Effects of chronic reduction in uterine blood flow on fetal and placental growth in the sheep

Pregnancy is associated with a significant increase in uteroplacental blood flow (UBF), which is responsible for delivering adequate nutrients and oxygen for fetal and placental growth. The present study was designed to determine the effects of vascular insufficiency on fetal and placental growth. Thirty-nine late-term pregnant ewes were instrumented to investigate the effects of chronic UBF reduction. Animals were split into three groups based on uterine blood flow, and all animals were killed on gestational day 138. UBF, which began at 851 +/- 74 ml/min (n = 39), increased in controls (C) to 1,409 +/- 98 ml/min (day 138 of gestation) and in the moderately restricted (R(M)) group to 986 +/- 69 ml/min. In the severely restricted (R(S)) group, UBF was only 779 +/- 79 ml/min on gestational day 138. This reduction in UBF significantly affected fetal body weight with R(M) fetuses weighing 3,685 +/- 178 g and R(S) fetuses weighing 2,920 +/- 164 g compared with C fetal weights of 4,318 +/- 208 g. Fetal brain weight was not affected, whereas ponderal index was significantly reduced in R(M) (2.94 +/- 0.09) and R(S) fetuses (2.49 +/- 0.08) compared with the value of the C fetuses (3.31 +/- 0.08). Placental weight was also significantly reduced in the R(M) group, being 302 +/- 24 g, whereas the R(S) group placenta weighed 274 +/- 61 g compared with the C values of 414 +/- 57 g. Fetal heart, liver, lung, and thymus were all significantly smaller in the R(S) group. Thus the present study shows a clear relationship between the level of UBF and both fetal and placental size. Furthermore, the observation that fetal brain weight was not affected, whereas fetal body weight was significantly reduced suggests that this experimental preparation may provide a useful model in which to study asymmetric fetal growth restriction.     

Relationship between corpora lutea or fetal number and plasma concentrations of progesterone and testosterone in mice

Blastocysts (1-14) were transferred unilaterally into 63 pseudopregnant mice which were killed on Day 17. Plasma progesterone concentrations were significantly (P less than 0.05) lower in animals with one fetus than in those with 2-5 or 9-14 fetuses. Plasma testosterone concentrations were correlated with fetal number in mice with 1-13 fetuses (P less than 0.001). The total placental content of chorionic gonadotrophin in 13 litters varied directly with the number in the litter (1-6), and was 1.67 +/- 0.15 ng/placenta. The number of corpora lutea per mouse was negatively correlated with mean CL volume per mouse (P less than 0.001), and the number of conceptuses was positively correlated with mean CL volume per mouse (P less than 0.001). The effect of conceptuses on the ovary was systemic. The relationship between plasma testosterone concentration and conceptus number may be due to gonadotrophins acting on the ovary, or androgens produced by the placenta or fetus.     

Inequality in function of the right and left ovaries and uterine horns of the mouse

Inequality in function of the left and right ovaries and uterine horns of mice was evaluated in three separate experiments. In Exp. 1, the effect of position in the reproductive tract on various reproductive characteristics was evaluated in 158 pregnant hybrid mice. Ovulation rate, number of fetuses, total fetal weight and total placental weight were higher (P less than 0.05) on the right than the left on Day 18 of pregnancy (vaginal plug = Day 1). In Exp. 2, the effect of previous sham or unilateral ovariectomy (right or left) in mated Swiss-Webster mice was compared with unoperated mated controls (N = 17-24/treatment). In control mice, ovulation rate, total fetal weight and ovarian weight were higher (P less than 0.05) on the right than left side. Surgery (sham or unilateral, ovariectomy) decreased (P less than 0.005) ovulation rates, number of fetuses, ovarian weights, total fetal weight and total placental weight on Day 18 of pregnancy. Unilateral ovariectomy decreased (P less than 0.05) ovulation rates and ovarian weights more than did sham operation. Ovulation rates were higher (P less than 0.01) when the left ovary was manipulated or removed rather than the right ovary. For Exp. 3, pairs of 8 hybrid mouse embryos each (morulae and blastocysts) were surgically transferred to the left and right uterine horns of the same (bilateral, N = 15) or different (unilateral, N = 28) Swiss-Webster recipients. In almost all incidences, embryo survival (to Day 18 of pregnancy) was twice as high (P less than 0.05) in right than left uterine horns. We conclude that the left and right ovaries and uterine horns are not equal in function in Swiss-Webster and a hybrid strain of mice.     

