Dissecting the role of the endoplasmic reticulum in trophectoderm development and blastocyst quality in yaks

Yak (Bos grunniens) are a unique bovine species adapted to extremely harsh environmental conditions in high-altitude regions. Despite their crucial roles in animal industry, knowledge of yak reproduction and embryonic development remains severely limited, and particularly, in vitro embryo production (IVEP) remains in its preliminary stage in yak. In the present study, we established a yak IVEP culture system and evaluated the developmental competence of yak embryos. We achieved cleavage and blastocyst rates of 71.04% and 29.18% in yak, respectively, which were comparable to those of cattle. However, pronounced developmental asynchrony was observed among yak blastocysts, with only 35.52% achieving complete hatching. Immunostaining assays revealed that abnormal blastocyst development was driven by impaired trophectoderm development rather than a reduction in total cell numbers. Compromised blastocyst expansion and hatching were characterized by inadequate proliferation and excessive apoptosis of trophectoderm cells. Transcriptome profiling identified core regulatory modules governing blastocyst development, including endoplasmic reticulum (ER) stress response, cell secretion and adhesion, and amino acid biosynthesis. Notably, underdeveloped blastocysts exhibited defective ER homeostasis relative to their normally developed counterparts. Supplementation experiments with IXA4 confirmed that enhanced ER function significantly promoted trophectoderm development and effectively reduced the proportion of unexpanded blastocysts. Collectively, our findings elucidate the critical role of ER function in regulating trophectoderm development and yak blastocyst quality. These results not only address a gap in understanding the mechanisms underlying yak embryonic development, but also provide new insights for optimizing IVEP protocols in other livestock species exposed to environmental stress.