Biomedical subjects

G J Burton

Publications and source records attributed to G J Burton.

At least 19 recordsLinked to original sources

PubMed1995

Placental histomorphometry and morphometric diffusing capacity of the villous membrane in pregnancies complicated by maternal iron-deficiency anemia.

OBJECTIVE: Our purpose was to determine the morphometric diffusing capacity of the human placental villous membrane in cases of maternal iron-deficiency anemia. STUDY DESIGN: Material from 10 term placentas from mothers with hemoglobin levels < 10 gm/100 cm3 was compared stereologically with control material from women with normal hemoglobin concentrations matched for age and parity. Data were compared with Student t test. RESULTS: Birth and placental weights were constant between the two groups. Maternal anemia was associated with a significant reduction in both volume and surface area of intermediate and terminal villi (157.8 vs 234.5 cm3 and 9.5 vs 14.9 m2, respectively). There was a concomitant rise in the volume of pathologic areas (32.1 vs 4.1 cm3). Harmonic mean thickness of the villous membrane was significantly less in the anemia cases (5.86 vs 7.15 microns), because of an increase in volume fraction of the fetal capillaries. Consequently, the morphometric diffusing capacity of the villous membrane was maintained (4.49 vs 5.31 cm3.min-1.mm Hg-1, t = 1.04, p > 0.05). CONCLUSIONS: There is a reduction rather than an enlargement of the placental villous tree in cases of maternal anemia. This may be because of underlying malnutrition. Nonetheless, the placenta adapts through thinning of the villous membrane so that diffusing capacity is maintained at normal levels.

Anemia, Iron-Deficiency↗

PubMed1995

Human term placental capillary endothelial cell specialization: a morphometric study.

In the human placental capillary endothelium, two cell types have previously been described with different organelle distribution patterns. This observation was based on examination of cross-sectional profiles. It is known, however, that organelle distribution may vary in different parts of a cell. The present study was undertaken to estimate morphometrically the pattern of organelle distribution in normal term placental capillary endothelium. Ten normal term placentae were processed routinely for electron microscopy. The volume fractions of the different organelles were determined in both the perinuclear and peripheral parts of the cells. In addition, comparisons were made between endothelial cells at the site of vasculosyncytial membranes (VSM) and those forming the remainder of the capillary wall (non-VSM). Multifactor ANOVA showed there was a significant difference between the perinuclear and the peripheral parts in the volume fractions of mitochondria, rough endoplasmic reticulum, intermediate filaments, all these being more in the perinuclear region, whilst vesicles and cytosol were more prevalent in the peripheral parts. In contrast, there was no significant difference between the regions of VSM and the non-VSM except for the volume fractions of microfilaments which were higher in the non-VSM region.

Capillaries↗

PubMed1995

Sonographic, stereological and Doppler flow velocimetric assessments of placental maturity.

OBJECTIVE: To determine whether sonographic assessments of increasing placental maturity are associated with changes in the theoretical diffusing capacity of the organ and with changes in vascular resistance. DESIGN: Placental maturity was assessed sonographically in 10 organs: five were classified as completely immature and five as completely mature. Doppler flow velocimetric recordings were obtained shortly before delivery performed by elective caesarean section. Placental biopsies taken immediately after delivery were analysed stereologically. RESULTS: No differences were observed in either placental weight or volume, but the morphometric diffusing capacity of the villous membrane was 40% higher in the mature than in the immature placentas. Doppler indices did not alter with increasing placental maturity. CONCLUSIONS: Normal term placentas differ significantly in their diffusion characteristics, even when organ weight and volume remain constant. Umbilical vascular resistance may be heavily influenced by the number of parallel circulatory units offered by the placental lobules.

Blood Flow Velocity↗

PubMed1994

Effects of hypobaric hypoxia on the fetoplacental unit: the morphometric diffusing capacity of the villous membrane at high altitude.