Effect of chlortetracycline supplementation during prebreeding and early gestation on age at puberty, ovulation rate, embryo survival and fetal development in gilts

A total of 166 crossbred gilts weighing approximately 87 +/- 1 kg was limit-fed (2.5 kg/d) a corn-soybean meal gestation diet containing either 0 or 220 ppm of chlortetracycline (CTC) from 157 +/- 1 d of age until 15 d after breeding. These gilts were slaughtered at 31 +/- 1 or 71 d +/- 1 d of gestation for evaluation of reproductive performance. Age (190 +/- 3 d vs 195 +/- 3 d) and body weights (106 +/- 2 kg vs 106 +/- 2 kg) at puberty were similar for control and CTC-fed gilts, respectively. Although not significant (P > 0.05), ovulation rate was higher in CTC-fed than in control gilts as assessed at both 31 d (14.2 +/- 0.7 vs 12.9 +/- 0.9, P = 0.31) and 71 d (13.9 +/- 0.6 vs 12.4 +/- 0.5, P = 0.10) of gestation. There was an increase (P = 0.04) in the number of live embryos for CTC-fed gilts at 31 d (12.1 +/- 0.7 vs 9.7 +/- 0.7) but not at 71 d (10.0 +/- 1.1 vs 9.6 +/- 1.0) of gestation. The mean uterine length, placental length, placental weight, fetal length, fetal weight, and allantoic fluid volumes were similar between the control and CTC-fed gilts. Results indicated that feeding CTC during prebreeding and early gestation did not influence the proportion or age of gilts at puberty. However, CTC feeding may have influenced a trend to-ward an increased ovulation rate and increased number of live embryos in gilts.     

The effect of prolonged hypobaric hypoxia on growth of fetal sheep

The effect of prolonged hypobaric hypoxia on fetal sheep was studied. Pregnant ewes were subjected to an atmospheric pressure of 429 torr from 30 days to 135 days gestation (long-term study). Average fetal weight for the hypoxaemic group (3.35 +/- 0.53 kg; n = 4; mean +/- SD) was significantly lower than for the controls (4.23 +/- 0.29 kg; n = 7; P less than 0.05). A short-term study was undertaken with fetuses (n = 8) which were catheterized at 110 days gestation and whose dams were subjected to hypobaric hypoxia from 120 to 141 days gestation. The mean carotid PO2 of fetuses in the hypoxic group was 12.7 +/- 0.7 torr compared to 22.7 +/- 0.7 torr for the control group (n = 9; P less than 0.001) throughout the period of treatment. Fetal arterial oxygen content fell from 6.5 +/- 1.7 to 4.9 +/- 0.4 ml/dl (P less than 0.05), but rose to control values after 7 days due to an increase in fetal haemoglobin concentration (9.6 +/- 1.1 to 13.0 +/- 1.9 g/dl, P less than 0.001) and packed cell volume (33 +/- 3 to 45 +/- 4%, P less than 0.001). In the hypoxaemic fetuses, pH fell initially from 7.34 +/- 0.02 to 7.28 +/- 0.03 (P less than 0.05) and then recovered to 7.32 +/- 0.03 within 24 h. Mean fetal weight of the short-term hypoxic group was 3.46 +/- 0.72 kg compared to 4.15 +/- 0.51 for the control group (P less than 0.05). Both long- and short-term hypoxia produced a similar reduction in fetal body weight. The adrenal glands were significantly heavier in the hypoxic fetuses than in controls. Placental weight was not effected by hypoxia, but exposure from 30 days gestation reduced the average size of cotyledons (P less than 0.05). It is concluded that the fetal sheep increases its ability to acquire and transport oxygen in response to chronic hypoxia, but this compensation is not sufficient to prevent growth retardation or changes to the pattern of tissue growth.     

The influence of parity and maternal preimmunization on fetal survival in mice

It is known that as parity increases, cell-mediated immunity to paternal antigens increases in mice. The article reports an experiment designed to compare the results of normal parity with artificial immunization simulating maternal sensitization to paternal antigens in mice. Virgin females were immunized at weekly intervals by intraperitoneal injections of 50 mcl of heparinized blood from males with whom they would eventually be mated. The immunized virgins and multiparous females were mated for 4 to 6 weeks after their last injection or litter. The total and live litter size increased with immunization; the total size with natural parity also increased though the live litter size slightly decreased with parity. The mean fetal death rate was relatively the same for both groups. Fetal weight increased with parity but not change was observed in placental weight. Both fetal and placental weight did not change with immunization. There was no increase in the expected number of male fetuses. The increased litter size in the immunized group may be due to increased ovulation rates.     

