The human epiblast is a lumenized and polarized PSC colony
The lumenized rosette architecture emerges during the transition from naïve to primed pluripotency. In response to extracellular matrix cues, naïve PSCs start to polarize. As transcriptional factors trigger primed pluripotency, hPSC form de novo intracellular actin-rich compartments called apicosomes, which accumulate Ca2+ and express apical proteins. As apicosomes regroup, concomitant mechanisms promoting water influx towards the center of the epiblast participate in the establishment of a fluid-filled lumen. The resulting rosette architecture displays apical-basal polarity: all apical domains are facing the lumen, while basal domains are facing the external environment of the epiblast.
Fig. Lumenogenesis & rosette expansion
Lumen formation is required for optimal proliferation and homogenous pluripotency
hPSCs in lumenized rosette conformation are characterized by prolonged undifferentiated proliferation without undergoing senescence or quiescence. Kim et al. showed in 20211 that inhibiting lumen formation in PSC colonies results in reduced proliferation, and that the lumen is required for the establishment of critical signalling pathways, such as the Nodal cascade. Although the composition of the luminal fluid and the signalling molecules found within this central cavity are not fully characterized, the lumen is known to enhance intercellular communication within the rosette, and this architecture is considered to contribute to the maintenance of stemness during hPSC exponential growth. In a 2021 interview, developmental biologist Marta Shahbazi reported that in vitro, PSCs that escape the 3D rosette architecture are differentiating, thus losing their pluripotent phenotype.
The rosette architecture participates in the fitness of pluripotent stem cells
In 2018, Knouse et al. demonstrated that polarized epithelial tissue architecture is required for high-fidelity chromosome segregation. Loss of polarity and disruption of tissue architecture around a central lumen are in fact correlated with mitotic failures, aneuploidy and accumulation of DNA damage in epithelial cells. This observation gains a particular relevance in the context of early human development, where genomic abnormalities are extremely common. In 2020, a single-cell RNA sequencing analysis reported that more than 70% of human embryos contain karyotypically abnormal cells at the pre-implantation stage, prior to epiblast formation.
Recent research has shown that the pluripotent rosette undergoes natural quality control through cell competition mechanisms, leading to the elimination of abnormal cells through apoptosis and autophagy In the mouse epiblast, 35% of PSCs were found to be eliminated by the onset of gastrulation. Several groups demonstrated that defective cells, harboring mis-patterning, karyotypic, or mitochondrial abnormalities are efficiently depleted in epiblast conformation. Normal diploid PSCs were also found to increase their proliferation to compensate for the elimination of abnormal cells in the epiblast. Overall, the competition between PSCs in the epiblast, which involves YAP/TAZ25, MYC23, p5324, and mTOR signalling pathways, is considered as a critical mechanism, safeguarding pluripotency and genomic integrity before germline commitment.