Webster H H, Lengi A J, Corl B A
Department of Dairy Science, Virginia Tech, Blacksburg, VA 24061.
JDS Commun. 2024 Apr 20;5(6):707-712. doi: 10.3168/jdsc.2023-0534. eCollection 2024 Nov.
Milk yield (MY) naturally declines after peak lactation. One approach to improving production is to increase lactation persistency, or the length of time a cow maintains lactation. To do so, cows must maintain functional secretory mammary epithelial cells (MEC), which synthesize milk. Some viable MEC are exfoliated from the udder into milk. The objective of this experiment was to identify the magnitude and direction of association between exfoliated MEC and MY, days in milk (DIM), and parity. Our hypothesis was that MY and the quantity of exfoliated MEC were negatively correlated. Therefore, milk from low-yielding, late-lactation, and primiparous cows would contain more MEC. Thirty-six multiparous and 12 primiparous Holstein cows were sampled once during lactation. Flow cytometry was used to identify hematopoietic cells and MEC in milk using an antibody specific for cluster of differentiation-45 (CD45) and an antibody against butyrophilin 1A1 (BTN), respectively. Nucleated cells were stained with Hoechst dye, and propidium iodide was used to identify dead cells. For statistical analysis, we analyzed the effect of parity, MY, and DIM, and DIM on the number and concentration of both total and live exfoliated MEC. Primiparous cows exfoliated fewer MEC and had a lower concentration of MEC in milk than multiparous cows regardless of lactation stage. This could reflect continued mammary growth in first lactation. Cows shed an average of 9.80 × 10 ± 6.90 MEC (LSM ± SEM) with a concentration of 4,582 ± 3,574 cells/mL in one milking. Milk yield was inversely related to concentration of both total and live MEC in milk. Similarly, cows in late lactation, which is associated with MY decline, exfoliated a greater number of MEC into milk. Primiparous cows had a lower MY and shed fewer total and live MEC in this study. The relationship of MY and DIM with MEC losses in milk may also contribute to declining MEC number in the mammary gland found in postpeak lactation. Understanding MEC exfoliation regulation could help develop husbandry practices that improve secretory cell retention in the mammary gland and increase lactation persistency.
泌乳高峰期过后，产奶量（MY）会自然下降。提高产量的一种方法是提高泌乳持续性，即奶牛维持泌乳的时长。为此，奶牛必须保持功能性的分泌型乳腺上皮细胞（MEC），这些细胞负责合成乳汁。一些有活力的MEC会从乳房脱落进入乳汁中。本实验的目的是确定脱落的MEC与MY、泌乳天数（DIM）和胎次之间关联的程度和方向。我们的假设是MY与脱落的MEC数量呈负相关。因此，低产、泌乳后期和初产奶牛的乳汁中会含有更多的MEC。在泌乳期间，对36头经产和12头初产的荷斯坦奶牛各进行了一次采样。分别使用针对分化簇45（CD45）的特异性抗体和抗嗜乳脂蛋白1A1（BTN）的抗体，通过流式细胞术鉴定乳汁中的造血细胞和MEC。有核细胞用Hoechst染料染色，碘化丙啶用于鉴定死细胞。为了进行统计分析，我们分析了胎次、MY和DIM以及DIM对总脱落MEC和活脱落MEC数量及浓度的影响。无论泌乳阶段如何，初产奶牛脱落的MEC数量较少，乳汁中MEC的浓度也低于经产奶牛。这可能反映了头胎泌乳期乳腺仍在持续生长。奶牛一次挤奶平均脱落9.80×10±6.90个MEC（最小二乘均值±标准误），浓度为4582±3574个细胞/毫升。产奶量与乳汁中总MEC和活MEC的浓度呈负相关。同样，与MY下降相关的泌乳后期奶牛，脱落到乳汁中的MEC数量更多。在本研究中，初产奶牛的MY较低，脱落的总MEC和活MEC数量也较少。MY和DIM与乳汁中MEC损失的关系，也可能导致泌乳高峰期后乳腺中MEC数量减少。了解MEC脱落调控机制有助于制定饲养管理措施，提高乳腺中分泌细胞的留存率并增加泌乳持续性。
