Comparative biochemistry of mouse and chick secondary-palate development in vivo and in vitro with particular emphasis on extracellular matrix molecules and the effects of growth factors on their synthesis.

A biochemical study analysing the wet weight, dry weight, water, protein and DNA content, collagen and GAG composition of all stages of the developing secondary palate in vivo and in vitro was undertaken to investigate differences between a species in which the palatal shelves elevate (mouse) and one in which they do not (chick). The effects of EGF, bFGF, PDGF and TFG-beta 1 on collagen and GAG synthesis by cultured mouse and chick palatal shelves of different embryonic stages were also studied. The total GAG content of developing mouse palatal shelves decreased with developmental time; heparan sulphate proteoglycan formed the major species in early palates but hyaluronan was the major species in mid-late palates. There was a peak of hyaluronan synthesis in embryonic palatal shelves in vitro at day 13 (T.21), i.e. immediately before shelf elevation. By contrast the total GAG content of chick palates increased with development; chondroitin-6-sulphate formed the major GAG species and there was no peak in hyaluronan synthesis. The water content of developing murine palates rose rapidly at day 14 (T.22), i.e. the time of shelf elevation. No such peak was seen in the chick, where the water content rose exponentially with developmental time. Mouse palates synthesized chondroitin-4-sulphate and novel proteins around the time of shelf elevation; chick palates synthesized chondroitin-6-sulphate and no novel proteins at any developmental stage. Collagen synthesis also peaked in vitro in T.21 murine palates. EGF markedly stimulated murine palatal collagens and GAG synthesis between stages T.20-T.22, but had no effect thereafter. Basic FGF had similar but smaller stage-related effects. PDGF had no effect on mouse palatal collagen and GAG synthesis whilst TGF-beta 1 inhibited GAG synthesis at T.21. The ratios of collagens I, III and V produced by mouse palates were unaltered by the growth factors. All the growth factors had no effect on chick palatal collagen synthesis at any stage and minimal effect on GAG synthesis; TGF-beta 1 stimulated it in early but inhibited it in mid- to late-stage chick palates. These data indicate that extracellular matrix molecule metabolism within the palate is markedly different in the two species studied and suggest that the differing profiles of such molecules may be regulated at certain developmental stages by specific growth factors.