Abstract
Disintegration of the midline epithelial seam (MES) is crucial for palatal fusion, and failure results in cleft palate. Palatal fusion and wound repair share many common signaling pathways related to epithelial-mesenchymal cross-talk. We postulate that chemokine CXCL11, its receptor CXCR3, and the cytoprotective enzyme heme oxygenase (HO), which are crucial during wound repair, also play a decisive role in MES disintegration. Fetal growth restriction and craniofacial abnormalities were present in HO-2 knockout (KO) mice without effects on palatal fusion. CXCL11 and CXCR3 were highly expressed in the disintegrating MES in both wild-type and HO-2 KO animals. Multiple apoptotic DNA fragments were present within the disintegrating MES and phagocytized by recruited CXCR3-positive wt and HO-2 KO macrophages. Macrophages located near the MES were HO-1-positive, and more HO-1-positive cells were present in HO-2 KO mice compared to wild-type. This study of embryonic and palatal development provided evidence that supports the hypothesis that the MES itself plays a prominent role in palatal fusion by orchestrating epithelial apoptosis and macrophage recruitment via CXCL11-CXCR3 signaling.
Highlights
Formation of the secondary palate requires adhesion by the midline epithelial edge (MEE) of both palatal shelves, formation of the transient midline epithelial seam (MES), disintegration of the MES, and fusion of the palatal shelves (Ackermans et al, 2011)
Quantitative real-time PCR confirmed the genotypes of mice by showing that heme oxygenase (HO)-2 mRNA was only present in samples from wild-type fetuses, and not in HO-2 KO fetuses (P < 0.001, Figure 5)
Deletion of HO-2 expression in mice leads to fetal growth restriction, severe malformations, and craniofacial anomalies, we found no evidence of disruption of palatal fusion in HO-2 KO fetuses
Summary
Formation of the secondary palate requires adhesion by the midline epithelial edge (MEE) of both palatal shelves, formation of the transient midline epithelial seam (MES), disintegration of the MES, and fusion of the palatal shelves (Ackermans et al, 2011). After disintegration of the MES the mesenchyme of the palatal shelves can fuse to form the secondary palate. Failure of epithelial adhesion between both palatal shelves (Dudas et al, 2006) or a lack of MES disintegration (GritliLinde, 2007; Iseki, 2011) will result in cleft palate with (CLP) or without cleft lip (CPO). The main hypotheses underlying MES disintegration involve epithelial cell migration to the oral or nasal epithelium (Jin and Ding, 2006), epithelial-to-mesenchymal transformation (EMT) (Nawshad, 2008), epithelial cell apoptosis (Vaziri Sani et al, 2005; Xu et al, 2006; Vukojevic et al, 2012; Lan et al, 2015), or a combination of these events (Iseki, 2011).
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