Thrombospondin-2 gene expression and protein localization during embryonic mouse palate development

The mammalian palate develops from projections of the paired maxillary processes termed palatal shelves. Shelf growth is an essential in normal palatal morphogenesis. Mesenchymal proliferation in the palatal shelves is modulated by transforming growth factor-β (TGF-β), among other growth factors. Several pathways effect TGF-β activation, including one which utilizes thrombospondin (TSP). TSP-1 is a major activator of TGF-β in vivo and has been localized in head mesenchyme, including palates. TSP-2 appears to inhibit TSP-1 activation of latent TGF-β by competitively binding the latent TGF-β. Here the TSP-2 mRNA transcript and the immunolocalization of TSP-2 protein with progressive palatogenesis were quantified. There was a significant (p<0.05) decline of TSP-2 transcript with palatal maturation; there was no evidence correlating the TSP-2 transcription with the amount of activated TGF-β. At the vertical shelf stage of palatogenesis, TSP-2 protein was found throughout the extracellular matrix of shelf mesenchyme. By the horizontal shelf stage, TSP-2 protein was principally localized to the ossification centres of the developing maxilla, both in extracellular matrix and bone; far less was seen in palatal shelves proper. These results suggest that TSP-2 is multifunctional during embryonic palate formation.