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De novo induction of a gene product during heterologous epithelial--mesenchymal interactions in vitro.

Mesenchymal specification of epithelial cytodifferentiation and morphogenesis has been considered to be a general feature of various epithelial-mesenchymal interacting systems (e.g., salivary gland, mammary gland, feather, hair, and tooth morphogenesis). In contrast, we have demonstrated that a mesenchyme can be induced by a heterologous epithelium to synthesize in quantity a specific gene product(s) unorthodox to the organ from which the mesenchyme was taken. Stage 22-23 avian limb bud epithelium induced 17-day embryonic mouse tooth mesenchyme to differentiate into cartilage. Peptide analysis (cyanogen bromide cleavage after purification of extracted collagen chains) demonstrated that heterologous tissue recombinations produced type II collagen [alpha(II)](3) (i.e., cartilage-type) in addition to type I collagen [alpha(I)](2)alpha(2). Intact or reconstituted mouse molar tooth organs synthesized type I collagen and type I trimer [alpha(I)](3) collagen. Immunohistochemical criteria using anti-type II collagen antibodies identified type II collagen in cartilage-like matrix within the mesenchymal component of heterologous tissue recombinants. Cartilage has never been described during in vivo or in vitro tooth tissue differentiation or associated with the pathology of dental papilla mesenchyme. These results support the hypothesis that epithelial-mesenchymal interactions during embryonic development can selectively induce de novo synthesis of unique gene products.