The primary multipotent pancreatic progenitor cells (MPCs) are initially undifferentiated and can give rise to both tip and trunk progenitor cells (reviewed in Jennings et al. 2015). Trunk progenitor cells are also known as bipotent pancreatic progenitor cells (bi-PPs) as they give rise to duct cells and endocrine progenitors (reviewed in Jennings et al. 2015).

As development progresses, at Carnegie stage CS19 (45-47 days post-conception, corresponding to post-conception week PCW7 or gestation week GW9) in humans, trunk MPCs located in the central trunks of the developing, branching pancreas, lose GATA4 expression but retain expression of FOXA2, NKX6-1, PDX1, and SOX9 (Jennings et al. 2013, reviewed in Jennings et al. 2015). GATA6 expression likely persists in trunk progenitors, as GATA6 is detected in endocrine progenitors (Shi et al. 2017, Chia et al. 2019), but the evidence specifically linking GATA6 to the bipotent state of human pancreatic progenitors is lacking. Characteristically, PTF1A expression in trunk progenitors is downregulated/undetectable (Villani et al. 2019; Olaniru et al. 2023). Based on a mouse pancreatic development study, Nkx6-1 and Ptf1a function as antagonistic lineage determinants forming a genetic switch for trunk/ductal/endocrine lineage and tip/acinar lineage, respectively (Schaffer et al. 2010), which is corroborated by mathematical modeling of pancreatic progenitor fate (Wang et al. 2020). Nkx6-2 can partially compensate for Nkx6-1 in promoting trunk/ductal/endocrine cell fate (Schaffer et al. 2010). HES1, a downstream target of Notch signaling that plays a role in repressing the PTF1A gene, is expressed in trunk progenitors (Mamidi et al. 2018; Olaniru et al. 2023) and helps to maintain their bipotent state (Mamidi et al. 2018). Expression of HES1 and NOTCH1 in pancreatic trunk progenitors may be stimulated by the complex of TEAD4 and YAP1, which are regulated by the HIPPO signaling pathway (Mamidi et al. 2018). Interaction of trunk bipotent progenitors with the extracellular matrix (ECM) determines whether they will maintain their bipotent state, eventually committing to the ductal cell fate, or whether they will assume the endocrine fate (Mamidi et al. 2018). Based on a study with human embryonic stem cell (hESC)-derived pancreatic progenitors, an ECM enriched for fibronectin and vitronectin promotes a spread-out cell morphology, maintenance of integrin alpha5-beta1 expression, activation of FAK kinase, nuclear YAP1 localization, and HES1 expression (Mamidi et al. 2018). By single-cell RNA sequencing (scRNA-seq)-based analysis of embryonic human pancreas from post-conception weeks (PCW) 4 to 11, the pancreatic ductal lineage, besides HES1, also expresses high levels of HES4 and HEY1, with HES4 expression increasing going from early trunk to ductal cells (Ma et al. 2023). NOTCH signaling, through NOTCH1, NOTCH2, and NOTCH3 expressed in trunk and duct cells, may be activated by NOTCH ligands DLK1 and JAG1 expressed by fibroblasts and pericytes in the developing pancreas (Ma et al. 2023). Yap1 gene deletion in mouse pancreatic epithelium leads to pancreatic hypoplasia and hypoglycemia due to increased endocrinogenesis associated with the depletion of bipotent trunk progenitors (Mamidi et al. 2018). An ECM enriched for laminin and collagen reduces integrin alpha5-beta1 expression, reduces FAK phosphorylation, and downregulates nuclear YAP1 in hESC-derived bipotent trunk progenitors (Mamidi et al. 2018). A pattern of laminin deposition in the developing mouse pancreas also suggest that laminin likely acts as an inducer of endocrine differentiation (Mamidi et al. 2018). In spread-out cells that maintain their bipotency and commit to the ductal lineage, the protein levels of NKX6-1 and PDX1 become low (Mamidi et al. 2018). YAP1 protein expression is maintained in the nuclei of mouse ductal cells (Cebola et al. 2015).

By scRNA-seq analysis of embryonic human pancreas from post-conception weeks (PCW) 4 to 11, the number of early tip and trunk cells gradually increases from PCW4 to PCW7 (Ma et al. 2023). Concordantly, scRNA-seq analysis of developing human pancreas between gestational weeks GW8-12 shows that trunk progenitors constitute the majority of pancreatic cells (>75%) at GW8 and GW10, but their proportion decreases to <10% at GW12 (Olaniru et al. 2023).

Cell fate decision analysis based on scRNA-seq of developing mouse pancreas distinguishes between trunk-early cells, which branch into trunk-duct cells (ductal progenitors) or trunk-endocrine progenitors (that will give rise to endocrine progenitors) (Yu et al. 2019). Mathematical modeling of stable and transition cell states associated with pancreas development based on a core endogenous transcription factor (TF) network regulating acinar, ductal, and endocrine fates that includes PDX1, PTF1A, NKX6-1, SOX9, HES1, NGN3, ARX, and PAX4 TFs similarly predicts the existence of a stable trunk progenitor state in between a multipotent progenitor transition state and a trunk stable state, which is supported by available scRNA-seq data (Wang et al. 2020). A scRNA-seq analysis of embryonic human pancreas from post-conception weeks (PCW) 4 to 11 similarly distinguishes between early trunk and trunk cells, with increasing expression of HES4, DCDC2, and CFTR going from early trunk to ductal cells (Ma et al. 2023). A finer grained annotation of trunk cell states may be possible when more experimental data becomes available.

Trunk bipotent pancreatic progenitor cells are characterized by expression of the following markers: