Maternal high density lipoproteins affect fetal mass and extra-embryonic fetal tissue sterol metabolism in the mouse

Previous studies have shown that antibodies to cubulin, a receptor on the yolk sac that binds high density lipoproteins (HDL) and cobalamin, induce fetal abnormalities. Mice with markedly low concentrations of plasma HDL-cholesterol (HDL-C) give birth to healthy pups, however. To establish whether maternal HDL-C has a role in fetal development, sterol metabolism was studied in the fetus and extra-embryonic fetal tissues in wild-type and apolipoprotein A-I-deficient mice (apoAI-/-). Maternal HDL-C content was markedly greater in apoAI+/+ mice prior to pregnancy and at 13 days into gestation. By 17 days into gestation, HDL-C content was similar between both types of mice. Fetuses from apoAI (-/- x -/-) matings were 16;-25% smaller than control mice at 13 and 17 days of gestation and contained less cholesterol. The differences in size and cholesterol content were not due to a lack of cholesterol synthesis or apoA-I in the fetus. In the yolk sac and placenta, sterol synthesis rates were approximately 50% greater in the 13-day-old apoAI-/- mice as compared to the apoAI+/+ mice. Even though synthesis rates were greater, cholesterol concentrations were 22% lower in the yolk sac and similar in the placenta of apoAI-/- mice as compared to tissues of wild-type mice. These data suggest that a difference in maternal HDL-C concentration or composition can affect the size of the fetus and sterol metabolism of the yolk sac and placenta in the mouse.     

Effect of maternal hypercholesterolemia on fetal sterol metabolism in the Golden Syrian hamster.

The fetus obtains a significant amount of cholesterol from de novo synthesis. Studies have suggested that maternal cholesterol may also contribute to the cholesterol accrued in the fetus. Thus, the present studies were completed to determine whether diet-induced maternal hypercholesterolemia would affect fetal sterol metabolism. To accomplish this, maternal plasma cholesterol concentrations were increased sequentially by feeding hamsters 0.0%, 0.12%, 0.5%, and 2.0% cholesterol. At 11 days into a gestational period of 15.5 days, cholesterol concentrations and sterol synthesis rates were measured in the three fetal tissues: the placenta, yolk sac, and fetus. In the placenta and yolk sac, the cholesterol concentration increased significantly when dams were fed as little as 0.12% cholesterol (P < 0.0167), and sterol synthesis rates decreased in dams fed at least 0.5% or 2% cholesterol, respectively (P < 0.0167). In the fetus, changes in fetal cholesterol concentration and sterol synthesis rates occurred only when dams were fed at least 0.5% cholesterol, which corresponded to a greater than 2-fold increase in maternal plasma cholesterol concentrations. When the cholesterol concentration in the fetal tissues in each animal was plotted as a function of maternal plasma cholesterol concentration, a linear relationship was found (P < 0.001).These studies demonstrate that sterol homeostasis in fetal tissues, including the fetus, is affected by maternal plasma cholesterol concentration in a gradient fashion and that sterol metabolism in the fetus is dependent on sterol homeostasis in the yolk sac and/or placenta.     

Yolk sac cholesteryl ester secretion rates can be manipulated in the Golden Syrian hamster: effect of yolk sac cholesterol concentrations

The yolk sac is one of two extra-embryonic fetal tissues that separates the fetal and maternal circulations. The yolk sac can secrete lipoprotein particles to the vitelline vessels, which supply yolk sac-derived nutrients to the embryo. The amount and composition of lipoproteins secreted from the rat yolk sac can be manipulated by fatty acid content and gestational age. The goals of the current studies were to determine, first, if tissue cholesterol concentration could mediate cholesterol secretion rate from the yolk sac and, second, if some of the secreted cholesterol could be derived from the maternal circulation. Golden Syrian hamsters were fed 2% added cholesterol to increase the yolk sac cholesterol concentration. Yolk sac explants secreted similar amounts of triglyceride and apolipoproteins B and E into the media regardless of yolk sac cholesterol concentration. In contrast, yolk sacs with greater cholesterol concentrations secreted 2.3-fold more cholesterol into the media as compared to control yolk sacs; the increase was found mostly as cholesteryl ester. At least part of the secreted cholesterol was maternally derived. These data demonstrate that yolk sac cholesterol concentration influences cholesterol secretion rates, and that at least some of the cholesterol secreted originates from the maternal circulation.     