OBJECTIVE: Our purpose was to determine whether the human placenta is able to adapt structurally to hypobaric hypoxia so as to increase its diffusing capacity. STUDY DESIGN: Material from 10 normal term placentas collected at each of three altitudinal levels in Kirghizstan was analyzed stereologically. Data were compared by one-way analysis of variance. RESULTS: Within the overall sample group birth weight and placental weight remained constant. There were no differences in villous and capillary surface areas or capillary length, but capillary volume increased from 37.9 cm3 in controls (700 m) to 50.8 cm3 at high altitude (2200 to 2800 m). The harmonic mean thickness fell from 6.9 microns in controls to 4.8 microns at high altitude. Consequently, the morphometric diffusing capacity of the villous membrane for oxygen increased from 4.22 to 7.56 cm3.min-1.mm Hg-1 (F = 6.45, p < 0.05). CONCLUSIONS: The placenta is capable of adapting to hypobaric hypoxia to increase its functional capacity for gaseous exchange. This is principally through a reduction in the thickness of the villous membrane.

Altitude↗

PubMed1994

The effects of maternal vascular pressure on the dimensions of the placental capillaries.

OBJECTIVE: The study sought stereological evidence for the sluice flow phenomenon in the human term placenta. DESIGN: Two series of experiments were conducted on caesarean delivered placentae. In both, two fetal arteries were cannulated and perfused. In the first series, five placentae were perfusion-fixed under arterial pressures of 40 mmHg and 60 mmHg, and an external pressure of 10 mmHg in the intervillous space. In the second series, five placentae were perfusion-fixed at arterial pressures of 70 mmHg and 90 mmHg with an external pressure of 30 mmHg. Stereological estimates relating to the size of the capillaries and the thickness of the villous membrane were then made. RESULTS: In each series, as the arterial pressure rose, there was an increase in the volume fraction and surface area of the fetal capillaries, and a concomitant decrease in the mean thickness of the villous membrane. These changes were negated as the external pressure rose, indicating the importance of the pressure differential between the capillary lumen and the intervillous space in determining the dimensions of the fetal capillaries. CONCLUSIONS: The study demonstrated that placental capillaries are elastic and hence deformable. If a sufficiently high pressure is generated in the intervillous space, they may therefore be compressed. This may explain the increase in umbilical vascular resistance demonstrated by doppler studies in pregnant women scanned in the supine position. In this situation the venous drainage from the intervillous space is believed to be impeded by the gravid uterus compressing the inferior vena cava.

Arteries↗

PubMed1994

Significance of changes in fetal perfusion pressure to factors controlling angiogenesis in the human term placenta.

The objective of the study was to determine the effects of mechanical factors on endothelial proliferation in the human placental villous vasculature. Individual fetal lobules were perfused with tissue culture medium at two different standard pressures (40 and 100 mm Hg). The perfused area was then removed and diced into small blocks which were quench frozen in liquid nitrogen. Cryostat sections were obtained and fixed in acetone at 4 degrees C. Proliferating cell nuclear antigen was then identified using Ki67 antibody as a marker. Proliferating nuclei were scored, using a light microscope, and a comparison made between the two pressures used. More proliferating endothelial nuclei were found at 100 mm Hg than at 40 mm Hg (P < 0.05). It is therefore concluded that mechanical factors may play a role in villous angiogenesis and the formation of terminal villi.

Blood Vessels↗

PubMed1993

A scanning electron microscopic study of the chick chorioallantoic membrane: cell death and the involvement of oxygen free radicals.

Cell death is a normal feature within the chick chorioallantoic membrane, occurring principally between days 10 and 14 of incubation. Samples of chorioallantoic membrane were obtained on days 6, 8, 10, 12 and 14 of incubation, after the creation of artificial air chambers on day 3. These were examined by scanning electron microscopy (SEM), transmission electron microscopy (TEM) after staining for acid phosphatase activity, and by light microscopy after demonstrating oxygen free radicals with nitro blue tetrazolium. On day 6, small defects in the plasmalemma, approximately 200 nm in diameter, could be seen by SEM. A sequence of events leading to complete destruction of the plasmalemma, with exposure of the nucleus and other cytoplasmic organelles, could be traced, and by day 8 the membrane was a mosaic of healthy cells and others in various stages of degeneration. TEM revealed that acid phosphatase activity was confined to the golgi apparatus and associated vesicles even in advanced stages of degeneration. By comparison, oxygen free radicals were demonstrated in individual cells from day 6 onwards. Application of superoxide dismutase and catalase to the epithelium using a nebuliser spray significantly reduced the amount of cell death seen by scanning microscopy on day 12. It is concluded that oxygen free radicals may mediate cell death in the chorioallantoic membrane.