Differential expression of the vascular endothelial growth factor-receptor system in the gravid uterus of yorkshire and Meishan pigs

Litter size in the pig is limited by uterine capacity, which is dependent on uterine size, placental size, and vascularity. Placentae of U.S. pig breeds, such as the Yorkshire, exhibit marked growth from mid to late gestation, increasing their surface area of endometrial attachment. In contrast, placentae of the prolific Chinese Meishan pig exhibit little growth from mid to late gestation; instead, they exhibit a marked and progressive increase in the density of placental blood vessels. Vascular endothelial growth factor (VEGF) is a potent angiogenic and permeability-enhancing factor that is produced and secreted by placentae of several species, including the pig. The activity of VEGF is mediated through two specific receptors (VEGF-R1 and VEGF-R2), both of which are expressed by placental and endometrial tissues in pigs and are thought to play a role in mediating increased vascularization and/or permeability at the fetal-maternal interface. The objectives of the present study were to determine concentrations of VEGF in fetal blood and placental fluids as well as placental and adjacent endometrial mRNA expression of VEGF, VEGF-R1, and VEGF-R2 on Days 30, 50, 70, 90, and 110 of gestation in Yorkshire and Meishan pigs. Day 90 Meishan conceptuses exhibited marked increases (P < 0.05) in placental VEGF mRNA expression as well as fetal blood and allantoic fluid concentrations of VEGF, which remained elevated through Day 110. In contrast, Yorkshire conceptuses failed to exhibit increases in placental VEGF mRNA expression or concentrations of VEGF in fetal blood or allantoic fluid until Day 110. Receptor mRNA expression patterns differed between Meishan and Yorkshire conceptuses, but no difference was found in their expression levels. Placental efficiency (fetal weight/placental weight) was higher (P < 0.05) on Days 90 and 110 in Meishan than in Yorkshire conceptuses. The earlier increase in VEGF protein and mRNA expression in the Meishan versus the Yorkshire conceptus may explain the previously reported increased vascularity and increased placental efficiency of this breed compared the Yorkshire breed.     

Myogenesis in sheep is altered by maternal feed intake during the peri-conception period

The effect of varying short-term maternal feed intake during the peri-conception period on the development of ovine fetal muscle at mid-gestation was investigated. Superovulated donor Merino ewes (n = 24) were fed a roughage/grain pelleted diet (10.1 MJME/kg dry matter) at either 1.5x maintenance (H; high) or 0.5x maintenance (L; low) from 18 days before until 6 days after ovulation. Embryos were transferred to recipient ewes (n = 60) on day 6. Singleton fetuses were collected on day 75 of gestation and placental weights, fetal body dimensions and fetal organ and muscle weights recorded. The number, type and size of muscle fibres and the dry matter, RNA, DNA and protein content in the semitendinosus muscle were determined. Maternal feed intake did not influence body dimensions, organ development or muscle weights in the fetus. However, L feed intake decreased total muscle fibre number in the fetus by approximately 20% (P = 0.06) compared to H feed intake. This resulted from a reduced secondary to primary fibre ratio (P < 0.05) and indicated that secondary fibre formation occurred at a reduced rate in L fetuses. In addition, protein:DNA ratio tended to be lower in muscles of L fetuses (P < 0.1). It is concluded that restricting feed intake over the peri-conception period reduces or delays myogenesis in fetal sheep. The potential mechanisms by which nutritional availability during this period may influence subsequent myogenic development are discussed.     