Transport of maternal cholesterol to the fetus is affected by maternal plasma cholesterol concentrations in the Golden Syrian hamster

The fetus has a high requirement for cholesterol and synthesizes cholesterol at elevated rates. Recent studies suggest that fetal cholesterol also can be obtained from exogenous sources. The purpose of the current study was to examine the transport of maternal cholesterol to the fetus and determine the mechanism responsible for any cholesterol-driven changes in transport. Studies were completed in pregnant hamsters with normal and elevated plasma cholesterol concentrations. Cholesterol feeding resulted in a 3.1-fold increase in the amount of LDL-cholesterol taken up by the fetus and a 2.4-fold increase in the amount of HDL-cholesterol taken up. LDL-cholesterol was transported to the fetus primarily by the placenta, and HDL-cholesterol was transported by the yolk sac and placenta. Several proteins associated with sterol transport and efflux, including those induced by activated liver X receptor, were expressed in hamster and human placentas: NPC1, NPC1L1, ABCA2, SCP-x, and ABCG1, but not ABCG8. NPC1L1 was the only protein increased in hypercholesterolemic placentas. Thus, increasing maternal lipoprotein-cholesterol concentrations can enhance transport of maternal cholesterol to the fetus, leading to 1) increased movement of cholesterol down a concentration gradient in the placenta, 2) increased lipoprotein secretion from the yolk sac (shown previously), and possibly 3) increased placental NPC1L1 expression.     

Tissue-specific and developmental expression of human transthyretin gene in transgenic mice

To analyze the regulation of transthyretin gene expression we have produced transgenic mice by microinjecting cloned human transthyretin genes into fertilized eggs of C57BL/6 mice. The 7.6-kilobase (kb) human transthyretin gene containing about 500 base pairs (bp) in the upstream region was used for microinjection. Seven out of nine transgenic mice had detectable amounts of human transthyretin in serum when analyzed by enzyme-linked immunosorbent assay. Transthyretin mRNA was detected in liver and yolk sac but not in other tissues including brain. The amount of mRNA was variable among transgenic mice and was about one-tenth of mouse endogenous transthyretin mRNA. Human and mouse transthyretin mRNAs were detected in liver of fetus and yolk sac at 13 days of gestation and unlike yolk sac the level of mRNA in liver increased gradually during development and reached the maximum at around 17 days of gestation. Human transthyretin was associated with mouse transthyretin to form tetramers as judged from the dilution curve of enzyme-linked immunosorbent assay and the spur formation in Ouchterlony assay.     

Vitamin D-dependent calcium-binding protein of mouse yolk sac. Biochemical and immunochemical properties and responses to 1,25-dihydroxycholecalciferol

