Abstract: This study aimed to compare the changes in rumen tissues morphology and transcriptome profile of the yak at different ages to explore the key genes and signaling pathways affecting the rumen development, and lay a foundation for further in-depth exploration of the rumen development mechanism of the yak. In this study, the histomorphological and transcriptomic analysis of the healthy yak's rumen at 5 age groups was performed, including non-ruminant stage (1 day old and 20 days old), transitional stage (60 days old) and ruminant stage (15 months old and 3 years old), 3 samples per group. The body and rumen weights of the samples were measured. The rumen tissue samples were collected and carried out histomorphology observation, the muscle layer thickness, papillae height and width were measured and counted. Total RNA was extracted from rumen tissue for sequencing, and transcriptome analysis was performed with 1-day-old as control. The differentially expressed genes were selected for RT-qPCR, the results of RT-qPCR was compared with the expression trend of transcriptome data. The results of histomorphological analysis showed that the rumen muscle layer thickness and papillae morphology of yaks from 1 day to 3 years old developed and differentiated to a great extent at the histomorphological level. The rumen weight and index increased significantly at 15 months and 3 years old (P<0.05). From 20 days old, the muscle layer thickness increased significantly with the increase of age (P<0.05). After 20 days old, the height and width of rumen papillae increased significantly with the increase of age (P<0.05). In the 20 days old group, transcriptome results showed that the pathways related to VFA metabolism were enriched in cholesterol synthesis, butyrate metabolism and PPAR signaling pathway; The pathways related to immunity were enriched in the immune response biological process and Th17 cell differentiation pathway; The pathways related to the metabolism of rumen epithelial cells xenobiotics was enriched in the metabolic pathway of cytochrome P450 exogenous substance. In the 60 days old group, the pathways related to VFA metabolism were enriched in the fatty acid metabolism, propionic acid metabolism pathway and pyruvate metabolism pathway; The pathways related to VFA absorption was enriched in mineral absorption. The 15 months old and 3-year-old groups enriched in ECM receptor interaction pathways that maintain the integrity and barrier function of rumen epithelium. HMGCS2, SLC26A3, PPARG, PPARD and CCL5 genes were selected from the above pathways, which related to the nutrient absorption, metabolism and immunity of rumen. The expression pattern of differentially expressed genes detected by RT-qPCR was consistent with the results of the transcriptome sequencing. The above results indicate that the rumen morphology of the yak develops early, and 20 days old is an important critical point for ruminal development; The development of ruminal muscle layer thickness was earlier than that of ruminal papillae, muscular thickness increased rapidly from 20 days old; and the rumen papillae developed rapidly after 20 days old; The rumen started VFA metabolism at 20 days old, which promotes the rapid and healthy development of rumen.

Key words: different ages, yak, rumen, histomorphology, transcriptome