The ultrastructure of the mesenchymal element of the palatal shelves of the fetal mouse.

The ultrastructure of the mesenchymal element of the palatal shelves of C3H mouse embryos aged 13 days 18 h and 14 days 18 h in utero was studied. At 13 days 18 h the mesenchyme showed a high density of cells. The cells contained a well developed system of rough endoplasmic reticulum and Golgi complex; many were ciliated and multi-vescular structures were common. By 14 days 18 h, many of the mesenchymal cells contained large numbers of glycogen particles. These cells also possessed long cytoplasmic processes which sometimes were seen to contain many filaments 5--7 nm in diameter. Some of the cells also contained a fine filamentous network just below the plasma membrane. Developing mononuclear and binucleate skeletal muscle cells containing myofibrils were present in the posterior region of the palate, with groups of cells which consisted of typical peripheral neurons and their surrounding satellite cells. Both the muscle and nerve cells were only observed in the 14 day 18 h material. It is suggested that both the myofibrils in the skeletal muscle cells and the filamentous network in the mesenchymal cells may play a role in shelf reorientation.