The salivary epithelium initiates as a solid mass of epithelial cells that are organized into a primary bud that undergoes morphogenesis and differentiation to yield bilayered acini consisting of interior secretory acinar cells that are surrounded by contractile myoepithelial cells in mature salivary glands. How the primary bud transitions into acini has not been previously documented. We document here that the outer epithelial cells subsequently undergo a vertical compression as they express smooth muscle α-actin and differentiate into myoepithelial cells. The outermost layer of polarized epithelial cells assemble and organize the basal deposition of basement membrane, which requires basal positioning of the polarity protein, Par-1b. Whether Par-1b is required for the vertical compression and differentiation of the myoepithelial cells is unknown. Following manipulation of Par-1b in salivary gland organ explants, Par-1b-inhibited explants showed both a reduced vertical compression of differentiating myoepithelial cells and reduced levels of smooth muscle α-actin. Rac1 knockdown and inhibition of Rac GTPase function also inhibited branching morphogenesis. Since Rac regulates cellular morphology, we investigated a contribution for Rac in myoepithelial cell differentiation. Inhibition of Rac GTPase activity showed a similar reduction in vertical compression and smooth muscle α-actin levels while decreasing the levels of Par-1b protein and altering its basal localization in the outer cells. Inhibition of ROCK, which is required for basal positioning of Par-1b, resulted in mislocalization of Par-1b and loss of vertical cellular compression, but did not significantly alter levels of smooth muscle α-actin in these cells. Overexpression of Par-1b in the presence of Rac inhibition restored basement membrane protein levels and localization. Our results indicate that the basal localization of Par-1b in the outer epithelial cells is required for myoepithelial cell compression, and Par-1b is required for myoepithelial differentiation, regardless of its localization.

唾液上皮细胞起始为上皮细胞的固体，这些细胞被组织成一个初级芽，芽经历形态发生和分化，产生双层腺泡，其由内部分泌腺泡细胞组成，在成熟的唾液腺中被收缩的肌上皮细胞所包围。以前尚未记录初级芽如何转变为痤疮。我们在这里记录，外部上皮细胞随后经历垂直压缩，因为它们表达平滑肌α-肌动蛋白并分化为肌上皮细胞。极化上皮细胞的最外层组装并组织基底膜的基础沉积，这需要极性蛋白Par-1b的基础定位。肌上皮细胞的垂直压缩和分化是否需要Par-1b是未知的。在唾液腺器官外植体中操作Par-1b之后，Par-1b抑制的外植体既显示出分化的上皮细胞的垂直压缩降低，又显示出平滑肌α-肌动蛋白水平降低。Rac1击倒和抑制Rac GTPase功能也抑制了分支形态发生。由于Rac调节细胞形态，因此我们研究了Rac在肌上皮细胞分化中的作用。抑制Rac GTPase活性显示出类似的垂直压缩和平滑肌α-肌动蛋白水平降低，同时降低了Par-1b蛋白水平并改变了其在外部细胞中的基础定位。对Par-1b进行基础定位所必需的ROCK抑制作用会导致Par-1b的错误定位和垂直细胞压缩的损失，但并未明显改变这些细胞中平滑肌α-肌动蛋白的水平。在Rac抑制作用下，Par-1b的过表达恢复了基膜蛋白水平和定位。我们的结果表明，Par-1b在外部上皮细胞中的基础定位是肌上皮细胞压缩所必需的，而Par-1b是肌上皮细胞分化所必需的，无论其定位如何。