Using both a radioimmunoassay and a radioreceptor assay, we have estimated the content of mouse epidermal growth factor-urogastrone (EGF-URO) in fetal mice at 111/2 to 171/2 days of gestation. Concurrently, the amount of specific EGF-URO receptor binding was determined in crude membrane fractions from the embryos. EGF-URO receptor binding is readily detected in membranes from the youngest embryos (day 11/2) and rises steadily up to parturition (18 days); the rise is more marked in embryo membranes derived from a potential target tissue, such as the maxilla and secondary palate. In the embryonic extracts, EGF-URO proved to be labile, requiring the presence of soybean trypsin inhibitor and sodium azide to stabilize the recovery of added EGF-URO in test samples. Even with added stabilizing agents, immunoreactive EGF-URO was barely detectable before day 141/2 (less than 20 fmol per embryo), whereas a substantial increase was observed from day 151/2 to 171/2 (from 70 to 200 fmol per embryo). In contrast, the radioreceptor assay detected appreciable amounts of an EGF-URO-like substance at 111/2 days (50 fmol per embryo) the values estimated by radioreceptor assay (about 0 old higher than by radioimmunoassay) also increase markedly between days 151/2 and 171/2 (on average from 500 to 3000 fmol per embryo). We conclude that during fetal mouse development there is an increase both in the receptors for EGF-URO and in a substance (presumably a fetal growth factor) that can occupy the receptor. The differences between the radioreceptor and radioimmunoassay estimates for the EGF-URO content suggest that the fetal form of mouse EGF-URO differs from the a ult molecule.Epidermal growth factor-urogastrone (EGF-URO) is a polypeptide of approximately 6000 daltons that is found in mice, in humans, and undoubtedly in a variety of other mammalian species (for reviews, see refs. 1-6). The recognized biological actions of EGF-URO are stimulation of cellular proliferation and inhibition of gastric acid secretion; in addition, the occurrence of receptors for EGF-URO in a large number of tissues, including the placenta (6, 7), and the effects of EGF-URO observed in organ cultures on the secondary palatal epithelium (8,9) suggest that EGF-URO plays an important role in embryonic development. Although the time course of the postnatal accumulation of EGF-URO in male mice has been observed [the submaxillary gland content rises to a maximum at about 50 days (10)], no detailed studies have been made in prenatal animals. Because of our interest in the effects of EGF-URO on palatal development, we have determined the content of both EGF-URO and its receptor at various stages of development. We have used both a radioreceptor and a radioimmunoassay to measure increasing levels of EGF-URO in embryos at 11/2 to 171/2 days of gestation, and we have concurrently measured increases in EGF-URO binding in membranes obtained from embryos at 12 to 14 days of gestation.
MATERIALS AND METHODSPolypeptides and Antibodies. Mouse EGF-URO was either isolat...