Regulation of erythropoietin production by the kidneys is central to the control of erythropoiesis. Uncertainty about the identity of the renal cells involved has been a major obstacle to understanding this mechanism. We have used sequence from the mouse erythropoietin locus to direct expression of a marker gene, SV40 T antigen, to these cells in transgenic mice. The transgenic constructs contained an oligonucleotide marker (Epo-M) or SV40 sequence (Epo-TAg) in the 5' untranslated region of the mouse erythropoietin gene, flanked on each side by 9 and 7.5 kb of DNA from the mouse erythropoietin locus. Anemia-inducible expression of Epo-M and Epo-TAg was observed in the kidney. In one of thirteen lines, homologous integration of Epo-TAg into the mouse erythropoietin locus occurred. In transgenic mice bearing Epo-TAg at homologous and heterologous insertion sites, renal expression was restricted to a population of cells in the interstitium of the cortex and outer medulla. Immunohistochemical characterization by light and electron microscopy shows that these are the fibroblast-like type I interstitial cells.

Purpose-To characterize the MRI relaxation properties of human umbilical cord blood at 1.5 Tesla.Methods-Relaxometry measurements were performed on cord blood specimens (N=88, derived from 6 caesarean deliveries) spanning a broad range of hematocrits (Hct=0.19-0.76) and oxygen saturations (sO 2 =4-100%), to characterize the dependence of T 1 and T 2 on these blood properties.Adult blood data (N = 31 specimens, derived from two volunteers) were similarly studied to validate our experimental methods by comparison with existing literature. Using biophysical models previously developed for adult blood, new model parameters were estimated, which relate Hct and sO 2 to the observed cord blood relaxation times.Results-Fitted biophysical models explained more than 90% of the variation in T 1 and T 2 . In general, T 2 relaxation times of cord blood were longer (by up to 35%) than those of adult blood, while T 1 relaxation times were slightly shorter (by up to 10%).Conclusion-The models and fitted parameters presented here can be used for calibration of future MRI investigations of fetal and neonatal blood physiology. This study is an important step in facilitating accurate, non-invasive assessments of fetal blood oxygen content, a valuable diagnostic parameter in the identification and treatment of fetal hypoxia.

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