The present studies were performed to further characterize a mouse yolk sac protein which is similar or identical to the vitamin D-dependent intestinal calcium-binding protein (CaBP). Yolk sac protein and purified rat intestinal CaBP displayed full identity upon immunodiffusion (Ouchterlony) using antiserum to the rat intestinal CaBP. Immunoreactive CaBP in yolk sac homogenates eluted from gel permeation columns with the low molecular weight peak of 45Ca2+ binding (Chelex assay), and the electrophoretic mobility of the protein was markedly increased by EDTA. On days 11-13 of gestation, the concentrations of immunoreactive CaBP in yolk sac were 4-5-fold higher than in placenta; by days 16-17, the concentrations in yolk sac and placenta were similar. Incubation of yolk sac with [3H]leucine demonstrated synthesis of immunoprecipitable [3H]CaBP. A single band of 3H-labeled protein was seen on sodium dodecyl sulfate gel electrophoresis of the immunoprecipitate. This protein co-migrated with radioactive placental CaBP with an apparent Mr of 10,050. Addition of 1,25-dihydroxycholecalciferol (calcitriol) to organ culture media with or without serum increased the amount and concentration of CaBP in yolk sac (p less than 0.001) at 48 h. CaBP synthesis in yolk sac appeared to be independent of calcitriol concentrations in the maternal circulation since injection of the hormone into the maternal compartment produced no change in yolk sac CaBP despite increases of maternal intestinal and renal CaBP. These studies demonstrate that yolk sac immunoreactive CaBP is synthesized in yolk sac and has an apparent molecular size and calcium-binding properties characteristic of mammalian vitamin D-dependent calcium-binding proteins. The in vitro response of yolk sac CaBP to calcitriol is the first evidence of a vitamin D effect on the fetal membranes and suggests one function for calcitriol receptors in these tissues.     

The rat visceral yolk sac internalizes maternal transferrin and secretes hydrolyzed products towards the fetus

The uptake of transferrin by the rat visceral yolk sac membranes, and the fate of this protein, were measured in a two-chambered system which allowed access to both surfaces of these membranes, i.e. that facing the maternal compartment and that facing the fetal compartment. 125I-labeled transferrin was internalized by the maternal surface of the visceral yolk sac but not by the fetal surface. Following internalization, this transferrin was degraded and the amino acids were secreted exclusively towards the fetal compartment. Transcytosis of intact transferrin was not detected in either direction. These results suggest that transport across the rat visceral yolk sac bound to maternally derived transferrin is not a major mechanism of iron transport in vivo. These results support a role for the visceral yolk sac in fetal metabolism, or supplying the fetus with amino acids derived from degradation of specific maternal plasma proteins, in this case, transferrin.     

Ontogeny of vitamin D action on the morphology and calcium transport properties of the chick embryonic yolk sac

The ontogeny of the calcium transport properties and hormonal modulation of the yolk sac membrane in amniote embryos is presently poorly understood. We investigated the role of 1,25-dihydroxyvitamin D3 (1,25-(OH)2D3) on plasma calcium values, yolk sac morphology and the ability of the yolk sac membrane to transport 45Ca from yolk to embryo. 1,25-(OH)2D3 treatment caused significant hypercalcaemia in 9-, 12- and 15-day embryos. Additionally, this hormone caused a hypertrophy of the endodermal cell layer that comprises the bulk of the yolk sac membrane. Both of these effects were the most dramatic in the 15-day embryo, the oldest age tested. 45Ca added to the yolk was transported into the blood rapidly across the yolk sac membrane. 1,25-(OH)2D3 significantly enhanced this transport in all age groups. [14C]Inulin was also taken across the yolk sac membrane, but at a slower rate than 45Ca; this transport was unaffected by 1,25-(OH)2D3. Thus, the yolk sac responds to 1,25-(OH)2D3 treatment both morphologically and functionally. The mechanism for transport appears to be a specific one, rather than a simple enhancement of non-specific endocytosis.     

The Role of the Yolk Sac in Copper Metabolism during Rat Embryogenesis

Using the immunoblotting method, the synthesis of two copper-transporting P1-type ATPases, ATP7A (a candidate for the product of the Menkes disease gene) and ATP7B (a presumed product of the Wilson disease gene), in the yolk sac cells of rat embryos at days 11 and 20 of embryogenesis was demonstrated. Concomitantly, yolk sac cells produce ceruloplasmin, a soluble copper-transporting glycoprotein, a proportion of which in secreted proteins progressively diminishes, attaining 5.2% at day 11 and 3.1% at day 20 of development. At different stages of embryogenesis, yolk sac cells synthesize two molecular forms of [¹⁴C]-ceruloplasmin, one of which is secreted towards the embryo, whereas the other, towards the decidual membrane. Two forms of ceruloplasmin secreted in polar directions differ in the rate of secretion. The role of the yolk sac as a key organ controlling the delivery and secretion of copper in the embryo during the postimplantation period is discussed.     

