Expression of the gene encoding transforming growth factor-a (TGFa) was examined in developing rat embryos by using a cloned TGFo cDNA as a hybridization probe. Northern blot analysis of RNA isolated from whole fetuses revealed that TGFaL mRNA was present at relatively high levels in 8-through 10-day-old embryos and then declined to the low or undetectable level, which is characteristic of adult tissues before birth. The level of TGFcc mRNA present during early gestation was similar to that present in retrovirus-transformed cells in culture, suggesting that TGFa expression is not highly localized in the embryo. These observations are consistent with the hypothesis that TGFat plays a role in development, possibly as a fetal growth factor.Transforming growth factors (TGFs) are potent mitogenic polypeptides produced by a variety of retrovirustransformed cells and certain tumors (17). In addition, TGFs confer on normal cells in culture some of the phenotypic properties associated with transformation, including loss of contact inhibition and growth in soft agar (3). Two distinct types of TGFs have been identified. TGFa is structurally related to epidermal growth factor (EGF) (6,11,12,15) and competes for binding to the EGF receptor (13). TGF,B, on the other hand, potentiates the growth-stimulating activity of TGFa and demonstrates no affinity for the EGF receptor (1).Recently, cDNA clones encoding human (4) and rat (9) TGFs's have been isolated; the nucleotide sequence of these cDNAs suggests that human and rat TGFs's are initially synthesized as polypeptides of 160 and 159 amino acids, respectively, and that these larger forms exist as transmembrane proteins. Release of the smaller TGFa from the larger form apparently does not occur through the action of a serine-like protease but, rather, through an unusual proteolytic cleavage between alanine and valine residues at both the amino and carboxy termini of the rat and human molecules.Using the cloned rat TGFa cDNA as a hybridization probe, we have previously shown that retrovirustransformed cells express a 4.5-kilobase TGFa mRNA (9). A similar-sized transcript is also detected at considerably lower levels in some adult rat tissues but not in the corresponding untransformed cell line. Here we report that Northern analysis of developing rat embryos indicates that TGFa transcripts are expressed at relatively high levels during early embryogenesis but then decline to low levels before birth. These results are consistent with the hypothesis that TGFa functions as a developmental growth factor.Pools of rat embryos were collected at various time points from time-mated Sprague-Dawley rats. RNA was isolated by homogenization of embryos or cells in 4 M guanidine * Corresponding author. Medicine, Chapel Hill, NC 27514. thiocyanate and sedimentation through 5.7 M CsCl (2, 5).Poly(A)' RNA was selected by chromatography on oligo(dT)-cellulose, and samples (5 ,ug) were electrophoresed through 1% agarose gels containing formaldehyde (10). RNA was transferred to nitrocellulose and...