Palatal process movement in the rat as demonstrated in frozen sections

During mammalian secondary palate development, movement of the lateral palatine processes from the vertical plane to the horizontal plane involves a complex interaction of the palatine processes and the tongue within a dynamic growing oronasal cavity environment. This study of pre-fixation facial profile photographs and frozen sections was undertaken to evaluate external and internal changes in the oronasal complex during secondary palate elevation without the shrinkage known to be present with routinhistological preparation of embryonic tissues. Frozen sections of Sprague-Dawley rat embryos between 15 and 17 days of (conceptual) age were prepared by hexane quenching and cryostat cutting. The results showed that, during the stages of palate development prior to shelf elevation, the tongue and mandible became positioned beneath the primary palate, and the vertical dimension of the oronasal cavity increased by the lifting of the nasomaxillary complex. The tongue and mandible maintained contact with the primary palate, whereas a space developed above the tongue in the middle and posterior palate regions. As the vertical dimension increased the volume of the palatomaxillary processes increased rapidly, the tongue became squeezed, and the palatine processes bulged medially above the level of the tongue. After shelf elevation extensive contact between the palatine processes was present, and the tongue became flattened. The results of this study support the observations of Lazzaro (1940) that rapid increase in shelf volume owing to increased intercellular volume contributes to movement of the processes above the tongue. But, rapid increase in shelf volume occurred contemporaneously with the time when the tongue and mandible outgrew the oronasal cavity and became positioned beneath the primary palate. Therefore, it would appear that the simultaneous occurrence of a lower and more forward tongue position, and an increased palatomaxillary process volume without change in maxillary width, contributed to the medial movement of the processes above the tongue.