Single-cell transcriptomics reveals dynamic cellular remodeling of porcine mammary gland during pregnancy

Pregnancy elicits extensive remodeling of the mammary gland to establish the alveolar network required for lactation, yet the cellular programs underlying this transition in pigs remain poorly resolved. Here, we generate a single-cell transcriptomic atlas of the porcine mammary gland across gilt, late pregnancy, and post-lactational stages. Differential expression analyses identify epithelial, fibroblast and endothelial compartments as the most dynamic during pregnancy, motivating higher-resolution analyses of these lineages. Epithelial trajectories toward alveolar fate exhibit induction of fibrillar collagen programs, including COL1A1, COL3A1 and COL5A1, implicating epithelial contributions to extracellular matrix assembly. Fibroblast analysis revealed a PGLYRP1⁺ fibroblast subtype enriched for the chemokines CCL4, CCL5 and CCL21, suggesting a role in immune cell recruitment. Endothelial cells undergo stage-specific metabolic reprogramming, with late pregnancy characterized by increased expression of glycolysis-associated genes, including IGF1, FLCN and P2RX7, as well as lipid metabolism-related genes such as RBP7, KITLG and CD36. Cross-species comparison reveals conserved cellular identities and key molecular features (ESR1, AR, FOXA1) across human, pig, and mouse mammary glands. Together, this atlas delineates the cellular logic underlying pregnancy-associated remodeling and serves as a resource for elucidating transcriptomic changes in the porcine mammary gland.