Effect of physiologic perfusion-fixation on the morphometrically evaluated dimensions of the term placental cotyledon.

OBJECTIVE: To estimate the in vivo dimensions of the fetal villous tree of the normal term placenta. METHODS: Dual-circuit perfusion-fixation of a cotyledon from eight normal term placentas was performed with random intra-cotyledon tissue sampling. Stereologic methods were used to derive estimates of villous (intermediate and terminal) surface area and volume, and star volume (a measure of villous volume). RESULTS: Villous surface area (mean 20.9 m2 [standard deviation 1.8]), capillary surface area (12.8 m2 [1.5]), villous volume (469 mL [40]), and capillary volume (80 mL [10]) values were all approximately 50% higher than reported previously. Star volume estimates ranged from 480 to 1350 microns 3. CONCLUSION: Tissue perfusion-fixation more accurately reconstructs the in vivo state, resulting in higher reference values than previously thought for the fetal villous tree dimensions. Up to one-quarter of fetoplacental blood volume is likely to be accommodated within the placenta at term.

[Development of the placenta and utero-placental circulation from the morphologic and functional viewpoint]. / Entwicklung der Plazenta und des utero-plazentaren Kreislaufes aus morphologischer und funktioneller Sicht.

All requirements of the conceptus, i.e. the fetus and the placenta are distributed by the maternal blood via the utero-placental vessels. Fetus and placenta need varying amounts of oxygen and nutrients at different stages of pregnancy. In the first half of pregnancy, the placenta, besides growth and development, fulfills the function of an endocrine gland whilst the nutrient supply of the fetus is of minor importance. In the second part of pregnancy, the placenta differentiates into an organ for exchange of gases and nutrients. Placental development may be divided into three stages: Implantation, growth and maturation. In the first half of pregnancy, the placental growth precedes the major fetal growth spurt, whereas, in the second half, the weight ratio placenta: fetus shifts in favour of the fetus and reaches 1:7 to 1:10 at the end of pregnancy. Growth and development of the placenta is the basis for fetal growth and it is not surprising that the weight ratio of the placenta and the newborn is positively correlated at the end of pregnancy. From the morphological point of view the development of the utero-placental circulation is characterized by a loss of the muscular elements in the wall of the peripheral parts of the spiral arteries supplying the endometrium and the intervillous space. These morphological changes functionally correlate with a reduction of resistance in the peripheral vessels. This can be visualized even doppler-sonographically as a reduction of pulsatility in the radial arteries.(ABSTRACT TRUNCATED AT 250 WORDS)

Cross-sectional features and three-dimensional structure of human placental villi.

Human placental villi from both normal and complicated pregnancies were examined by both light and scanning electron microscopy. The findings provide evidence that histological features such as syncytial sprouts, bridges, and a net-like arrangement of villi represent tangential sections of irregularly shaped villi rather than proliferative activity of the villous surface. Hence the two-dimensional appearance of paraffin and semithin sections has to be interpreted three-dimensionally in comparison with the respective scanning electron micrographs. In the light of these findings the various types of villous maldevelopment are summarized in a diagram which may be used as an aid for pathological diagnosis.

Interpretation of syncytial sprouts and bridges in the human placenta.

Human placental villi from normal and pathological material from the eighth to the 40th week of gestation were examined by light, scanning and transmission electron microscopy. Trophoblastic specializations--such as syncytial sprouts of early and late pregnancy, syncytial knots and syncytial bridges--were classified into proliferative and degenerative processes or artefacts caused by tangential sectioning. In early pregnancy and in diabetes mellitus most syncytial sprouts represent the initial phases in the development of villi. In late pregnancy, in particular in pre-eclampsia, most syncytial knots, sprouts and bridges are histological artefacts, caused by tangential sectioning of the trophoblastic surface. The chance of producing such artefacts is increased with increasing section thickness and with increasing branching and tortuosity of the villi. However, a small proportion of the syncytial knots, sprouts and bridges in the last-trimester placenta, those containing coarse pyknotic nuclei, are trophoblastic specializations of a probably degenerative character.

The fetal vascularisation of term human placental villi. II. Intermediate and terminal villi.