Long-term hypoxia increases leptin receptors and plasma leptin concentrations in the late-gestation ovine fetus

This study was designed to test the hypothesis that long-term hypoxia (LTH) increases fetal plasma leptin and fetal adipose or placental leptin expression and alters hypothalamic and adrenocortical leptin receptor (OB-R) expression. Pregnant ewes were maintained at high altitude (3,820 m) from day 30 to approximately 130 days of gestation. Reduced Po2 was maintained in the laboratory by nitrogen infusion through a maternal tracheal catheter. On day 132, normoxic control and LTH fetuses underwent surgical implantation of vascular catheters (n=6 for each group). Five days after surgery, maternal and fetal arterial blood samples were collected for leptin, insulin, and glucose analysis. Placental tissue, periadrenal fat, and fetal hypothalami and adrenal glands were collected from additional control (n=7) and LTH (n=8) fetuses for analysis of leptin mRNA by quantitative, real-time, RT-PCR (qRT-PCR). There was a significant (P<0.03) elevation in fetal plasma leptin in the LTH fetuses (3.5+/-0.7 ng/ml) vs. control (1.1+/-0.1 ng/ml). There were no differences in either glucose or insulin concentrations between the two groups. Periadrenal adipose leptin mRNA was significantly higher in the LTH group compared with control, as was placental leptin expression. The levels of leptin mRNA in adipose were approximately 70 times higher vs. placenta. LTH significantly reduced expression of OB-Ra (short-isoform) in the hypothalamus (P=0.0156), while resulting in a significant increase in adrenal OB-Rb (long-form) expression (P<0.03). Our data suggest that leptin is a hypoxia-inducible gene in the ovine fetus and OB-R expression is altered by LTH. These changes may be responsible in part, for our previously observed alterations in fetal hypothalamic-pituitary-adrenal function following LTH.     

Continuous measurement of biparietal distance in the intact and hypophysectomized fetal sheep using ultrasound

Piezoelectric transducers were implanted into the parietal bones of intact (n = 4) and hypophysectomized (n = 8) fetal sheep of approximately 110-120 days gestational age (term 145-150 days). Intertransducer distance was determined by measuring the time taken for an ultrasonic pulse, generated by one transducer, to elicit a piezoelectric response in an opposing transducer. The limit of sensitivity of the timer was +/- 0.033 microsec. The ultrasonic velocity through fetal sheep brain tissue was 1549.6 +/- 2.2 m.s-1 (SEM; n = 33). This velocity remained constant throughout the entire period studied in both intact and hypophysectomized fetuses. At this velocity, the sensitivity of the measuring device was +/- 0.05mm. The ultrasonic transit time was measured daily between 0900 and 1100h until term in all fetuses. Three hypophysectomized fetuses were allowed to remain in utero until day 163 of gestation. The mean biparietal distance growth rate prior to day 135 for the intact and hypophysectomized fetuses was 0.25 +/- 0.03 and 0.27 +/- 0.025 mm/day respectively. These values were not significantly different (P greater than 0.05). A significant decrease (P less than 0.05) in growth rate was detected in both experimental groups between days 135 and 147 and was more pronounced in the sham (0.05 +/- 0.04 mm/day) than in the hypophysectomized (0.14 +/- 0.03 mm/day) group. However, the growth rate of the sham animals after day 135 was not significantly different from that of the hypophysectomized animals. In the three hypophysectomized fetuses killed at day 163 the biparietal distance growth was maintained at 0.12 +/- 0.005 mm/day. We conclude that fetal biparietal distance growth is pituitary independent from day 110 of gestation and that this technique for measuring distance is a valid and extremely accurate method for the continuous measurement of this parameter of fetal growth and may have further applications in other areas of growth research.     

Cellular accumulation of heat shock protein (Hsp) 72i in fetuses of trained rats

Forty-five Sprague-Dawley rats (60-80 days old) were randomly placed into one of three groups: sedentary pregnant control (PC); prepregnancy trained animals that exercised throughout pregnancy (PR); and nonpregnant trained animals (NPR). Each exercising animal ran at approximately 60-70% aerobic capacity (VO2max) for 1 hour/day up to and including day 18 of gestation (term = 21 days). On day 20 of gestation, fetuses were excised from each pregnant animal and scrutinized for gross abnormalities. In 3 randomly chosen fetuses from each litter, brain, heart, kidney, hind limb, and placental tissues were removed to assess the accumulation of the inducible isoform of the 70-kilodalton heat shock protein (Hsp 72i). No significant differences were detected between fetal hearts, hind limbs, or placental tissues of PC or PR groups. No Hsp 72i signal could be detected in fetal kidney or brain tissues from either pregnant group. Results indicate that maternal core temperature did not reach the threshold that would induce either gross fetal abnormalities or a fetal heat shock protein response. However, fetal and placental growth was reduced by the exercise protocol.     

