TGF-BETA-3 PROMOTES TRANSFORMATION OF CHICKEN PALATE MEDIAL EDGE EPITHELIUM TO MESENCHYME IN-VITRO

Epithelial-mesenchymal transformation plays an important role in the d isappearance of the midline line epithelial seam in rodent palate, lea ding to confluence of the palate, The aim of this study was to test th e potential of the naturally cleft chicken palate to become confluent under the influence of growth factors, such as TGF beta 3, which are k nown to promote epithelial-mesenchymal transformation, After labeling medial edge epithelia with carboxyfluorescein, palatal shelves (E8-9) with or without beak were dissected and cultured on agar gels, TGF bet a 1, TGF beta 2 or TGF beta 3 was added to the chemically defined medi um. By 24 hours in culture, medial edge epithelia form adherent midlin e seams in all paired groups without intact beaks, After 72 hours, sea ms in the TGF beta 3 groups disappear and palates become confluent due to epithelial-mesenchymal transformation, while seams remain mainly e pithelial in control, TGF beta 1 and TGF beta 2 groups, Epithelium-der ived mesenchymal cells are identified by carboxyfluorescein fluorescen ce with confocal microscopy and by membrane-bound carboxyfluorescein i solation bodies with electron microscopy, Labeled fibroblasts complete ly replace the labeled epithelia of origin in TGF beta 3-treated palat es without beaks, Single palates are unable to undergo transformation, and paired palatal shelves with intact beaks do not adhere or undergo transformation, even when treated with TGF beta 3. Thus, physical con tact of medial edge epithelia and formation of the midline seam are ne cessary for epithelial-mesenchymal transformation to be triggered, We conclude that there may be no fundamental difference in developmental potential of the medial edge epithelium for transformation to mesenchy me among reptiles, birds and mammals. The bird differs from other amni otes in having developed a beak and associated craniofacial structures that seemingly keep palatal processes separated in vivo, Even control medial edge epithelia partly transform to mesenchyme if placed in clo se contact, However, exogenous TGF beta 3 is required to achieve compl ete confluence of the chicken palate.