The mechanical role of the cranial base in palatal shelf movement: an experimental re-examination.

Straightening of a cranial base flexure observed in the midsagittal presphenoid region of mice has been postulated to play a major role in palate closure by providing the internal shelf force for palatal shelf movement. The straightening is thought to take place during the 8 h immediately preceding closure. This mechanical model assumes that straightening of the cranial base flexure is transmitted laterally to the alar regions of the sphenoid and then downward to the palatal shelves. According to this hypothesis, if the flexure did not straighten or the force generated by this form change was not transmitted to the palatal shelves, movement to the horizontal plane should not take place. Physical disruption of the area of the flexure should remove the potential for form change through force transmission. SwissWebster mouse fetuses which were 18 or 24 h prior to palate closure were obtained and the heads removed. Tongue and brain were then removed from each head. In the experimental specimens a midsagittal lesion, averaging 407 /tm in length and 84 /tm in width, extending from the nasal septum through the craniopharyngeal area was made. This lesion ablated the flexure region of the cranial base far in advance of the time straightening is thought to take place. Heads of litter-mates with intact cranial bases served as controls. Specimens with intact and ablated flexures were suspended in a submerged, circulating culture system which was continuously gassed. After 18 h of culture, palatal shelf elevation had occurred in fetuses of both ages whether or not their cranial bases were intact. Palatal shelf elevation in vitro does not require an intact cranial base.