During vertebrate embryonic development complex morphogenetic events drive the formation of internal organs associated with the developing digestive tract. The foregut organs derive from hepatopancreatic precursor cells that originate bilaterally within the endoderm monolayer, and subsequently converge toward the midline where they coalesce to produce the gut tube from which the liver and pancreas form. The progenitor cells of these internal organs are influenced by the lateral plate mesoderm (LPM), which helps direct them towards their specific fates. However, it is not completely understood how the bilateral organ precursors move toward the embryonic midline and ultimately coalesce to form functional organs. Here we demonstrate that the zebrafish homeobox gene hoxb5b regulates morphogenesis of the foregut endoderm at the midline. At early segmentation stages, hoxb5b is expressed in the LPM adjacent to the developing foregut endoderm. By 24 hpf hoxb5b is expressed directly in the endoderm cells of the developing gut tube. When Hoxb5b function is disrupted, either by morpholino knockdown or sgRNA/Cas9 somatic disruption, the process of foregut morphogenesis is disrupted, resulting in a bifurcated foregut. By contrast, knockdown of the paralogous hoxb5a gene does not alter gut morphology. Further analysis has indicated that Hoxb5b knockdown specimens produce endocrine pancreas cell types, but liver cells are absent. Finally, cell transplantation experiments revealed that Hoxb5b function in the endoderm is not needed for proper coalescence of the foregut at the midline. Together, our findings imply that midline morphogenesis of foregut endoderm is guided by a hoxb5b-mediated mechanism that functions extrinsically, likely within the LPM. Loss of hoxb5b function prevents normal coalescence of endoderm cells at the midline and thus disrupts gut morphogenesis.

在脊椎动物胚胎发育过程中，复杂的形态发生事件驱动与发育中的消化道相关的内脏器官的形成。前肠器官来源于肝胰腺前体细胞，这些前体细胞在双侧内胚层单层内起源，随后向中线汇合，在那里它们合并形成形成肝脏和胰腺的肠管。这些内部器官的祖细胞受到外侧板中胚层 (LPM) 的影响，这有助于引导它们走向特定的命运。然而，尚不完全了解双侧器官前体如何向胚胎中线移动并最终合并形成功能器官。在这里，我们证明了斑马鱼同源框基因hoxb5b调节中线前肠内胚层的形态发生。在早期分割阶段，hoxb5b在与发育中的前肠内胚层相邻的 LPM 中表达。到 24 hpf hoxb5b直接在发育中的肠管的内胚层细胞中表达。当 Hoxb5b 功能被 morpholino 敲低或 sgRNA/Cas9 体细胞破坏时，前肠形态发生过程被破坏，导致前肠分叉。相比之下，旁系同源hoxb5a的击倒基因不会改变肠道形态。进一步分析表明，Hoxb5b 敲低标本产生内分泌胰腺细胞类型，但不存在肝细胞。最后，细胞移植实验表明，内胚层中的 Hoxb5b 功能对于中线前肠的适当合并不需要。总之，我们的研究结果表明，前肠内胚层的中线形态发生是由hoxb5b介导的外在功能机制指导的，可能在 LPM 内。hoxb5b功能的丧失阻止了中线内胚层细胞的正常合并，从而破坏了肠道形态发生。