Transcriptome sequencing reveals the regulatory mechanisms during the development of small white follicles to small yellow follicles in laying hens

) levels in SYFs increased by 131.6% and 22.2%, respectively, compared to SWFs. At the transcript level, SYFs exhibited upregulation of apolipoprotein D (ApoD), apolipoprotein A1 (APOA1), apolipoprotein L (APOLD1), PPAR and very low-density apolipoprotein (VLDL) mRNA, alongside downregulation of occludin, which was further confirmed by increased VLDLR and PPAR-γ protein abundance and decreased occludin levels. In summary, our results revealed that the SWFs to SYFs transition proceeds with pronounced changes in transcriptional profiles associated with angiogenesis, sex steroid biosynthesis and yolk deposition. The increased steroid hormone production likely enhances the capacity for yolk synthesis. Furthermore, the elevated vascularization in the theca layer of SYFs in coupled with a reduction in tight junctions may facilitate the transport of yolk precursors to access the follicles.