Central nervous system lesions in the Wistar rat fetus following direct fetal injections of cadmium

During embryogenesis, maternal administration of cadmium (Cd) produces teratogenic effects, including hydrocephalus (HC), whereas later in gestation (during the fetal period), such effects have not been reported. Since there is little placental transfer of Cd late in gestation, such differences in response could be due to a lower Cd concentration in the fetus compared with the embryo after maternal Cd exposure, or could be due to a decreased sensitivity of the fetal central nervous system (CNS) to Cd. To test the susceptibility of the late gestational CNS to Cd, day 19 (sperm plug = day 0) rat fetuses were directly injected i.p. with CdCl2 (165, 100, 50 nmoles/fetus in 5 microliters saline). All fetuses in one horn were treated with Cd, while fetuses in the other horn were treated with saline. Fetuses were collected on day 21, grossly examined, weighed, fixed in Bouin's fixative, and later razor sectioned. Cd did not affect fetal viability or body weight. However, Cd caused a dose-dependent increase in hydrocephalus, with the total number of fetuses showing moderate to severe HC being 0/45, 0/11, 6/10, and 18/20 for controls, low, medium, and high doses, respectively. Mild HC was noted in one control and two low Cd fetuses. Brain necrosis was correlated with hydrocephalus, being observed in 0/45, 0/11, 5/10, and 16/20 fetuses, respectively. In medium-dose fetuses without HC or brain necrosis, extravasation of erythrocytes was noted histologically within the cortical parenchyma, suggesting that hemorrhaging may lead to brain necrosis and hydrocephalus in Cd-exposed fetuses. Thus, the fetal CNS is susceptible to the toxic effects of Cd.     

Antenatal administration of celecoxib,a selective cyclooxygenase (COX)-2 inhibitor,appears to improve placental perfusion in the pregnant rabbit

To investigate the effects of celecoxib on fetal growth, and placental prostanoid and nitric oxide (NO) production in fetal rabbits, pregnant rabbits received celecoxib (30 mg/kg per day) from 13 to 20 days (Cel-A), from 13 to 28 days (Cel-B), or vehicle from 13 to 28 days gestation. Fetal body and organ weights, and measurements of linear growth were recorded. The placentas were weighed and analyzed for prostaglandins (PGs), NO oxidation products (NOx), and total cellular protein levels. Placental prostaglandin E2 (PGE2) and NOx levels increased (P < or = 0.05), while thromboxane B2 levels were suppressed (P < or = 0.01) in Cel-B group. Tail length and brain weight were greater, while lung weights were lower in the Cel-B group (P < or = 0.05). Maternal administration of celecoxib appears to preferentially increase placental vasodilators and decrease placental TxA2, suggesting that the drug may increase uteroplacental perfusion without adverse fetal outcome.     

Fetal growth and placental diffusing capacity in guinea pigs following long-term maternal exercise

To determine the critical level of maternal exercise which produces effects on fetal weight and placental diffusing capacity, we measured the relationship between increasing levels of exercise and its effect on the fetus. Hartley guinea pigs with dated pregnancies were exercised on a treadmill (9.7 m/min at a 6.5% gradient) at one of five exercise levels (0, 15, 30, 45, and 60 min/day). We measured placental diffusing capacity for carbon monoxide (DPCO) fetal body and organ weights, placental weight, and maternal body and heart weights near term (63-64 days). Fetal body weight, kidney weight, and placental weight decreased as a function of increasing exercise level, decreasing 13, 13, and 21% respectively at 60 min/day exercise. DPCO1 decreased from a control value of 2.92 +/- 0.23 to 2.33 +/- 0.10 ml. min-1 torr-1 kg fetal wt in the 15 min/day exercise group, 2.17 +/- 0.08 in the 30 min/day group 2.16 +/- 0.11 in the 45 min/day group, and 2.65 +/- 0.31 in the 60 min/day exercise group. The decrease in placental weight along with the decrease in DPCO per kg of fetal weight suggests that with progressive maternal exercise the fetus is compromised by a smaller than normal placenta with less diffusing capacity.     

