Onset of endogenous synthesis of epidermal growth factor in neonatal mice

We have analyzed mouse fetuses and neonates for the presence of epidermal growth factor (EGF)-specific mRNA. No detectable EGF-specific mRNA was found in fetuses, fetal membranes, or placentae from Day 9 of gestation through birth or in the early neonatal period. While the kidneys begin to produce EGF specific transcripts by two weeks postpartum, the salivary glands begin to produce detectable levels of EGF mRNA only after weaning and even then at levels far below the adult amount. Reports of EGF and EGF-related material in rodent fetuses failed to determine whether this material was of maternal or fetal origin. We now conclude that authentic EGF in these embryos is probably of maternal origin. We have performed experiments designed to determine whether EGF can be transported into the fetus. A small percentage of 125I-EGF administered to pregnant females either systemically or directly into the uterine arteries reached the fetus itself. The uterus and the placenta attained a high level of labeling, whereas the amniotic fluid and yolk sac were virtually devoid of the tracer. In the neonatal period, milk may be the physiologically relevant source of EGF. We have found that 125I-EGF ingested by neonates was absorbed into the circulation, reached many internal organs, and was eventually excreted in the urine. Previously demonstrated EGF receptors in mouse embryonic cell types may be activated by either alpha type transforming growth factor or maternal EGF transported via the placenta.