Fate of unfused medial edge epithelia in rat fetuses with experimentally induced cleft palate: I. From 16.3 to 17.7 days of gestation.

The fate of unfused medial edge epithelia (MEE) was studied in rat (Sprague-Dawley) fetuses with experimentally induced cleft palate. Clefting was the result of amniotic sac puncture and the age of fetuses was individually determined ( = MA-age). The MEE (at five selected areas), the oral palatal, and the nasal septum epithelia of six specimens (with individual ages of 16.3, 16.8, 16.9, 17.0, 17.5, and 17.7 days) were illustrated by scanning electron and light microscopy. At day 16.3 of MA-age, the MEE was covered with cells of a regular shape. In the anterior elevation area, visible signs of cell death and surface alterations could not be observed until day 17.7. In the middle elevation area, a small region of exposed mesenchyme occurred at the level of the first and second rugae between day 16.9 and 17.5 of MA-age. At day 17.7, reepithelialization was complete. In the posterior elevation area and the anterior and posterior soft palate area, a narrow band of surface alterations (ie, surface accumulations of cell blebs, filopodial-like structures, and loss of villi-like cell boundaries) appeared cranial to a thickened termination of the hard palate epithelium between days 16.9 and 17.7 of MA-age. The residual MEE remained undifferentiated until day 17.5 of MA-age. At this point in time, small ciliated cells appeared in the MEE-domain near the borderline to the nasal epithelium. It is concluded that the fate of the unfused MEE differs in the various areas located along the anteroposterior shelf axis, so-called "programmed cell death" is restricted to a small zone in the middle elevation area, surface alterations observed along the thickened termination of the oral palate epithelium possibly reflect phenomena of epithelial rearrangement between the latter and the undifferentiated MEE, and the MEE remains undifferentiated, at least until day 17.5 of MA-age.