Secondary cartilage in the murine medial pterygoid plate has a critical role in the pathfinding of the tensor veli palatini

Abstract

The medial pterygoid plate plays a critical role in mammalian craniofacial function during suckling and swallowing. The plate supports the tensor veli palatini (TVP) muscle, which stiffens the soft palate to create a posterior seal. Despite its functional importance, the developmental origins and structural integration of the medial pterygoid plate and associated pterygoid hamulus remain incompletely understood. In this study, we investigated the ontogeny and lineage of the medial pterygoid plate using wildtype and conditional knockout mice with immunofluorescence and lineage tracing. Analysis from embryonic day (E)14.5 to E17.5 confirmed that the medial pterygoid plate formed as a bipartite structure, which later fused to the basisphenoid. The dorsal region of the medial pterygoid plate ossified via Runx2-dependent intramembranous bone formation, while the ventral region formed as a secondary cartilage, undergoing Sox9-dependent chondrogenesis, followed by endochondral ossification. The pterygoid hamulus was evident at E14.5 as a condensation of Sox9-positive mesenchyme at the end of the medial pterygoid plate. Confirming the different modes of development, the ventral and dorsal parts of the medial pterygoid plate showed distinct timing and pattern of collagen remodelling, as shown by B-CHP. Lineage tracing with Wnt1Cre;tdTom and Mesp1Cre;tdTom mice demonstrated that the entire pterygoid process formed from neural crest-derived mesenchyme. In keeping with this, conditional loss of Runx2 in the neural crest lineage disrupted ossification of the dorsal part of the medial pterygoid plate, whereas conditional loss of Sox9 abolished chondrogenesis of the ventral part of the medial pterygoid plate and the pterygoid hamulus. Notably, TVP muscle fibres were able to maintain their orientation around the residual cartilage in conditional Wnt1creRunx2flfl mutants, while the TVP in conditional Wnt1creSox9flfl mutants formed a ball of cells that failed to extend towards the palatal region. The ventral portion of the medial pterygoid plate and hamulus is therefore required to guide early muscle pathfinding. These findings establish the medial pterygoid plate as a compound craniofacial element with distinct ossification modes and an important role interacting and directing neighbouring tissues.