Pharmacological studies on the potentiation of phenytoin teratogenicity by acetaminophen

An in vivo murine model was developed to measure maternal phenytoin biotransformation along with the covalent binding of phenytoin to fetal tissues in the same fetuses which were assessed for fetal anomalies. Acetaminophen was administered to pregnant CD-1 mice 1 hour prior to phenytoin, both given i.p. at varying doses and gestational times between days 11 and 13. Dams were killed between days 12 and 19. Metabolites reflecting the enzymatic bioactivation of phenytoin were quantified in maternal plasma and urine with high-performance liquid chromatography (HPLC). Acetaminophen pretreatment caused a threefold increase in phenytoin-induced fetal cleft palates without increasing resorptions. The covalent binding of radiolabeled phenytoin to fetal and placental tissues measured on day 13 was increased twofold and threefold, respectively, by acetaminophen pretreatment. Phenytoin covalent binding measured on day 16 was significantly increased in the livers of fetuses with cleft palates, but not in the livers of dams with fetuses having cleft palates. Binding to fetal brain on day 16 was over fourfold higher than that in maternal brain. Acetaminophen pretreatment differentiated dams into poor and extensive metabolisers of phenytoin, with only the latter group carrying fetuses with cleft palates. The incidence of fetal cleft palates correlated positively with maternal urinary levels of phenytoin (r = +.81, P less than .01) and its dihydrodiol metabolite (r = +.61, 0.05 less than P less than .1), and negatively with levels of para-hydroxylated phenytoin (r = -.85, P less than .01). These findings related both to the mechanism of phenytoin teratogenicity and its potentiation by acetaminophen.     

Erythropoietin responses to progressive blood loss over 10 days in the ovine fetus

Long-term loss of fetal blood can occur with fetomaternal hemorrhage, vasoprevia, or placental previa. Our objective was to determine the effects of progressive fetal blood loss over 10 days on fetal plasma erythropoietin (EPO) concentration and its relationship to arterial PO(2), hematocrit, and the volume of blood loss. Late-gestation fetal sheep (n = 8) were hemorrhaged daily at a rate of 1 ml/min over 10 days. The extent of hemorrhage differed in each fetus and ranged from 30 to 80 ml/day, with the cumulative volume removed ranging from 78 to 236 ml/kg estimated fetal weight. Four fetuses served as time controls. EPO concentration measurements were by radioimmunoassay. Statistical analyses included regression, correlation, and analysis of variance. We found that EPO and arterial PO(2) were unchanged until the cumulative hemorrhage volume exceeded 20-40 ml/kg. Once this threshold was exceeded, plasma EPO concentration increased progressively throughout the study and averaged 14.3 +/- 3.2 times basal values on day 10. EPO concentration, arterial PO(2), and hematocrit changes were related curvilinearly to cumulative hemorrhage volume (P < 0.01), whereas the relationship between plasma EPO and arterial PO(2) was log linear (P < 0.001). We conclude that 1) fetal plasma EPO concentration and arterial PO(2) are insensitive to a slow, mild-to-moderate blood loss over several days; 2) unlike the rapid return of EPO to normal within 48 h after acute hemorrhage, fetal EPO concentration undergoes a progressive increase with moderate-to-severe blood loss over several days; 3) the long-term hemorrhage-induced changes in EPO are best correlated with arterial PO(2); and 4) the fetal EPO response to hemorrhage does not appear to be limited by the fetus's ability to produce EPO.     

Relation between mating or ovulation and the duration of gestation in dogs

The effects of day of mating and litter size on gestation length in dogs were studied in 36 beagle bitches (age 2-10 yr). The day that plasma progesterone concentrations exceeded 2 ng/mL was considered the day of ovulation; dogs were randomly assigned to be bred once, 1-5 days after the estimated day of ovulation. The interval from mating to parturition was negatively correlated with the number of days from estimated ovulation to mating (P < 0.01). Gestation length (interval from ovulation to parturition) was almost constant at 63.9 +/- 0.2 days (mean +/- S.E.M.), with no significant relationship between the number of fetuses and the duration of gestation.     

