Medial edge epithelial cell fate during palatal fusion

To explain the disappearance of medial edge epithelial (MEE) cells during p alatal fusion, programmed cell death, epithelial-mesenchymal transformation , and migration of these cells to the oral and nasal epithelia have been pr oposed. However, MEE cell death has not always been accepted as a mechanism involved in midline epithelial seam disappearance. Similarly, labeling of MEE cells with vital lipophilic markers has not led to a clear conclusion a s to whether MEE cells migrate, transform into mesenchyme, or both. To clar ify these controversies, we first utilized TUNEL techniques to detect apopt osis in mouse palates at the fusion stage and concomitantly analyzed the pr esence of macrophages by immunochemistry and confocal microscopy. Second, w e in vitro infected the MEE with the replication-defective helper-free retr oviral vector CXL, which carries the Escherichia coli lacZ gene, and analyz ed beta-galactosidase activity in cells after fusion to follow their fate. Our results demonstrate that MEE cells die and transform into mesenchyme du ring palatal fusion and that dead cells are phagocytosed by macrophages. In addition, we have investigated the effects of the absence of transforming growth factor beta(3) (TGF-beta(3)) during palatal fusion. Using environmen tal scanning electron microscopy and TUNEL labeling we compared the MEE of the clefted TGF-beta(3) null and wild-type mice. We show that MEE cell deat h in TGF-beta(3) null palates is greatly reduced at the time of fusion, rev ealing that TGP-beta(3) has an important role as an inducer of apoptosis du ring palatal fusion. Likewise, the bulging cells observed on the MEE surfac e of wild-type mice prior to palatal shelf contact are very rare in the TGF -beta(3) null mutants. We hypothesize that these protruding cells are criti cal for palatal adhesion, being morphological evidence of increased cell mo tility/migration. (C) 2000 Academic Press.