Ex utero 3D culture system illuminates post-implantation development and functions of the mouse yolk sac.

Embryonic implantation initiates crucial developmental events, yet technical limitations hinder comprehensive investigation. Although whole embryo culture (WEC) enables ex utero studies, maintaining stable development from blastocyst to organogenesis remains a challenge. A key factor during early organogenesis is the yolk sac (YS), which plays a central role in providing nutritional support to the developing embryo. Deciphering the mechanisms underlying YS development and function is essential for optimizing WEC systems and uncovering the molecular and cellular dynamics of this stage. Here we present a novel 3D bionic culture platform that accurately recapitulates in utero YS development, including mesodermal, vascular, and endodermal differentiation. Comprehensive histological, molecular, and single-cell RNA-sequencing (scRNA-seq) analyses confirm the fidelity of this ex utero YS (eYS) model to its in utero counterpart. Beyond its established role in hematopoiesis, our study reveals additional functions of the YS in glycometabolism, lipometabolism, proteometabolism, coagulation, vascular development, substance transport, and barrier formation. This platform combines operational simplicity with high reproducibility, bypassing the uterine barrier to enable direct visualization of YS development and hematopoiesis. Our findings provide a robust tool for investigating the mechanisms of mammalian embryogenesis and diseases related to YS development, offering valuable insights into the early stages of life.