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Biomedical subjects

M R Jackson

Publications and source records attributed to M R Jackson.

144 records · Page 8Linked to original sources
PubMed1984

Stabilities of some constituents of marmoset (Callithrix jacchus) plasma under various conditions of storage.

The stability of various marmoset (Callithrix jacchus) plasma constituents was investigated after storage at room temperature, 4 degrees C, and -20 degrees C. The method of sequential analysis ensured that the between-run bias of the methods of analysis used was drastically reduced, and the definitions of stability were linked to the imprecision of these methods. Optimal conditions for storage for as long as 48 h depended on the analyte being measured. Room temperature was optimal for cholinesterase and acetylcholinesterase; 4 degrees C for protein, albumin, alanine aminotransferase, isocitrate dehydrogenase, sorbitol dehydrogenase, lactate dehydrogenase, and glutamate dehydrogenase; and -20 degrees C for glutathione reductase and alkaline phosphatase. For aspartate amino-transferase and gamma-glutamyltransferase, either 4 degrees C or -20 degrees C would be suitable. Reasons are advanced for some conflicting reports in the published work, and we emphasize the need to investigate each analyte and species separately.

Animals↗
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Morphometric studies on villi in human term placentae and the effects of altitude, ethnic grouping and sex of newborn.

Stereological methods were used to test whether or not altitudinal, ethnic and sex differences in birthweight in Bolivia are associated with differences in placental morphology. Estimates of the volumes, surface areas, lengths and diameters of villi and of fetal capillaries were obtained by analysing paraffin sections of tissue. The average highland placenta contains villi of smaller overall volume, surface area and length. These villi possess a less voluminous fetal vascular space, and the capillaries are smaller in mean diameter. The average Amerindian placenta has longer but thinner capillaries than the average placenta delivered by non-indigenous women. Altitude and ethnic grouping interact to have an effect on total villous length, whilst ethnic grouping and sex interact to influence fetal capillary length. These structural differences are discussed in the context of their likely impact on placental functioning, notably on exchange by passive diffusion.

Acclimatization↗
PubMedDate not supplied

On the factors which contribute to thinning of the villous membrane in human placentae at high altitude. I. Thinning and regional variation in thickness of trophoblast.

Sections of human term placentae delivered at low and high altitude to indigenous and non-indigenous populations in Bolivia were analysed by stereological methods. Estimates were obtained of the arithmetic and harmonic mean thicknesses of villous membrane and of trophoblast. These values were then used to derive indices of thickness variation. No significant ethnic, sex or interaction effects were found. At high altitude, the villous membrane was significantly more variable in thickness owing to a lower harmonic mean but the same arithmetic mean thickness. Alterations in the arithmetic and harmonic mean thicknesses of the trophoblast contributed to the thinning of the villous membrane. The trophoblast was thinner and more irregular in thickness at high altitude. These findings are discussed in the context of placental diffusing capacity and the possible mechanisms for formation of vasculosyncytial membranes.

Altitude↗
PubMedDate not supplied

On the factors which contribute to thinning of the villous membrane in human placentae at high altitude. II. An increase in the degree of peripheralization of fetal capillaries.

The contribution made by fetal capillary peripheralization to the thinning of the villous membrane seen in human placentae from high-altitude pregnancies is examined by stereological methods. Variables characterizing the shape of the villous core and the spatial relationships between trophoblast and capillaries are quantified. They shed light on the relative importance of dynamic versus mechanistic processes of villous membrane attenuation. Highland villi differ from lowland villi in several ways. On average, they possess a thinner barrier due to closer approximation of capillaries to overlying trophoblast; in consequence, the villous core is more irregular in outline and its surface (that of the inner aspect of the trophoblast) exceeds in area that of the outer aspect of the trophoblast. These results suggest that the dynamic process (protoplasmic streaming within syncytiotrophoblast) cannot alone explain thinning of the villous membrane. A mechanistic process (capillary peripheralization and obtrusion into the trophoblastic epithelium) is sufficient to account for the differences observed, although the possibility that both processes operate concurrently cannot be discounted. This report completes a study into factors contributing to villous membrane thinning at high altitude.

Altitude↗
PubMedDate not supplied

Quantitative description of the elaboration and maturation of villi from 10 weeks of gestation to term.