Cultured chick yolk sac epithelium: structure and IgG surface binding

The chick yolk sac endoderm transports maternal immunoglobulin G (IgG) from the yolk into the embryo during development, providing the newly hatched chick with passive immunity until it becomes immunocompetent. To study this transport process, chick yolk sac endodermal cells isolated from embryos of 6 to 18 days of incubation were grown in vitro on a collagen substrate. The cultured cells possessed a remarkable structural similarity to the in vivo tissue and reformed a polarized confluent epithelium with tight junctions and desmosomes joining the cells at their apical margins. In addition, the cells exhibited apical microvilli, numerous phagolysosomes in the cytoplasm and retained the expression of the yolk sac endoderm-specific enzyme marker, cysteine lyase. Importantly, the cultured cells retained the ability to specifically bind IgG as demonstrated by indirect immunofluorescence. Chicken IgG bound to the cultured cells at 4 degrees C in a diffuse pattern that clustered into a punctate pattern when a second antibody was used. Cultures from yolk sacs of day 6 through day 18 of development all demonstrated this immunofluorescent labeling for at least 14 days in culture. These results demonstrate that cultured yolk sac endoderm maintains its differentiated morphology and ability to bind IgG.     

Metabolism of cholesterol in the tissues and blood of the chick embryo

Three artificially inseminated laying White Leghorn hens were given 35-50 micro c of cholesterol-4-(14)C intravenously. Their subsequently produced eggs contained cholesterol-(14)C-labeled yolks. Some of the fertilized eggs were analyzed for cholesterol content and radioactivity. Other eggs were incubated until hatching. The specific activity of the cholesterol contained in the serum and tissues of newly hatched chicks was determined and compared with that of yolk sac, which was taken as representative of egg yolk cholesterol before its metabolic transfer into the chick embryo. The specific activities of cholesterol in intestine, liver, serum, heart, and skeletal muscle and the whole chick were 95-98% of that in yolk sac, but that of brain cholesterol was only 11% of this value. These results indicate that whereas most of the cholesterol in the chick originated from the egg yolk, cholesterol biosynthesis was active in the brain and provided about 90% of its cholestero content. Newly hatched chicks were found to be hyperlipemic compared with older chicks and had fatty livers with a high cholesterol content. Desmosterol was found in 9- and 15-day old chick embryos but not in the newly hatched chicks, in which the only sterol was cholesterol.     

A comparison of the metabolism of eighteen-carbon 13C-unsaturated fatty acids in healthy women

Altered use of different dietary fatty acids may contribute to several chronic diseases, including obesity, noninsulin-dependent diabetes mellitus, and cardiovascular disease. However, few comparative data are available to support this link, so the goal of the present study was to compare the metabolism of [(13)C]oleate, [(13)C]alpha-linolenate, [(13)C]elaidate, and [(13)C]linoleate through oxidation and incorporation into plasma lipid fractions and adipose tissue. Each tracer was given as a single oral bolus to six healthy women. Samples were collected over 8 days, and (13)C was analyzed using isotope ratio mass spectrometry. At 9 h postdose, cumulative oxidation was similar for [(13)C]elaidate, [(13)C]oleate, and [(13)C]alpha-linolenate (19 +/- 1%, 20 +/- 4%, and 19 +/- 3% dose, respectively). Significantly lower oxidation of [(13)C]linoleate (12 +/- 4% dose; P < 0.05) was accompanied by its higher incorporation into plasma phospholipids and cholesteryl esters. Abdominal adipose tissue was enriched with [(13)C]alpha-linolenate, [(13)C]elaidate, or [(13)C]linoleate within 6 h. The percentage linoleate in plasma phospholipids correlated positively with [(13)C]linoleate and [(13)C]elaidate oxidation, indicating a potential role of background diet. Conversion of [(13)C]linoleate and [(13)C]alpha-linolenate to longer chain polyunsaturates was a quantitatively minor route of utilization.     

