D cycle alters mammary development during the prepartum period, and indicate that disruption of clocks may affect milk production.

Keywords: circadian, mammary development.