Epithelial-Mesenchymal Interactions During Odontogenesis .1. Isolation Of Several Intercellular Matrix Low Molecular Weight Methylated Rnas

Abstract The present results and those previously reported (Slavkin et al., 1969a) show that embryonic tooth primordia synthesize in organ culture four discrete methylated RNA species (7 S, 5 S, 4 S, and 2 S) isolated from the intercellular matrix interposed between epithelia and mesenchyme. These RNA species were first separated by gel exclusion chromatography and subsequently resolved with polyacrylamide gel disc electrophoresis. Resolution required employment of a variety of nucleic acid precursors, labeled methyl-methionine, liquid scintillation spectrometry, and autoradiography. The labeled precursors are initially incorporated into both epithelial and mesenchymal cells, primarily in the germinative or cervical region of the tooth primordia. Subsequently, small amounts of RNase labile material(s) are observed over the intercellular matrix; presumably transferred from both cell types. Tritiated thymidine is not found as a component of the intercellular matrix; deoxyribose is not found in the isolated matrices. Various labels cannot be incorporated into the intercellular matrix in the absence of associated cell populations. The kinetics of labeling within the components, both cell populations and matrix, have been investigated using autoradiography (Slavkin et al., 1969a). On the basis of grain counts, only 2% of the total grain density incorporated within this epithelial-mesenchymal system is transferred into the intercellular matrix; Dactinomycin inhibits this process. This specific intercellular matrix has recently been shown to enhance homotypic cellular differentiation in vitro (Slavkin et al., 1969b). Ultrastructural findings indicate the presence of membrane-bound, electron dense material (500–3000 A diameter) within the matrix prior to phenol extraction; this material is not present in phenol-extracted matrices. Whether the extracted intercellular RNAs are to be found within the membrane-bound material observed within the matrix in situ (Slavkin et al., 1969b,c) is not as yet certain. Further studies designed to evaluate this biological activity (Slavkin et al., to be published) suggest that the activity is RNase sensitive. As to the mechanism(s) for the transfer of RNA or possibly ribonucleoprotein during epithelial-mesenchymal interactions, the present experiments do not provide adequate evidence with which to understand the developmental specificity of the isolated RNA species.