Vessel arrangement and vessel structure of the intermediate and terminal villi of 50 human normal term placentas have been studied by means of semithin histology, three-dimensional reconstruction of serial sections as well as scanning electron microscopy of vessel casts. The reliability of the methods applied has been checked by a morphometrical comparison of the luminal diameters obtained. The mature intermediate villi are characterized by the presence of 1 to 2 terminal arterioles as well as 1 to 2 postcapillary venules, and a few moderately coiled, mostly narrow capillaries, some of which belong to the so-called paravascular network. The remaining capillaries are continuous with the capillary loops of the terminal villi. The fetal vessels of the terminal villi are represented by capillary loops only, parts of which are sinusoidally dilated, reaching diameters up to 50 micrometers. Depending on the method, the mean vessel diameter of the terminal villi is 12.3 (vessel casts) or 14.5 micrometers (semithin sections). The capillaries of the terminal villi are arranged in such a way that 3 to 5 terminal villi are supplied by the same, multiply coiled capillary loop. The average capillary length of the paravascular net is 1,000 to 2,000 micrometers, that of the terminal villus capillary loops 3,000 to 5,000 micrometers. The extent of sinusoidal dilation rises with increasing capillary length, indicating that the main functional importance of the sinusoids is the reduction of blood flow resistance.

The structure of the tupaia placenta. I. Histology and vascularisation.

The placenta of the tupaia is bidiscoidal, labyrinthine and endotheliochorial in structure. The two placental discs show a characteristic partition into three broadly fusing main lobules on the mesometrial, and some very small accessory lobules on the antimesometrial side. The accessory lobules are separated incompletely from each other. Their inner lobular structure is rather homogeneous. All vessels seem to exhibit a rather random distribution. The maternal vessels of the three main lobules are regularly arranged in such a way that each lobule can be subdivided into three zones. The lobular centre is the maternal inflow area. The intermediate zone surrounding the centre is composed of maternal and fetal capillaries; the peripheral capsule collects the maternal blood into coarse networks of veins. Each lobule corresponds to one maternal circulatory unit in which up to ten fetal circulatory units are included. The fetal vertical lobular arteries are evenly distributed. Fine networks of fetal capillaries spread out centrifugally towards small collecting veins. Opposite the maternal veins the fetal ones do not form meshworks. Within the main lobules fetal and maternal capillaries are arranged parallel, antiparallel or in varying angles to each other, resulting in a complex crosscurrent flow system.

The structure of the tupaia placenta. II. Ultrastructure.

Both placental discs of the tupaia placenta consist of three large mesometrial and numerous additional smaller antimesometrial lobules. The larger lobules can be subdivided into a central (maternal arterial inflow), intermediate (maternal capillaries, exchange area), and a peripheral zone (maternal veins). The ultrastructure of the intermediate lobular zone has been studied from day 21 until day 41 of pregnancy. The materno-fetal barrier is endotheliochorial in structure. The voluminous maternal endothelium is highly differentiated. The adjacent basement membrane, separating endothelium and trophoblast, is constantly interrupted by endothelial and trophoblastic extensions, forming intense interdigitations between both tissue layers. The trophoblast is a multinuclear continuous layer, syncytial or plasmodial in nature. Cytotrophoblast is missing in all stages studied so far. Towards term, the trophoblast differentiates into three distinct types, arranged beneath each other in a mosaic-type pattern: trophoblast with rough e.r. and trophoblast with smooth e.r. as dominating organelles, as well as thin trophoblastic lamellae, almost devoid of organelles. The connective tissue is composed of very compact, epithelium-like cells, whereas connective tissue fibres and interstitial space are virtually absent. Macrophages, undifferentiated fibroblasts and highly differentiated fibroblasts can be observed. There are structural transitions between connective tissue and neighbouring fetal capillary endothelium.

The fetal vascularisation of term human placental villi. I. Peripheral stem villi.

The fetal vascularisation of the most peripheral three generations of stem villi has been studied by means of semithin serial sectioning and scanning electron microscopy of vessel casts in 50 human term placentas. The vessel types have been classified according to the structure of the vessel walls. Stem villi with a diameter of 200-400 microns are characterized by one smaller artery and one small vein, some arterioles and venules and capillaries of the paravascular net. Stem villi of about 150 microns diameter contain arterioles and muscular venules besides the paravascular capillary network. The last generation of stem villi measuring 80 to 100 microns in diameter exhibit a terminal arteriole and a collecting venule as well as up to ten paravascular capillaries. The luminal width of the arterial and venous stem vessels is considerably smaller than described for other vascular beds. This may partly be due to postpartal vascoconstriction. The capillaries of the paravascular net normally to not show sinusoidal dilation. They are arranged as long, hairpin-like, poorly branched loops connecting the arterial and venous stem vessels to each other.