Acid Phosphatase↗

PubMed1993

Star volume estimates of the intervillous clefts in the human placenta: how changes in umbilical arterial pressure might influence the maternal placental circulation.

The study estimated the mean volume of the clefts between adjacent villi in the normal term human placenta, and the effect on this of increasing the fetal perfusion pressure, using a new stereological tool called the 'star volume estimator'. This enables the measurement of irregular and complex structures, including both particles and cavities, in a mathematically defined and unbiased manner. To achieve this, a total of ten term placentae delivered by caesarean section were obtained. Four fetal arteries supplying opposite quadrants of the placental disc were perfusion-fixed under standard pressures of 40, 60, 80 and 100 mmHg respectively. Stereological estimates relating to the star volume of the clefts between the villi, and to the volume density of the intervillous space were obtained. There was a significant rise in the star volume of the intervillous clefts from 26.8 x 10(4) m3 at 40 mm Hg to 75.1 x 10(4) m3 at 100 mmHg (F = 75, df = 1.38, P < 0.05). The volume density of the intervillous space remained unchanged, thus obviating the possibility of fluid leakage into the intervillous space accounting for this change. It is concluded that the fetal vasculature provides hydraulic support to the villous tree, and that changes in the umbilical perfusion pressure can therefore alter the disposition of the villi within the intervillous space. As fetal blood pressure rises, for example during acute hypoxic episodes, the villi will move apart. The enlargement of the clefts between adjacent villi will have a secondary effect upon the maternal circulation, promoting more even perfusion of the intervillous space at higher overall flow rates.(ABSTRACT TRUNCATED AT 250 WORDS)

Blood Pressure↗

PubMed1992

Development of the chick chorioallantoic capillary plexus under normoxic and normobaric hypoxic and hyperoxic conditions: a morphometric study.

Fertile eggs from the domestic fowl were incubated under normobaric normoxic (21% O2), hypoxic (14% O2), and hyperoxic (40% O2) conditions in order to examine the influence of the prevailing oxygen level on the growth and maturation of the chorioallantoic membrane. Eggs were sampled at regular stages throughout incubation for morphometric analysis. Under normoxic conditions, maturation of the capillary plexus occurred in two distinct stages, both of which contributed to a reduction in the thickness of the air-blood barrier. Between days 6 and 10, the capillaries sprouted and fused to form a dense plexus. Subsequently, between days 10 and 14, this plexus invaginated into the chorionic epithelium. Differentiation of the chorioallantoic membrane appeared maximal by the end of this period. Hypoxia resulted in diminished growth of the embryo and chorioallantoic membrane, but in accelerated maturation of the capillary plexus. Hyperoxia had a less marked effect but appeared to retard the final invagination of the plexus, resulting in a thicker air-blood barrier.

Allantois↗

PubMed1992

Formation of vasculo-syncytial membranes in the human placenta.

Vasculo-syncytial membranes are localised areas of the placental villous membrane where the thickness of the barrier separating the maternal and fetal circulations is reduced to as little as 1-2 microns. Consequently, they are believed to be important sites for diffusional exchange. The morphological appearances suggest that they are caused by the obtrusion of locally dilated segments of the fetal capillaries into the trophoblast layer. This study sought quantitative evidence for the hypothesis by performing stereological analyses on vasculo-syncytial membranes at the electron microscopic level. The results confirmed that a strong relationship existed between the thickness of the capillary endothelium and that of the overlying stromal and trophoblastic tissue at these sites (r = 0.47, P < 0.001), indicating that some asymmetrical stretching or remodelling of the capillary wall was involved. Comparisons were also made between the thickness of the trophoblastic, stromal and endothelial components of the villous membrane in villi obtained from the central and from the peripheral parts of placental lobules, where vasculo-syncytial membrane formation is accentuated. The mean thickness of each component was lowest in the samples from the peripheral region, although the differences only proved to be statistically significant for the stromal layer (P = 0.01). Both sets of data lend quantitative support to the hypothesis that vasculo-syncytial membrane formation is the result of obtrusion of locally dilated segments of the fetal capillaries. The way in which this may be linked to changes in the dynamics of the fetal circulation as gestation advances is discussed.

