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Query: UMLS:C0851184 (thinning)
 11,252 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

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.
Placenta
PMID: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. 336 94

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.
Placenta
PMID: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. 336 90

A common assertion in the literature is that Langhans cells in placental villi decline in number during gestation but this is a misinterpretation which may be caused by the greater growth of villous surface area compared with trophoblast volume. To test this possibility, human placentae were collected at 12-41 weeks of gestation for a cross-sectional study on the packing density of nuclei within villous trophoblast. Numbers of nuclei in the cyto- and syncytiotrophoblast were estimated using a design-based stereological device, the physical disector (parallel pairs of sections). Surface areas were estimated in order to assess the overall growth of villous arborizations. Packing densities of nuclei were calculated and expressed as numbers/1000 microns 2 of villous surface. Densities decreased during gestation and this can be explained by expansion of villous surface area and thinning of trophoblast. The biggest drop in packing density of cytotrophoblast nuclei (30 per cent) occurred between 17-21 and 22-26 weeks and this period coincided with the largest changes in villous surface area (62 per cent increase) and trophoblast thickness (30 per cent decrease). Results are consistent with the notion of an epithelial proliferative unit of constant volume and comprising about nine syncytiotrophoblast nuclei per Langhans cell.
Placenta 1994 Dec
PMID:Quantitative evidence for the spatial dispersal of trophoblast nuclei in human placental villi during gestation. 788 24

The aim was to determine whether the fetal vasculature of the human placenta adapts in a uniform manner to different forms of hypoxic stress. Stereological analyses were performed on the intermediate and terminal villi of placentae obtained from each of the following conditions; high altitude (up to 2800 m), maternal iron-deficiency anaemia and pre-eclampsia. These conditions were taken to represent hypoxic, anaemic and ischaemic hypoxia, respectively. In each situation, there was a trend for the volume fraction of the fetal capillaries to be higher than in the controls, although the differences were statistically significant only for the cases of hypoxic and anaemia hypoxia. This was due, in part, to capillaries being of larger diameter. Evidence for capillary proliferation was inconsistent. At high altitude there was a trend for the capillary length density to be raised, but there was no change in the ratio of capillary:villous length. In both maternal anaemia and pre-eclampsia the capillary length density remained constant, but the ratio of capillary:villous length was significantly raised in the former. These results suggest that dilatation of the capillary sinusoids, with accompanying thinning of the villous membrane, is the principal adaptation to hypoxia under the conditions studied.
Placenta 1996 Jan
PMID:Stereological evaluation of vascular adaptations in human placental villi to differing forms of hypoxic stress. 871 Aug 13

Villous development and maturation depend on fetoplacental angiogenesis but detailed baseline quantitative data on underlying structural events are lacking. The present aim was to analyse temporospatial patterns of villous and fetal vascular growth using microscopical sections of placentae at different periods of gestation. The emphasis is on acquiring absolute rather than relative data. Random tissue samples collected at 10-41 weeks of gestation were processed for stereological analyses. Growth strategies in 'stem', 'intermediate' and 'terminal' villi (classified according to villous diameter and nature of fetal vessels) were evaluated in terms of total volumes, lengths and mean cross-sectional areas. Total volume, total and relative length, mean cross-sectional area and shape, vascular endothelial cell number and mean cell size (squame surface area) were used to examine capillary growth. Volumes of 'intermediate' and 'terminal' villi increased significantly due to elongation and thinning. Villous maturation involved differential growth of capillaries which grew linearly without changes in mean cross-sectional area. Longitudinal growth was due to endothelial cell proliferation and increases in mean cell area. Events were characterized by a common inflection point at about mid-gestation. Usually, an early slow growth phase was followed by one of faster growth but capillary : villus length ratios and capillary shape factors tended to peak at mid-gestation. Findings confirm that angiogenesis is linked to villous growth and maturation, and is biphasic with dramatic changes in vascular content and arrangement occurring around mid-gestation. Capillary growth occurs preferentially and is solely by increase in total length driven by continuous proliferation and, later, by endothelial cell remodelling. Though providing no evidence for branching versus non-branching angiogenesis, these observations are consistent with the paradigm that fetoplacental angiogenesis occurs in two phases, the switch being associated with changes in uteroplacental oxygen tensions, local growth factor receptors and their ligands.
Placenta 2002 Nov
PMID:Fetoplacental angiogenesis during gestation is biphasic, longitudinal and occurs by proliferation and remodelling of vascular endothelial cells. 1239 14

The diversity of placental structures in Eutherian mammals is such that drawing generalizations from the definitive forms is problematic. There are always areas of reduced interhaemal distance whether the placenta is epitheliochorial, synepitheliochorial, endotheliochorial or haemochorial. However, the thinning may be achieved by different means. The presence of a haemophagous area as an iron transport facilitator is generally associated with endotheliochorial placentae but is also found in sheep and goats (synepitheliochorial) and in tenrecs and hyaenas (haemochorial). Although similar chorioallantoic placentae are found within families, structure begins to diverge at the ordinal level and there is little correlation at the supraordinal level of phylogeny. Differences in formation and function of the yolk sac provide additional variation. There would appear to be considerable adaptive pressure for development or retention of the haemochorial type of chorioallantoic placenta. This type of placenta has several possible drawbacks including more ready passage of fetal cells to the maternal organism and, should the haemochorial condition be achieved early, oxidative stress. At any rate no animal larger than the human and gorilla has this type of placenta. The endotheliochorial condition is found in animals as large as the bears, manatee and elephants. In addition to the ungulates, the epitheliochorial condition is present in the largest animals with the longest gestation periods, the whales. Considering the length of time since the early stages of mammalian evolution, it is probable that few unmodified structural features are present in any currently surviving mammal. Nevertheless, more complete studies of divergent types of mammalian placenta should help our understanding of mammalian interrelationships as well as placental function.
Placenta 2004 Apr
PMID:What can comparative studies of placental structure tell us?--A review. 1503