We have examined the expression pattern of the PG-Lb/epiphycan gene that encodes a small leucine-rich repeat proteoglycan during mouse embryonic development, PG-Lb/epiphycan mRNA transcripts were first detected at E12.5 days postcoitus (dpc) at high levels in structures that were developing cartilage elements. The gene is expressed in a very specific temporal and spatial fashion in cartilaginous structures, To examine PG-Lb/epiphycan gene expression during cartilage development in more detail, we performed in situ hybridization on hindlimb sections at specific stages of mouse embryonic development, The expression of PG-Lb/epiphycan was compared to that of collagen type II and collagen type X, which are early and late markers for cartilage development, respectively. The expression of PG-Lb/epiphycan occurs later than collagen type II in cartilage development, but its expression appears in the growth plate before and is excluded from the zone of hypertrophic chondrocytic cells expressing collagen type X, An antibody against PG-Lb/epiphycan localized the protein within the entire growth plate of the E17.5 dpc embryonic hindlimb cartilage including the hypertrophic zone where PG-Lb/epiphycan gene expression is turned off. Our results show that PG-Lb/ epiphycan gene expression is an intermediate marker for chondrogenesis, and that the protein can be localized to the extracellular matrix surrounding resting, proliferating, and hypertrophic chondrocytes by immunofluorescence histochemistry. (C) 1999 Wiley-Liss, Inc.