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In order to identify receptor molecules that participate in the growth and differentiation of chondrocytes, we cloned a number of cDNA fragments from HCS-2/8 chondrocytic cells, by using tyrosine kinase-specific primers for amplification. The mRNA expression of one such receptor, ErbB4, was increased by connective tissue growth factor/hypertrophic chondrocyte-specific gene product (CTGF/Hcs24), which promotes all stages of the endochondral ossification in vitro. ErbB4 expression was observed through all stages of chondrocytic differentiation in vitro, corresponding to the wide distribution of CTGF/Hcs24 target cells. Furthermore, positive signals for erbB4 mRNA were detectable throughout most populations of chondrocytes, in growth and articular cartilage in vivo. These results demonstrate for the first time that ErbB4 is expressed in chondrocytes and may play some roles in chondrocytic growth and differentiation along with CTGF/Hcs24.

Spatial and temporal expression pattern of retinoic acid receptor (RAR) genes was investigated in mouse finger bones during development by an in situ hybridization method with riboprobes synthesized from a human cDNA of the RAR-alpha. We found that the RAR genes are expressed intensively and specifically in calcifying fronts of the mouse finger bones, whereas the expression pattern is rather uniform in the limb buds and cartilage matrices of the embryonic fingers. Our findings are consistent with the fact that vitamin A is essential for normal mammalian bone development.

We found, by an in situ hybridization method with riboprobes synthesized from human cDNA of the retinoic acid receptor (RAR), that the RAR genes (predominantly gamma-subtype) are intensively expressed in the epidermis of normal and psoriasic human skins, and also in keratinizing fronts of 4-day-old mouse skins, nail matrices and hair follicles. Thus, target cells of retinoic acid in the skins are concluded to be keratinocytes, which is quite consistent with the fact that retinoic acid regulates keratinization of epidermis in vivo and also modulates expression of the keratin gene in vitro.