Fig. 8

Model of effects of Satb1 deletion on enamel formation. During tooth development, the expression of SATB1 starts in PABs, which attach to the dentin matrix. PABs further elongate and polarize as they differentiate into SABs. Circumferential F-actin belt-like structures are assembled along apical poles, mostly attributing to the locally increased claudin 1 (CLDN1). These F-actin belt-like structures not only physically connect elongated ameloblasts into a sturdy sheet, but also control the contents to be secreted as a sieve. As SABs start to secrete enamel matrix, cells move away from the matrix leaving a cytoplasmic protrusion surrounding by the organic matrix. This protrusion is a histological landmark of SAB, so-called Tomes’ process (TP). The F-actin networks built along the cytoplasmic membrane of the apical surface, likely through the interactions between transmembrane integrins and extracellular matrix proteins, could generate pushing force to facilitate the membrane protrusion and serve as tracks for transporting vesicle/A (secretory vesicles containing amelogenins). TP is responsible for the directional secretion of enamel matrix proteins and the formation of one enamel rod. When Satb1 gene is deleted, PABs are significantly shorter and have barely detectable CLDN1 at the apical pole. Not surprisingly, no F-actin belt orderly assembles at the apical pole of PABs. SABs are continuously shorter compared to controls with significantly reduced CLDN1, deformed F-actin architecture, and no Tomes’ process at its apical end. These defects are believed to account for the amelogenin retention in Satb1^−/− ameloblasts and a thin, hypomineralized and disordered enamel phenotype. It is worthy to note that often several claudin family members are co-expressed and interact with each other to regulate the tight junction structure in epithelial cells. This is a simplified schema that primarily focuses on the correlation between SATB1 with CLDN1