Hyaluronic acid is required for palatal shelf movement and its interaction with the tongue during palatal shelf elevation.

Palatal shelf elevation is an essential morphogenetic process that results from palatal shelf movement caused by an intrinsic elevating force. The nature of the elevating force remains unclear, but the accumulation of hyaluronic acid (HA) in the extracellular matrix (ECM) of the palatal shelves may play a pivotal role in developing the elevating force. In mammals, HA is synthesized by hyaluronic acid synthases (HAS) that are encoded by three genes (Has1-3). Here, we used the Wnt1-Cre driver to conditionally disrupt hyaluronic acid synthase 2 (Has2) in cranial neural crest cell lineages. All Has2 conditional knockout (cko) mice had cleft palate due to failed shelf elevation during palate development. The HA content was significantly reduced in the craniofacial mesenchyme of Has2 cko mutants. Reduced HA content affected the ECM space and shelf expansion to result in a reduced shelf area and an increased mesenchymal cell density in the palatal shelves of Has2 cko mutants. We examined palatal shelf movement by removal of the tongue and mandible from unfixed E13.5 and early E14.5 embryonic heads. Reduced shelf expansion in Has2 cko mutants altered palatal shelf movement in the medial direction resulting in a larger gap between the palatal shelves than that of littermate controls. We further examined palatal shelf movement in the intact oral cavity by culturing explants containing the maxilla, palate, mandible and tongue (MPMT explants). The palatal shelves elevated alongside morphological changes in the tongue after 24-h culture in MPMT explants of early E14.5 wild type embryos. On the contrary, shelf elevation failed to occur in MPMT explants of age-matched Has2 cko mutants because the tongue obstructs palatal shelf movement, suggesting that reduced shelf expansion could be essential for the palatal shelves to interact with the tongue and overcome tongue obstruction during shelf elevation. Has2 cko mutants also showed micrognathia due to reduced HA content in the mandibular mesenchyme including Meckel's cartilage. Through 3D imaging and morphometric analysis, we demonstrate that mandibular growth results in a significant increase in the vertical dimension of the common oral-nasal cavity that facilitates palatal shelf movement and its interaction with the tongue during shelf elevation.