The role of the yolk sac in copper metabolism during rat embryogenesis

Using the immunoblotting method, the synthesis of two copper-transporting P1-type ATPases, ATP7A (a candidate for the product of the Menkes disease gene) and ATP7B (presumed product of the Wilson disease gene), in the yolk sac cells of rat embryos at days 11 and 20 of embryogenesis was demonstrated. Concomitantly, yolk sac cells produce ceruloplasmin, a soluble copper-transporting glycoprotein, a proportion of which in secreted proteins progressively diminishes, attaining 5.2% at day 11 and 3.1% at day 20 of development. At different stages of embryogenesis, yolk sac cells synthesize two molecular forms of [14]C-ceruloplasmin, one of which is secreted towards the embryo, whereas the other, towards the decidual membrane. Two forms of ceruloplasmin secreted in polar directions differ in the rate of secretion. The role of the yolk sac as a key organ controlling the delivery and secretion of copper in the embryo during the postimplantation period is discussed.     

Human placenta secretes apolipoprotein B-100-containing lipoproteins

Supply of lipids from the mother is essential for fetal growth and development. In mice, disruption of yolk sac cell secretion of apolipoprotein (apo) B-containing lipoproteins results in embryonic lethality. In humans, the yolk sac is vestigial. Nutritional functions are instead established very early during pregnancy in the placenta. To examine whether the human placenta produces lipoproteins, we examined apoB and microsomal triglyceride transfer protein (MTP) mRNA expression in placental biopsies. ApoB and MTP are mandatory for assembly and secretion of apoB-containing lipoproteins. Both genes were expressed in placenta and microsomal extracts from human placenta contained triglyceride transfer activity, indicating expression of bioactive MTP. To detect lipoprotein secretion, biopsies from term placentas were placed in medium with [(35)S]methionine and [(35)S]cysteine for 3-24 h. Upon sucrose gradient ultracentrifugation of the labeled medium, fractions were analyzed by apoB-immunoprecipitation. (35)S-labeled apoB-100 was recovered in d approximately 1.02-1.04 g/ml particles (i.e. similar to the density of plasma low density lipoproteins). Electron microscopy of negatively stained lipoproteins secreted from placental tissue showed spherical particles with a diameter of 47 +/- 10 nm. These results demonstrate that human placenta expresses both apoB and MTP and consequently synthesize and secrete apoB-100-containing lipoproteins. Placental lipoprotein formation constitutes a novel pathway of lipid transfer from the mother to the developing fetus.     

Biliary phospholipid secretion is not required for intestinal absorption and plasma status of linoleic acid in mice.

Biliary phospholipids have been hypothesized to be important for essential fatty acid homeostasis. We tested this hypothesis by investigating the intestinal absorption and the status of linoleic acid in mdr2 Pgp-deficient mice which secrete phospholipid-free bile. In mice homozygous (-/-) for disruption of the mdr2 gene and wild-type (+/+) mice, dietary linoleic acid absorption was determined by 72 h balance techniques. After enteral administration, [(13)C]-linoleic acid absorption was determined by measuring [(13)C]-linoleic acid concentrations in feces and in plasma. The status of linoleic acid was determined in plasma and in liver by calculating the molar percentage of linoleic acid and the triene:tetraene ratio. Although plasma concentration of [(13)C]-linoleic acid at 2 h after enteral administration was significantly lower in (-/-) compared to (+/+) mice (P相似文献   

The development of the hemopoietic system: colony-forming splenic units in mouse embryogenesis

