Ultrastructural evidence of contractile systems in mouse palates prior to rotation

Abstract

Previous studies have shown that the palatal shelves of mouse embryos synthesize the contractile proteins actin and myosin at a rate equal to that of tongue just prior to shelf movement (day 14.5). The purpose of this study was to examine the morphology of the palatal shelves for evidence of a contractile system. Myosin ATPase histochemistry was performed on frozen sections of day-14.5 fetal mouse heads. Three areas of the palatal shelves gave a positive reaction: 1) A reaction product typical of skeletal muscle on the oral side of the posterior palate (region 1); 2) a “heavy-diffuse” reaction product on the tongue side extending from the top mid-palate to the posterior end (region 2); and 3) a “light-diffuse” reaction product along the oral epithelium in the mid-palate (region 3). Electron microscopy of excised day-14.5 palates was carried out after fixation in glutaraldehyde or an acrolein-dichromate solution. Region 1 contained a large area of developing and adultlike skeletal muscle. In the area of region 2 a large population of filamentous-rich mesenchymal cells was observed. In addition, large neurons coursing through both contractile systems were noted. Preliminary observations in region 3 indicated the possibility of a primitive (nonmuscle) contractile system in that area. The contractile and nervous systems in the palate, prior to rotation, indicate the possibility that an innervated embryonic muscle system may provide the “intrinsic shelf force” to rotate the shelves.