Alternative splice form of type II procollagen mRNA (IIA) is predominant in skeletal precursors and non‐cartilaginous tissues during early mouse development

Type II collagen, generally considered to be characteristic of cartilage, has been localized in specific non-cartilaginous structures during embryogenesis and development of the skeleton. Type II procollagen is synthesized in two different forms generated by alternative splicing of exon 2 in the precursor mRNA transcript. One form (type IIA procollagen) contains a large cysteine-rich domain in the NH2-terminal propeptide, while the second form (type IIB procollagen) does not. These two forms are spatially expressed during development and chondrogenesis with the type IIB procollagen mRNA primarily expressed by chondrocytes while the IIA form is expressed in chondroprogenitor cells (Sandell et al. 1991 J. Cell Biol. 114:1307-1319). The present study demonstrates that the early non-cartilage expression, by somites, mesenchymal and epithelial cells, is predominantly the alternate splice form, type IIA procollagen mRNA. Later in development, the type IIB mRNA splice form is expressed by chondrocytes. During the development of intramembranous bones, such as the mandible, type IIA procollagen mRNA is also expressed. In this tissue, the splice form does not switch to type IIB mRNA and no cartilage is formed. These results show that expression of type IIA mRNA, whether by epithelial or mesenchymal cells, precedes formation of overt skeletal structures.