Localization and time of appearance as well as dynamics of quantitative changes of splenic colony-forming units (CFU-S) in mouse (C57BL/6 X CBA)F1 embryos were studied. Cells taken from the whole embryo (day 8), yolk sac and embryo per se (day 9), and also liver (day 10) were injected into the lethally irradiated syngenic mice. 7-8 days after the injection the spleens were fixed and the number of macrocolonies was counted. Statistically significant number of CFU-S was detected starting from day 10 of development, first in the embryo (30-33 somites), then in yolk sac and blood (37-38 somites) and liver (after the 40 somites stage). Rapid increase of CFU-S number during days 11-12 (two-fold increase in about 4.6 hours) suggest that not only active proliferation of CFU-S but also maturation of CFU-S precursors take place.     

Sister-chromatid exchange analyses in rodent maternal, embryonic and extra-embryonic tissues: Transplacental and direct mutagen exposures

Sister-chromatid exchange (SCE) analyses were conducted in maternal, embryonic and extraembryonic tissues of pregnant rats and mice. The various tissues were substituted in vivo with 5-bromodeoxyuridine (BrdU) by implantation of a BrdU tablet in pregnant animals at mid-gestation. Following maternal exposure to 5–20 mg/kg cyclophosphamide, embryonic liver cells demonstrated dose-dependent SCE increases up to 10-fold that of control. Rat embryos revealed little intralitter variability for this transplacental effect. Maternal marrow and yolk sac cells examined in the rat also underwent significant increases in SCE, although to different extents. While marrow SCE frequencies were similar to those of embryo liver, yolk sac SCE frequencies were generally much lower.

SCE analyses were also conducted in rat yold sac cells substituted in vivo with BrdU and subsequently explanted to whole-embryo culture. In vitro exposure to cyclophosphamide at concentrations up to 100 μg/ml had no SCE-inducing effect. However, similar exposures to phosphoramide mustard, a presumed metabolite of cyclophosphamide, caused dose-dependent increases in SCE up to 8-fold higher than control at 2 μg/ml. Thus, cyclophosphamide appears to require maternal metabolic activation in order to cause an increased SCE frequency in yolk sac cells. The system described permits versatile SCE analyses which can help to define relative maternal and embryo tissue-specific sensitivities to chemical-induced genetic damage.     

In vivo investigation of the placental transfer of (13)C-labeled fatty acids in humans

Placental fatty acid transfer in humans in vivo was studied using stable isotopes. Four pregnant women undergoing cesarean section received 4 h before delivery an oral dose of [(13)C]palmitic acid (PA), [(13)C]oleic acid (OA), [(13)C]linoleic acid (LA), and [(13)C]docosahexaenoic acid (DHA). Maternal blood samples were collected at -4 h (basal), -3 h, -2 h, -1 h, 0 h, and +1 h relative to time of cesarean section. At the time of birth, venous cord blood and placental tissue were collected. Fatty acid composition was determined by gas-liquid chromatography and isotopic enrichment by gas chromatography-combustion-isotope ratio mass spectrometry. (13)C-enrichment of fatty acids in the nonesterified fatty acids (NEFA) of cord plasma tended to be higher than in NEFA of placenta, with statistically significant differences for the nonesterified OA and DHA ([(13)C]PA, 0.024 +/- 0.011 vs. 0.001 +/- 0.001; [(13)C]OA, 0.042 +/- 0.008 vs. 0.005 +/- 0.003; [(13)C]LA, 0.038 +/- 0.010 vs. 0.008 +/- 0.002; [(13)C]DHA, 0.059 +/- 0.009 vs. 0.010 +/- 0.003). The ratio of tracer fatty acid concentrations of placenta to maternal plasma was significantly higher for [(13)C]DHA than for the other fatty acids ([(13)C]PA, 7.1 +/- 1%; [(13)C]OA, 3.8 +/- 0.4%; [(13)C]LA, 9.2 +/- 1.3%; [(13)C]DHA, 25.9 +/- 3.4%). These results suggest that only a part of the placental NEFA participated in fatty acid transfer, and that the placenta showed a preferential accretion of DHA relative to the other fatty acids.     

