Endogenous epidermal growth factor regulates the timing and pattern of embryonic mouse molar tooth morphogenesis

The tooth organ provides a model for discrete patterns of morphogenesis over short periods of developmental time. Studies were designed to test the hypothesis that endogenous epidermal growth factor (EGF) functions to regulate multiple cusp molar tooth morphogenesis during embryonic mouse development. The relative levels of endogenous EGF and EGF receptor (EGFR) transcripts were determined in both enamel organ epithelia and dental ectomesenchyme by reverse transcription-polymerase chain reaction (RT-PCR) assays. EGF and EGFR were localized by immunohistochemistry; both antigenic determinants were demonstrated on the same odontogenic cells in cultured tooth explants. To examine EGF-mediated signal transduction, cap stage mouse molar tooth organs (E16) were cultured in serumless, chemically-defined medium as either (i) controls, or supplemented with (ii) tryphostin (an EGF receptor kinase inhibitor), (iii) tyrphostin plus exogenous EGF, and (iv) exogenous EGF. Antisense oligodeoxynucleotide (ODN) strategy was used to investigate the functions of endogenous EGF employing (i) non-treated control, (ii) sense ODN control, (iii) antisense ODN, (iv) exogenous EGF, (v) sense ODN with exogenous EGF, and (vi) antisense ODN with exogenous EGF. Tyrphostin inhibited DNA synthesis and produced a significant decrease in the volume of the explants. These effects were recovered by addition of exogenous EGF. Antisense ODN inhibition resulted in abnormal cusp formations, decreased DNA synthesis, total DNA, RNA and protein content, and decreased stellate reticulum and tooth explant volumes. The decreased tooth size was not uniform, the most pronounced effect was in the stellate reticulum. This pattern of changes was not seen when antisense ODN treatment was supplemented with exogenous EGF. These results suggest that during cap stage of odontogenesis endogenous EGF acts to stimulate DNA synthesis, which increases the cell number of specific phenotypes within the enamel organ epithelia, and thereby regulates molar tooth morphogenesis.