expression of collagen alpha1i mrna variants during tooth and bone formation in the rat collagen alpha1i mrna is composed of two variants of 5 and 6 kb differing in the length of the 3 untranslated region in this work the nucleotide sequences of the two rat mrna variants were compared and their expression pattern in cells forming bone dentin and cementum were analyzed the sequences were determined from cdna inserts of tooth and bone libraries plus directly from pcr fragments obtained from bone a total of 5721 bases of the rat collagen alpha1i sequence from cdna of tooth and bone was determined all sequences of the short variant were represented in the long variant only the alternatively polya additions gave rise to the variants in hard tissue two oligonucleotides were chosen as probes one of which recognized on northern blots the two bands of 5 and 6 kb and the other the 6kb variant only the oligonucleotides were used in in situ hybridization experiments for study of the distribution of the variants in different extracellular matrixforming cells osteoblasts odontoblasts and cementumassociated cells were closely examined in sections from rat maxillae from 2 to 25 days of age a similar or identical pattern of mrna expression was observed with both oligonucleotides indicating that the two mrna variants were coexpressed in all cases