Michael Luckhardt

The fetal vascularisation of term human placental villi

SummaryVessel 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 μm. Depending on the method, the mean vessel diameter of the terminal villi is 12.3 (vessel casts) or 14.5 μm (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 μm, that of the terminal villus capillary loops 3,000 to 5,000 μm. The extent of sinusoidal dilation rises with increasing capillary length, indicating that the main functional importance of the sinusoids in the reduction of blood flow resistance.

Three-Dimensional Representation of the Fetal Vessel System in the Human Placenta

As compared to the maternal vascularization of the human placenta, the fetal vessels have been of minor interest throughout the past two decades. This is evident, too, from the monograph by Ramsey and Donner (1980) dealing with placental vascularization and circulation. About 5% of the volume is devoted to the fetal aspects. Among the 14 “areas of ignorance” recommended for future work, the fetal vessels of the placenta are not mentioned. This is no criticism of this outstanding monograph, but rather a strong argument that the classical studies by Boe (1953) and Arts (1961) have been fundamental, clear and sufficiently detailed to meet the requirements for the interceding 20 to 30 years.

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.

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.

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

SummaryThe 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 μm 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 μm diameter contain arterioles and muscular venules besides the paravascular capillary network. The last generation of stem villi measuring 80 to 100 μm 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.

Comparative Morphological Aspects of Placental Vascularization

The placental vascular architecture is interesting not only from the morphological point of view, but also as a basis for functional interpretation. The materno-fetal vascular arrangement in the placental exchange areas is one important criterion for the effectiveness of transplacental transport.

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 μm3. 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.

The structure of the tupaia placenta

SummaryThe placenta of the tupaia is bidiscoidal, labyinthine and endotheliochorial in structure. The two placental dises 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.