The expression of mRNAs for collagen types I, II, III and for aggrecan core protein was studied in developing human femoral cartilage by in situ hybridization, with special attention given to the cartilage covered by the perichondrium and to the articular surface. In parallel, the synthesis of the related proteins was monitored by immunohistochemistry. The cells metabolically active for type I and type III collagen expression were identified by hybridization using [32P]-labeled cDNA clones coding for human alpha 1(I) and alpha 1(III), respectively. Type II collagen and core protein mRNAs were detected by hybridization with specific [32P]-labeled oligonucleotide probes. In the femoral heads of one 22-week old fetus and of one newborn, our in situ hybridization and immunohistochemical analysis revealed that chondrocytes located immediately subjacent to the perichondrium produced collagen types I, II, III as well as aggrecan; whereas only type II collagen and aggrecan gene expression was detected deeper in the cartilage covered by the perichondrium. This observation supports the hypothesis that the inner cell layers of perichondrium are chondrogenic, with a transient state where cells express all the markers studied here. At the articular surface different patterns of expression were observed at the two developmental stages. After 22 weeks of fetal development only collagen types I and III were expressed by the surface zone cells while in the newborn cartilage, these cells expressed all the molecules studied (collagen types I, II, III and cartilage proteoglycan). At both ages the underlying cartilage cells expressed only the cartilage-specific molecules (type II collagen and aggrecan). Thus a progressive transformation of cartilaginous matrix occurs with time from the deep cartilage up to the surface by addition of new components, i.e. aggrecan and type II collagen. These results supplemented by an immunofluorescence analysis on 20-, 26- and 38-week old fetal femoral heads suggest that expression of collagen and aggrecan in the cartilage covered by the perichondrium and in the cartilage at the articular surface are subject to different regulatory mechanisms during development. Furthermore, the appearance of hybridizable core protein and type II collagen mRNAs at the articular surface, closely followed by the appearance of the proteins for which they code, indicates that core protein and type II collagen expression is regulated primarily at the transcriptional level in this region. Finally, the similar topography observed for the expression of these two proteins suggests that the genes for these two major constituents of cartilage matrix are coordinately regulated during growth of articular cartilage.