The visceral yolk sac--an important site of synthesis and secretion of apolipoprotein B containing lipoproteins in the feto-placental unit of the rat.

Rat fetuses exhibit a high serum LDL concentration at term. Delivery caused a marked decrease of the LDL apolipoprotein (apo) B concentration independent of whether this occurred on days 21, 22 or 23 of gestation. The interruption of the yolk sac circulation by a ligature in situ for 6 h led to the same alterations of the LDL-apo B concentration as Caesarean section. Immunoelectronmicroscopic studies provided evidence that the epithelial cells of the visceral yolk sac exhibited electron dense LDL-sized and apo B containing particles which were localized over the compartments of the Golgi complexes, endoplasmatic reticulum, secretory vesicles and intercellular spaces, but not over the cell nuclei, mitochondria or lysosomes. ApoB containing LDL-sized particles could be obtained by ultracentrifugation from the disrupted material of the microsomal fraction of yolk sac homogenates. Isolated segments of the yolk sac membranes were capable to secrete apoB containing lipoproteins floating in the d less than 1.020 g/ml as well as in the d = 1.020-1.064 g/ml fraction with a 10-fold higher amount of apoB in the higher density class. Incorporation experiments with [35S] methionine gave evidence that these lipoproteins were at least partially provided with newly synthesized apoB predominantly found in the LDL fraction. The size of the negatively stained particles in the d = 1.020-1.064 g/ml fraction secreted from yolk sac segments corresponded to that of LDL from fetal rat serum. In contrast their acylglycerol content was significantly higher, whereas the percentage contribution of total cholesterol and protein was markedly reduced in comparison with serum LDL of the fetus. In summary, biochemical and ultrastructural studies provide clear cut evidence that the rat yolk sac is able to synthesize and to deliver apo B containing lipoproteins in the density ranges of VLDL, IDL and particular of LDL thus contributing to the supply of serum lipoproteins in the rat fetus. By recalculation of recent tracer kinetic data (Plonné et al. (1990) J. Lipid Res. 31, 747) using a mathematical step function model it was possible to assess the contribution of the rat yolk sac to the LDL influx into the fetal serum.     

Cadmium-induced forelimb ectrodactyly: a proposed mechanism of teratogenesis

We have attempted to elucidate the mechanism of cadmium teratogenesis utilizing inbred mouse strains sensitive (C57BL/6J) or resistant (SWV) to the embryotoxic effect of this common heavy metal contaminant. Carbonic anhydrase activity of whole-embryo homogenates was moderately depressed in C57BL/6J mice compared to a slight and transient decrease in the resistant SWV mice. Embryonic erythrocytes were similarly examined, and the cadmium did not have any effect on carbonic anhydrase activity in either strain. Likewise, histochemical examination of carbonic anhydrase activity did not reveal any effect of cadmium in the embryos of their strain. Generally, the zinc concentration of embryos was not affected by cadmium administration. However, increased levels of zinc were observed in cadmium-exposed yolk sacs of both strains suggesting that cadmium produces an adverse effect on yolk sac function. Untreated C57BL/6J units (embryo plus surrounding extraembryonic membranes), embryos, and yolk sacs had much lower hemoglobin concentrations than those observed in untreated SWV units, embryos, and yolk sacs. Additionally, cadmium exposure significantly decreased C57BL/6J embryonic hemoglobin levels on gestation day 10 (PM) and increased C57BL/6J yolk sac hemoglobin levels on gestation days 10 (AM) and 10 (PM). No difference in hemoglobin concentration was observed between untreated and cadmium-treated SWV embryos or yolk sacs. We propose that cadmium induces forelimb ectrodactyly by creating an acidotic embryonic environment and that the primary site at which cadmium exerts its teratogenic effect might be the yolk sac.