Single-cell transcriptomic insights into lipoprotein transport in yolk sac membrane endodermal epithelial cells of chicken embryos

Efficient lipid mobilization from yolk is critical for avian embryonic development, yet the cellular mechanisms governing lipoprotein transport in yolk sac membrane endodermal epithelial cells remain poorly characterized. Single-cell RNA sequencing was performed on 9,037 embryonic day 4 and 6,884 embryonic day 7 chicken yolk sac membrane cells to investigate lipoprotein biosynthesis pathways. Analysis identified 17 transcriptionally distinct clusters, including 3 endodermal epithelial cell subtypes. Cluster 2, characterized by high expression of epithelial markers (CDH1, EPCAM) and HDL biosynthesis genes (APOA1, ABCA1, LCAT), emerged as the primary site of HDL assembly. Apolipoprotein A1 ranked third in expression whereas apolipoprotein B ranked only 63rd, indicating a prominent HDL-biogenesis program in EECs. Gene ontology enrichment revealed upregulation of cholesterol metabolism and lipoprotein particle receptor binding pathways from embryonic day 4 to 7. Temporal expression analysis demonstrated progressive increases in HDL-related genes (APOA1, ABCA1, LCAT, SOAT1) throughout incubation, peaking at embryonic day 20, while VLDL-related genes (APOB, MTTP, SAR1B) showed sustained upregulation. Plasma analysis at embryonic day 19 confirmed HDL concentration (194.78 ± 30.13 mg/mL, mean ± SEM) significantly exceeded VLDL concentration (65.43 ± 13.82 mg/mL; P < 0.01), representing approximately 75% of measured lipoproteins. These findings reveal that HDL, rather than VLDL alone, plays a substantial role in lipid redistribution during late chicken embryogenesis, with HDL serving as the primary carrier of cholesteryl esters and phospholipids while VLDL remains the dominant triacylglycerol transporter, highlighting complementary lipoprotein functions and species-specific metabolic adaptations critical for embryonic development.