EGF Receptor Expression in Mineralized Tissues: AnIn SituHybridization and Immunocytochemical Investigation in Rat and Human Mandibles

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Abstract
There is extensive evidence that growth factors play a central part in the autocrine/paracrine regulation of cell growth and differentiation in mineralized tissues. In order to investigate involvement of the EGFr receptor (EGFr) in forming mineralized tissues, its expression was studied by in situ hybridization and immunocytochemistry in mandibles of growing rats, as well as in human embryos. In Hertwig's epithelial root sheath of rat molar, EGFr mRNAs appeared strongly expressed, while dental pulp and dental follicle showed weak labeling. The lingual epithelium of rat incisor showed strong labeling, which decreased after epithelial dislocation. Cells of the adjoining lingual dental pulp and dental follicle, as compared to epithelium, contained a low level of EGFr mRNAs. In contrast, a significant signal with antisense RNA probe was observed in bone. Sense RNA probes provided a regular background or no labeling. Undifferentiated cells located in the periosteum and endosteal spaces were labeled. EGFr mRNAs were also present in osteoblasts and in lesser amounts in some osteocytes. In rat and in human bone, both osteoblasts and osteocytes were positive on immunostaining. Similarly in the Hertwig's root sheath, EGFr immunostaining and in situ hybridization labeling were closely related. These data show that different patterns of EGFr expression in forming mineralized tissues are tissue-and stage-specific. However, in all these cells, the present in situ investigation supports the assumption that EGFr is involved in the early stages of cellular proliferation and differentiation. This report also suggests that EGFr may play a role in differentiated and mature cells of mineralized tissues.