Pax6 regulates craniofacial form through its control of an essential cephalic ectodermal patterning center

Abstract

Summary: Normal patterning and morphogenesis of the complex skeletal structures of the skull requires an exquisite, reciprocal cross-talk between the embryonic cephalic epithelia and mesenchyme. The mesenchyme associated with the jaws and the optic and olfactory capsules is derived from a Hox-negative cranial neural crest (CNC) population that acts much as an equivalence group in its interactions with specific local cephalic epithelial signals. Craniofacial pattern and morphogenesis is therefore controlled in large part through the regulation of these local cephalic epithelial signals. Here, we demonstrate that Pax6 is essential to the formation and maturation of the complex cephalic ectodermal patterning centers that govern the development and morphogenesis of the upper jaws and associated nasal capsules. Previous examinations of the craniofacial skeletal defects associated with Pax6 mutations have suggested that they arise from an optic-associated blockage in the migration of a specific subpopulation of midbrain CNC to the lateral frontonasal processes. We have addressed an alternative explanation for the craniofacial skeletal defects. We show that in Pax6SeyN/SeyN mutants regional CNC is present by E9.25 while there is already specific disruption in the early ontogenetic elaboration of cephalic ectodermal expression, associated with the nascent lambdoidal junction, of secreted signaling factors (including Fgf8 and Bmp4) and transcription factors (including Six1 and Dlx5) essential for upper jaw and/or nasal capsular development. Pax6 therefore regulates craniofacial form, at stages when CNC has just arrived in the frontonasal region, through its control of surface cephalic ectodermal competence to form an essential craniofacial patterning center. genesis 49:307-325, 2011. V V C 2011 Wiley-Liss, Inc.

Fgf8 dosage determines midfacial integration and polarity within the nasal and optic capsules

“…To molecularly characterize the FNP re-orientation, we analyzed the expression of genes with regionally distinct patterns and known involvement in craniofacial development (Bei and Maas, 1998; Beverdam et al, 2001; Britanova et al, 2006; Compagnucci et al, 2011; Depew et al, 1999, 2000a, b; Depew and Compagnucci, 2008; Depew and Simpson, 2006; Grifone et al, 2005; Lanctot et al, 1997, 1999; Qu et al, 1999; Ruf et al, 2004; Satokata et al, 2000; Satokata and Maas, 1994; Szeto et al, 1999; Zirzow et al, 2009; Zou et al, 2004). We were principally interested in understanding the presumptively mature λ-junction at E10.5 as this time-point provides a useful read out of the essential molecular baüplan at the root of subsequent craniofacial morphogenesis (Depew et al, 2002b).…”

“…Moreover, the development of the neuronal component of the optic system is apparently not required for optic capsular development, as exemplified by the loss of Pax6 (Matsuo et al, 1993; Osumi-Yamashita et al, 1997; Compagnucci et al, 2011). While ocular tissue defects likely characterize Fgf8 null/Neo mutants, we did not investigate them outside of the skeletal system; we did, however, note changes in gene expression (e.g., Barx2 , Sox9 , Raldh3 ) in early optic and peri-ocular cells as well as significant changes in apoptosis in the optic primordia, including in the optic stalk and the surrounding mesenchyme (Supplementary Figure 1).…”

“…Elaboration of this cross-talk is manifested in the induction and maintenance of intricate patterns of gene expression, the spatial and temporal details of which underlie the precise translation of patterning processes and information into discrete, appropriate skeletal morphologies. The lambdoidal junction (λ-junction), formed where the maxillary division of the first branchial arch (mxBA1) meets the medial (mFNP) and lateral (lFNP) frontonasal processes, exemplifies a morphologically and molecularly intricate epithelial-mesenchymal craniofacial interface (Depew and Compagnucci, 2008; Compagnucci et al, 2011). The organization of the λ-junction is complex and embodies the future positions of the choanae, the upper lips and (premaxillary) incisors, the primary and secondary palates, and the optic (OPC) and nasal (NSC) capsules (Figure 1).…”

“…The organization of the λ-junction is complex and embodies the future positions of the choanae, the upper lips and (premaxillary) incisors, the primary and secondary palates, and the optic (OPC) and nasal (NSC) capsules (Figure 1). The functionality and morphology of each of these structures depends on the appropriate orientation and polarity (i.e., relative development along the medio-lateral, dorso-ventral and rostro-caudal axes) of the craniofacial primordia associated with the λ-junction during development (Tamarin and Boyde, 1977; Compagnucci et al, 2011). …”

Osterix is required for cranial neural crest-derived craniofacial bone formation

“…In addition, Dlx5 is expressed in CNC for the development of craniofacial hard tissues, and Dlx5-deficient mice show defective craniofacial structures, frontonasal development, and maxillae and mandibles, which suggests that Dlx5 is required for craniofacial development and that it regulates the patterning of craniofacial morphology through the CNC [21]. Mutations in Pax6, a crucial regulator of craniofacial development, cause craniofacial skeletal defects that arise from the arrested migration of midbrain CNC-derived cells to the lateral frontonasal region [22,23]. During craniofacial development in mouse, Osx is expressed in condensed mesenchymes of the developing frontal bone, maxilla, and mandible [9].…”