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Fetal Marking in Utero 1

In utero marking of a fetus to detect it among litter mates after birth became a necessity in our laboratory. The technique needed to be one which had no adverse effect on the fetus marked. Vital staining of fetuses has been attempted with varying results (1, 2). Most mammalian fetuses that have been checked were found to drink amniotic fluid while in utero. The technique is dependent upon fetal drinking in utero (1, 3–5) and absorption of the dye into the blood stream. Most “success” has been obtained with Trypan Blue. However, mammalian fetuses have a low tolerance limit for the dye. Materials and Methods. A saturated solution of Blue Dextran 2000 (2.506 g dissolved in 63–65 cc of H2O) (furnished by Pharmacia Fine Chemicals, Inc., Piscataway, N. J.) and Cibacron Blue F 3G-A dye (18 mg/cc of H2O) (furnished by CIBA, Basal, Switzerland) were prepared. As far as we could determine the latter dye is the one used to make the Blue Dextran 2000. Coeliotomies were performed on pregnant females, exposing the uterine horns with their fetuses; 0.1 cc of the dye was injected through the uterine wall into the amniotic fluid of selected fetuses with a #27 hypodermic needle. Results. We surveyed many “vital” stains for marking fetal cotton rats (Sigmodon hispidus). All ended with failure except the saturated solution of Blue Dextran 2000 and the Cibacron Blue F 3G-A (6). Each dye marked a fetus equally well. The fetuses were not vitally stained, but the dye was concentrated in the lower intestine. The dye could be seen with ease through the abdominal walls in the inguinal area. Two females were killed before parturition to ascertain that the marked fetus was indeed the one whose amniotic fluid received the injection.