Stereological methods have been applied to a cross-sectional sample of human placentae collected at 10-41 weeks of gestation in order to provide a quantitative description of the growth and maturation of villi. Random tissue sections were analysed to derive volumes, surface areas, lengths, diameters and membrane thicknesses for villi and their fetal capillaries. Expansion of the total volume and surface area of villi can be explained by a dramatic linear growth of terminal villi which begins at about the middle of the second trimester. Growth of intermediate villi also occurs but to a more limited extent. Linear growth is accompanied by villous maturation which involves increases in the relative volume of capillaries and in villous capillarization coupled with decreases in villous diameter, capillary diameter and harmonic thickness of the villous membrane. These findings confirm that placental growth and development depend greatly on growth and maturation of terminal villi. They do not confirm sinusoidal dilation of fetal vessels as a generalized phenomenon. They also support the contention that changes in effective diffusion distances across the villous membrane have real adaptive significance.

Analysis of Variance↗
PubMedDate not supplied

Oxygen diffusive conductances of human placentae from term pregnancies at low and high altitudes.

A morphometric model of oxygen diffusion is employed to calculate the partial, total and specific conductances of human placentae from low- and high-altitude term pregnancies. Placentae were obtained from indigenous and non-indigenous populations and the diffusion pathway dissected into six tissue compartments. Birthweights were reduced at high altitude but were greater in natives versus non-natives. The altitudinal differences were associated with changes in placental diffusive conductances. The partial conductance of the villous trophoblast was conserved but the conductance on the maternal side was increased as was the conductance of the villous stroma. Fetal conductances were conserved (plasma) or diminished (erythrocytes). We conclude that birthweights are reduced at high altitude despite the attempts to increase the total placental diffusive conductance for oxygen. The mechanisms underlying these adaptations are discussed.

Adaptation, Physiological↗
PubMedDate not supplied

Changes in oxygen diffusive conductances of human placentae during gestation (10-41 weeks) are commensurate with the gain in fetal weight.

Evidence is adduced that the oxygen diffusive conductance of the human placenta increases during gestation. Results are based on placentae from 10 weeks to term. The diffusion pathway is analysed as six discrete tissue compartments arranged in series. Stereological and physicochemical data are used to determine partial, total and specific diffusive conductances for each organ and for its tissue compartments. During gestation, maternal and fetal vascular spaces became more voluminous and exchange surface areas of villi and capillaries expanded. Both were manifestations of placental growth and development. Maturation of villi was evident as decreases in effective diffusion distances across the trophoblast and villous stroma. These alterations contributed to improvements in partial and total conductances which continued towards term. Changes within the trophoblast and stroma were particularly influential in determining total conductance. Those on maternal and fetal sides of the diffusion pathway were less important. The rise in total conductance was related directly to the rise in fetal weight. Specific diffusive conductance was constant throughout gestation.

Diffusion↗
PubMedDate not supplied

Stereological studies on the true thickness of the villous membrane in human term placentae: a study of placentae from high-altitude pregnancies.

Stereological principles were used to calculate functionally significant dimensions of the human villous membrane, its arithmetic mean thickness (Ta) and its harmonic mean thickness (Th). The former is proportional to tissue mass and oxygen consumption, the latter to diffusional resistance. For a group of 15 term placentae from uncomplicated pregnancies at high altitude, the average values were Ta = 4.44 micron, Th = 3.56 micron and Ta/Th = 1.26. The latter figure provides a useful quantitative expression for the efficiency of the membrane in gas and metabolite diffusion. It implies that vasculosyncytial membranes and syncytial knots decrease resistance to diffusion by 26 per cent, compared with that of a membrane with uniform thickness throughout. The methods are simple to apply and provide better estimates of true thickness than do measurements confined to thin sections.

Altitude↗
PubMedDate not supplied

Microscopical morphology of the human placenta and its effects on oxygen diffusion: a morphometric model.

A combination of stereological and physicochemical data was used to estimate the oxygen-diffusing capacity of the human term placenta. The effects on this morphometric diffusing capacity of isolated changes in placental structure were investigated by permitting volumes, surface areas and harmonic mean diffusion distances to deviate from normal baseline (term) values. Diffusion performance could be monitored effectively by estimating only three structural quantities. The most influential variable was the harmonic mean thickness of the villous membrane followed by the surface areas of villi and of fetal capillaries. Blood space volumes and plasma diffusion distances had negligible effects on diffusing capacity. Conclusions are discussed in the context of published findings on changes in placental anatomy which occur during gestation, abnormal pregnancies and pregnancies at high altitude.

Chorionic Villi↗