In this study we investigated erythropoiesis and fetal liver protein secretion during late embryonic (Day 24 and Day 30) and early fetal (Day 40) development in pigs from domestic white crossbred (WC) gilts with a normal (intact; INT) or crowded (unilateral hysterectomized/ovariectomized; UHO) uterine environment, or from prolific Chinese Meishan (MS) gilts. Increased fetal weight, fetal liver weight, placental weight, total red blood cells, hematocrit, blood hemoglobin content, and maternal plasma erythropoietin (EPO) levels were observed as gestation advanced. Cultured fetal liver secretion of transferrin and a protein of Mr 12500 and pI 7.5 also increased as gestation advanced. Fetal plasma EPO declined between Day 30 and Day 40. Differential counts of circulating erythroid precursors revealed a decline in basophilic erythroblasts and polychromatic erythroblasts between Day 24 and Day 40, an increase in orthochromatic erythroblasts on Day 30 followed by a drop on Day 40, and an increase in the percentage of reticulocytes/ erythrocytes from < 1.0% to approximately 90% of circulating red blood cells between Day 24 and Day 40. Differences among the treatment groups included a lower fetal survival percentage in UHO (vs. INT or MS) on Day 40, and higher maternal hematocrits, fetal weights, fetal hematocrits, fetal EPO levels, and liver transferrin secretion in WC vs. MS pigs. MS pigs had a lower percentage of polychromatic erythroblasts overall and a higher percentage of orthochromatic erythroblasts on Day 24 followed by a higher percentage of erythrocytes on Day 40 than WC pigs, suggesting a more mature erythron (circulating red blood cells plus erythropoietic tissue) in the MS pigs. Covariate analysis indicated that MS had larger placentae per unit of body weight than did WC. Conclusions were that 1) Days 24-40 of gestation is a critical time for fetal erythropoiesis in pigs as well as survival in a crowded uterine environment, 2) the MS breed may differ in the development of the fetal erythropoietic system because of altered fetal or uterine physiology, and 3) the UHO procedure did not significantly affect erythropoiesis in the fetuses studied but did alter fetal survival and the relationship between fetal weight and both hematocrit and hemoglobin on Day 40.

Two distinct forms of endometrial folate binding protein (FBP) cDNAs were isolated using reverse transcription-polymerase chain reaction and 3' and 5' rapid amplification of cDNA ends (RACE) procedures. On the basis of the absence or presence of an intact glycophosphatidylinositol linkage site in the C terminus of the predicted amino acid sequences, the two forms appear to encode secreted and membrane-bound forms of FBP. The cDNAs for the putative secreted and membrane forms encoded 252- and 249-amino acid proteins, respectively, that were 73% identical with each other and were 66-82% identical with other known FBPs. However, the nucleotide sequences within the 5' untranslated region and from codons 224 and 223 of the secreted and membrane forms, respectively, to the 3' ends of each RNA, were divergent. The divergence in the 3' ends of the two cDNAs was exploited to determine changes in concentrations of each mRNA in the endometrium during the estrous cycle and early pregnancy. Northern blots of endometrial total RNA probed with a putative secreted FBP specific probe indicated that mRNA concentrations do not change during early pregnancy. In contrast, blots probed with a putative membrane FBP specific probe indicated that mRNA concentrations increase dramatically from Day 15 to Day 24 of pregnancy. Finally, N-terminal amino acid sequencing of FBP purified from Day 15 pregnant uterine flushings matched the secreted form of FBP mRNA. These data are consistent with a role for putative secreted and membrane-bound forms of FBPs in the transport of folate to the developing swine conceptus during early pregnancy.

In a previous report, it was suggested that intrauterine crowding impaired fetal erythropoiesis and that fetal erythropoiesis was accelerated in Meishan pigs during early pregnancy. Because these conclusions were based on limited numbers of observations, the present experiment was undertaken to provide a more extensive investigation of these phenomena. Intact white crossbred gilts, unilaterally hysterectomized-ovariectomized (UHO) white crossbred gilts, and intact Meishan gilts (n = 13 to 16 per group) were mated after at least one estrous cycle of normal duration (17 to 23 d). Gilts were laparotomized at d 35 of pregnancy, the uterine horns were exteriorized and opened near each fetus, and a blood sample was collected from each fetus. The uterine horn was then surgically removed, and each fetus and placenta was weighed. All fetal blood samples were measured for hematocrit, red blood cell number, and hemoglobin. Erythropoietin and the percentages of nucleated cells and reticulocytes were also measured in blood samples from the largest and smallest living fetus in each litter. Fetal hematocrits were not affected by treatment. Blood cell counts were greater (P < 0.01) in fetuses of Meishan gilts than in White crossbred intact or UHO gilts. Hemoglobin was less (P < 0.01) in fetuses of Meishan gilts than in fetuses of White crossbred intact or UHO gilts. The percentages of nucleated (immature) cells and reticulocytes were both less (P < 0.01) in fetuses of Meishan intact gilts. Erythropoietin was also lower (P < 0.01) in fetuses of Meishan gilts. As observed previously, fetal weight was correlated (r = 0.38; P < 0.01) with blood hemoglobin concentration. These results confirm that fetal erythropoiesis in Meishan gilts is accelerated compared with White crossbred gilts. These results are consistent with the hypothesis that faster blood cell development could be beneficial to fetal survival in swine.