Capillaries↗

PubMed1992

The effect of smoking in pregnancy on early placental morphology.

Histomorphometric and ultrastructural investigations were performed on the placental tissue of 20 heavy smokers and 20 non-smokers. All pregnancies were artificially terminated at 9-14 weeks' gestation. When compared with values in non-smokers, smokers had significantly increased arithmetic and harmonic mean thicknesses (P less than .005 and P less than .05, respectively) of the villous membrane, and arithmetic and harmonic mean thicknesses (P less than .05 for both) of the trophoblastic layer. No significant difference was found between the groups for the mean number of capillary profiles per villous profile or for the mean volume fraction of the villi occupied by the trophoblast, the villous stroma, or the fetal capillary lumina. At the ultrastructural level, the mean number of areas presenting with syncytiotrophoblastic necrosis was significantly (P less than .001) higher in the smoking than in the non-smoking group, whereas no difference was observed for the mean number of areas demonstrating trophoblastic basal membrane thickening. These results suggest that smoking during the first months of pregnancy induces morphologic changes, which may explain biologic disturbances observed during early gestation and also later in pregnancy.

Capillaries↗

PubMed1992

Capillary volume fraction is the principal determinant of villous membrane thickness in the normal human placenta at term.

The thickness of the villous membrane is known to be an important factor in determining the morphometric diffusing capacity of the placenta at term. As yet it is not certain how areas of the villous membrane specialised for gaseous exchange, the vasculo-syncytial membranes, differentiate. One mechanism suggested is that they arise through obtrusion of the fetal capillaries contained within the stromal core. As a result the bulk of the overlying trophoblast is displaced laterally, reducing the thickness of the villous membrane to as little as 1-2 microns at these points. To test this hypothesis the relationship between the vascularity of the villi, as determined by the villous capillary volume fraction, and the mean thickness of the villous membrane was investigated. Data were taken from a recent study in which placental villi were biopsied from normal term placentae within 1 min of caesarean delivery and at 5, 10, 15 and 20 min thereafter. Since intuitively the membrane has both a maximum and a minimum thickness a sigmoid relationship was fitted to the data using least squares regression analysis. Estimates of arithmetic and harmonic mean thicknesses were then predicted from the capillary volume fraction for a large number of placentae using data from previous studies. These all employed similar stereological techniques but were performed over a number of years by several workers in this laboratory. The predicted values were tested against the measured values using paired 't' tests, but no significant differences (P greater than 0.05) were detected.(ABSTRACT TRUNCATED AT 250 WORDS)

Capillaries↗

PubMed1991

Thinning of the placental villous membrane during maintenance in hypoxic organ culture: structural adaptation or syncytial degeneration?

This study was undertaken in order to reinvestigate claims that the human placenta is able to adapt structurally under hypoxic stress so as to facilitate materno-fetal oxygen exchange. The results confirmed that thinning of the villous membrane does occur after maintenance for 10 h in an atmosphere of 6% oxygen, and that this effect is partially reversed by maintenance for a further 10 h under normoxic conditions (21% O2). However, throughout these experiments the values for both the arithmetic and the harmonic mean thicknesses of the villous membrane altered in an identical fashion. This indicates that the thinning observed was not the result of redistribution of the syncytiotrophoblast around the villous surface, with movement away from the sites of the fetal capillaries as previously postulated. Rather, it must have been caused by a reduction in the total mass of the syncytium, resulting from either involution and partial degeneration of its organelles, or some form of fluid-shift phenomenon. Whatever the cause, these effects were only partially reversed after further culture for 10 h in well-oxygenated conditions. Thus whilst there is an accumulating body of evidence to suggest that the placenta can adapt structurally to hypoxia in vivo, it is apparent that the in vitro organ culture system is not a suitable model with which to investigate the process.

Analysis of Variance↗

PubMed1991

A technique for correlative scanning and transmission electron microscopy of individual human placental villi: an example demonstrating syncytial sprouts in early gestation.

