Lactating dairy cows heavily rely on mammary gland functionality to maximize milk production. The number and activity of secretory mammary epithelial cells (MEC) plays a pivotal role in defining the synthesis potential of the gland. This dissertation aimed to investigate the effects of increased milking frequency (IMF), heat stress (HS), and cell heterogeneity as key contributors to the regulation of mammary gland milk synthesis capacity in lactating Holstein cows. The first study evaluated the implementation of IMF with 2x and 4x udder halves at early and mid-lactation for 21 and 20 d on milk yield (MY) and its association with changes in cistern and alveolar capacity. Results showed that udder halves milked 4x produced 2.27 kg more MY. Additionally, cows milked during early and mid-lactation had increased cistern capacity, while alveolar capacity remained unaffected. This suggests that increased cistern capacity may support MY enhancement through possible systemic responses caused by IMF. The second study examined the effects of 4 days of HS on mammary gland tissue structure, MEC number, and activity using a pair feeding model. Heat stress reduced MY of 4.3 kg/d. At the tissue level, HS decreased alveolar area and increased alveoli number and nucleated MEC per area. Gene expression analysis revealed unaffected activity-related targets but showed reduced phosphorylation of protein synthesis (pSTAT5) and cell survival (pS6K1) markers, as well as upregulation of an autophagosome-related protein (LC3 II). These findings indicate impaired pathways that could explain the reduction in MY after acute HS. The final study utilized single-cell RNA sequencing (scRNA-seq) to characterize the heterogeneity of epithelial and immune cell subpopulations in milk. Analysis revealed multiple subpopulations with distinct gene expression profiles, including different subtypes of mammary epithelial cells expressing representative marker genes (CSN3, CSN2, CSN1S1, CSN1S2, and LALBA) and immune cell types such as T cells, granulocytes (including neutrophils), macrophages, and B cells. Understanding the populations of hematopoietic cells in milk provides valuable insights into mammary gland function during lactation. The investigation of factors influencing cell number and activity in MEC is crucial for optimizing milk production and maintaining udder health. By identifying and addressing these factors, dairy farmers and researchers can implement strategies to enhance mammary gland function, improve milk production efficiency, and ensure the overall well-being of dairy cows. / Doctor of Philosophy / Milk production capacity in dairy cows relies on specialized cells in the mammary gland called secretory mammary epithelial cells (MEC). This study investigated how management practices, environmental factors, and individual cow factors affect the regulation of milk synthesis in Holstein cows. In the first study, we compared milking frequency in udder halves milked two times or four times per day during early and mid-lactation. The cows that were milked four times produced 2.27 kg/d of additional milk. This perhaps happened because the mammary gland's storage capacity increased with more frequent milking. Next, we studied the effects of short-term heat stress on the structure of the mammary gland tissue and the number and activity of MEC. Heat stress lowered milk production by 4.3 kg/d. We observed changes in the size and number of certain cells in the mammary gland, which likely affected the observed milk production findings. We also noticed differences in the activity of proteins related to protein production, cell survival, and the recycling of cell materials. In the final part of the study, we used single-cell characterization techniques to examine the different types of MEC and immune cells in milk. We found that there are various subgroups of MEC, as well as different types of immune cells such as T cells, neutrophils, macrophages, and B cells. Understanding the variety and abundance of these cell populations helps us learn more about how the mammary gland works during milk production. Studying the factors that influence the number and activity of MEC is essential for optimizing milk production in dairy cows. By identifying and addressing these factors, dairy farmers and researchers can develop strategies to enhance mammary gland function, improve milk production efficiency, and ensure the overall well-being of dairy cows.
| Identifer | oai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/116197 |
| Date | 01 September 2023 |
| Creators | Perez Hernandez, Gabriela |
| Contributors | Dairy Science, Corl, Benjamin A., Daniels, Kristy Marie, Rhoads, Robert P., Wong, Eric A. |
| Publisher | Virginia Tech |
| Source Sets | Virginia Tech Theses and Dissertation |
| Language | English |
| Detected Language | English |
| Type | Dissertation |
| Format | ETD, application/pdf |
| Rights | In Copyright, http://rightsstatements.org/vocab/InC/1.0/ |
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