Effect of hypoxia on the initiation of secondary wool follicles in the fetus

The development of secondary wool follicles in single fetal sheep subjected to hypobaric hypoxaemia was studied. One group of pregnant ewes were exposed to 57.1 kPa from 30 to 135 days gestation. Fetal weights (mean +/- s.d.) for the hypoxaemic group (3.35 +/- 0.53 kg; n = 4) were significantly lower than for the controls (4.19 +/- 0.31 kg; n = 3, P less than 0.05). At 110 days gestation, a second group had arterial and venous catheters surgically implanted into the ewe and fetus and skin samples were taken from the fetus. At 120 days gestation (10 days after surgery) these animals were subjected to hypoxia for 20 days, at a level to maintain fetal carotid pO2 between 1.47 and 1.87 kPa (mean carotid pO2 for the control fetuses was 2.84 +/- 0.28 kPa). Fetal weight at 140 days was not significantly different in the hypoxaemic and control groups. Morphometric analysis revealed that the secondary to primary follicle ratio (S:P) was less in both groups of hypoxaemic fetuses than in their respective controls. Although hypoxia for 20 days did not significantly alter fetal weight, it produced a low S:P ratio similar to the longer-term hypoxaemic animals. It is concluded that hypoxia has a marked effect in reducing the initiation of secondary follicles in the last third of gestation.     

Effect of empty uterine space on birth intervals and fetal and placental development in pigs

A substantial loss of embryos occurs between Days 30 and 40 of pregnancy in the pig under crowded intrauterine conditions, but it is not clear whether this loss affects the growth of adjacent conceptuses. Birth intervals are known to increase with decreasing litter size, but the factors responsible are unknown. Two possibilities are that increased birth weight associated with reduced litter size and the empty uterine space and resulting constricted uterine regions that occur in pigs with small litters may impair piglet delivery. To address these, pregnant gilts were laparotomized on Day 35 of pregnancy and one or two fetuses were manually crushed through the uterine wall on the ovarian or cervical end of each uterine horn to create an empty uterine space behind or in front of the litter of piglets, respectively, in relation to the route of delivery from the uterus. A subset of gilts was slaughtered at 105 days of gestation to confirm that the empty uterine spaces were successfully created and to determine their effects on placental and fetal weights of adjacent conceptuses. At slaughter, the lengths of all externally visible empty constricted regions of the uterus were measured. The uterine horns were opened and the lengths of each placenta were measured from the umbilicus toward the ovary and toward the cervix to assess whether placentas developed symmetrically, and then each fetus and placenta was weighed. Fetal crushing successfully created constricted empty uterine regions on the ovarian and cervical ends of the uterine horns. Ovarian-side placental lengths were greater than cervical-side for conceptuses adjacent to fetuses crushed on the ovarian end of the horn. Cervical-side placental lengths were greater than ovarian-side for conceptuses adjacent to fetuses crushed on the cervical end. Both placental and fetal weights were greater (10% and 6%, respectively, P<0.05) for conceptuses adjacent to crushed fetuses compared to nonadjacent conceptuses. Remaining gilts were farrowed to determine the effect of litter size, average birth weights, and treatment on birth intervals of piglets, which were monitored using 24-h video surveillance. The negative association between number of piglets born alive and average birth interval was confirmed and was not explained by litter size-induced reduction in litter average birth weights. Birth intervals and stillbirth rate did not differ between cervically- and ovarian-treated gilts. These results indicate that conceptus loss on Day 35 of gestation can benefit the growth of adjacent placentas and fetuses, but the benefit is small. Increased average birth weight and the presence of empty uterine space that occurs when litter size is reduced does not fully explain the effect of litter size on birth intervals.     

Analysis of metopirone-induced hypertrophy of adrenals in fetal rats encephalectomized in utero

Pregnant rats were treated with 30 mg metopirone (M) each day for 2 days and autopsied on the third day in various gestational periods (Days 18-20, 19-21, and 20-22). Control rats were treated with saline alone (S). The adrenals of intact fetuses in M-treated dams were significantly heavier than those of intact fetuses in S-treated dams in every experimental period. In both M- and S-treated dams, the adrenals of encephalectomized (E) fetuses were lighter than those of intact littermates. However, in the experimental period of Days 18-20 and 19-21, the adrenals of E fetuses in M-treated dams were slightly but significantly heavier than those of similar E fetuses in S-treated dams. In contrast, in the experimental period Days 20-22, there was no significant difference in the weight of adrenals of E fetuses of M- and S-treated dams. These changes in fetal adrenal weight were reflected histologically in parallel changes in the size of adrenocortical cells. The observations suggest that the fetal adrenal hypertrophy following maternal treatment with metopirone can occur to some extent independent of the fetal brain, but that close to the end of gestation the hypertrophy occurs completely under the control of the fetal brain.