Correlating the surface appearances of certain features with their internal structure is made particularly difficult in the human placenta by the complex three-dimensional branching pattern of the villous tree. This places a possible limitation on the use of the scanning electron microscope in this field, both for basic research purposes and as a tool in pathological diagnosis. To help overcome this problem, a technique for handling individual placental villi has been devised. By attaching single villi to glass pipette tips it has proved possible to scan the villi, embed them in resin and then section them in a known pre-determined orientation. Exact correlations between the surface appearances and the internal structure, as seen with either the light or transmission electron microscope, can then be drawn. This paper describes the technique and, using an example based on syncytial sprouts in early pregnancy, illustrates the precision afforded by the method.

Chorionic Villi↗

PubMed1990

On the varied appearances of the human placental villous surface visualised by scanning electron microscopy.

Scanning electron microscopy enables extensive areas of the human placental villous tree to be viewed at high resolution in a relatively quick and easy manner. It is therefore an invaluable aid to the study of normal placental structure, and may have a potential role to play in pathological diagnosis. However, with increasing experience an awareness of the possible artefacts that may be introduced during fixation and tissue preparation is gradually developing. Some of these artefacts are common to other organs and applications of scanning electron microscopy, whereas others result from the unique structure and three-dimensional configuration of the placenta. Examples of the second category are poorly documented, and yet their occurrence may also significantly influence the appearance of the tissue as seen by light or transmission electron microscopy. This paper illustrates examples of this problem, and emphasises some of the factors that must be considered when interpreting both scanning and transmission electron micrographs of the villous surface.

Chorionic Villi↗

PubMed1989

Morphometric differences between the placental vasculature of non-smokers, smokers and ex-smokers.

The aim of this study was to determine whether cigarette smoking during pregnancy has an adverse effect upon the placenta's capacity for gaseous exchange. Using recently developed stereological techniques, in conjunction with perfusion fixation, computer-assisted measurements were made on the placentas of 15 non-smokers, 15 moderate smokers, 15 heavy smokers and 13 ex-smokers, 7 of whom stopped smoking during the course of the pregnancy. Compared with the placentas of non-smokers and of those who stopped before pregnancy, it was found that the placentas of smokers and of those who stopped after conception exhibited a reduced capillary volume fraction, and an increased thickness of the villous membrane. Although they must impair gaseous exchange across the placenta, these changes were less severe than suggested by previously published reports. Nonetheless it is clear that in order to prevent these changes women should stop smoking before conception rather than during the course of a pregnancy.

Adult↗

PubMed1989

The chorioallantoic capillary plexus of the chicken egg: a microvascular corrosion casting study.

The chorioallantoic membrane of the avian egg serves as the principal organ of respiratory gaseous exchange for the embryo until close to hatching. It lies closely apposed to the inner shell membrane and contains an extremely dense capillary plexus supplied by the allantoic blood vessels. This study applied the microvascular corrosion casting technique to investigate the three-dimensional arrangement of the plexus at various stages of incubation. Casts were produced between days 6 and 14 of incubation, and their appearances were compared with those obtained from traditionally sectioned material and from freeze-cleaved specimens. By day 6 the capillary network was remarkably profuse but showed considerable regional variation in vessel density. In some areas there were only short capillary buds whereas in other areas fusion had taken place so that a true plexus was formed. By day 10 the capillaries had become confluent to such a degree that the cast consisted of a thin sheet of resin perforated only by an array of small irregularly shaped orifices. These corresponded closely in size to the intervening columns of chorionic epithelial cells seen in the sectioned material. It is clear from the appearances of the casts that the capillary surface density becomes maximal at approximately day 10 of incubation. From then on in incubation any increase in the diffusing capacity of the chorioallantoic membrane must be the result of either an increase in its overall surface area, or a decrease in the thickness of the air-blood barrier.

Allantois↗

PubMed1987

The fine structure of the human placental villus as revealed by scanning electron microscopy.

Scanning electron microscopy of the placenta has a history of only twenty years. During that time, however, there have been dramatic advances in instrument technology coupled with the refinement of preparative techniques designed to reduce fixation artifacts to a minimum. As a result many of the early claims must be amended or suitably qualified, and this is one aim of the present review. Much new data on the internal structure of the placental villus is also presented. By means of the partial digestion technique it is now possible to describe the three dimensional configuration of the various components of the villous tree. This review will consider these sequentially, starting externally with the gross morphology of the villi and ending with replicas of the fetal vasculature produced by corrosion casting.

Chorionic Villi↗