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The pancreas develops from dorsal and ventral epithelial extensions at the foregut/midgut boundary in Xenopus embryos. Endocrine and exocrine specification is thought to occur from a pool of uniform precursor cells. While the genetic network controlling endocrine specification and differentiation has been the object of extensive investigations, the corresponding mechanism leading to the exocrine pancreas is much less understood. Here, we report on the identification and characterisation of a novel molecular marker for the early exocrine pancreas in Xenopus embryos. Xenopus pancreatic protein disulfide isomerase is expressed in both dorsal and ventral pancreatic buds. By whole mount in situ hybridization it is detected as early as stage 39 in the exocrine lineage of the developing pancreas; RT-PCR reveals onset of expression as early as stage 35/36.
Fig. 2. Expression of XPDIp in Xenopus embryos between stages 39 to 46. (A) A left side view with expression of XPDIp in the dorsal (red arrow) and ventral (black arrow) pancreatic buds. Note the posterior position of the dorsal bud relative to the ventral bud. (B) A right side view of the same embryo as shown in panel A; note the proximity of the dorsal and ventral pancreatic buds on this side of the embryo. The dim appearance of the dorsal bud in right side view as compared to the left side view is as a result of a slight shift of the dorsal bud to the left of the embryo. (C) A cross-section of a stage 39 embryo (at the level indicated by dotted lines in panel B) showing the dorsal pancreatic bud (red arrow) and the two ventral buds (black arrows) not yet fused at this stage. Note the slight ventral expansion of the dorsal bud on the right side, and the corresponding dorsal shift of the rightventral bud. R and L indicate the right and left side of the embryo, respectively. (D) A right side view showing the fused dorsal and ventral pancreatic buds in a stage 40 embryo. (E–K) Serial sections from anterior to posterior of the embryo shown in panel D. Note the fusion of the two ventral buds in panel F. Inspection of panel G reveals that a part of the right half of the dorsal pancreas is fused to the rightventralpancreas. Panel H illustrates the dorsal bud in the medial position of the embryo as it extends from the right to the left half; the left half of the dorsal bud is not visible in G and H because it is positioned more posteriorly. Panel I-K reveal the left half of the dorsal pancreas, which lies posterior to the right half. (L–P) Expression of XPDIp in embryos between stages 41 to 46. L, M are leftlateral views; N, O are antero-ventral views and P a right-ventral view. The dorsally derived pancreas is indicated with a red arrow and the ventrally derived pancreas with a black arrow.
Fig. 3. A comparative expression analysis of XPDIp and other pancreas specific genes. (A) In situ hybridization showing xPDIp and insulin expressing cells in embryonic and adult pancreas. (i)–(iii) Double in situ hybridization staining of insulin (red) and XPDIp (blue) expressing cells in the adult and embryonic pancreas. XPDIp/insulin co-staining of stage 39 dorsal embryonic pancreas, stage 44 embryonic pancreas as well as the adult Xenpouspancreas shows that expression of XPDIp is largely restricted to the exocrine pancreatic lineage. (iv) and (v) show the expression of XPDIp and insulin respectively on neighbouring section of stage 39 embryo. The two sections are 5 μm apart, with (iv) anterior to (v). The arrows indicate groups of cells, which express insulin (v) and the corresponding exclusion of XPDIp expression in such cells (iv). (B) Comparison of the temporal expression patterns of XPDIp, amylase, elastase, trypsinogen, insulin and XlHbox8 by RT-PCR.
