Die Ultrastruktur der Kapillarwand in der menschlichen Placenta zur Zeit der Schwangerschaftsmitte

Ultrastructure of the normal human placenta

In this review, the ultrastructure of the normal human chorionic villus is examined and illustrated. The outer covering of trophoblast, comprising the generative cytotrophoblast and the multinucleated syncytiotrophoblast which is derived from it, is described, including such features as the microvillous surface, vesicles and vacuoles, endoplasmic reticulum and secretory droplets. The structure, composition, development and inclusions of the trophoblastic basement membrane are discussed, and the ultrastructure of the various components of the stroma, including reticulum cells, fibroblasts, Hofbauer cells, capillaries and the non-cellular matrix are illustrated and described, with special reference to their inter-relationships and function.

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.

Intercellular junctions in the full term human placenta. II. Cytotrophoblast cells, intravillous stroma cells and blood vessels

Intercellular junctions within the villous stroma and the cytotrophoblastic layer of the human full term placenta were investigated using thin sectioning and freeze-fracturing. Numerous maculae adherentes (desmosomes) were found between the cytotrophoblast cells and the syncytiotrophoblast. This junction type was also seen connecting adjacent cytotrophoblast cells. Large gap junctions were frequently observed in contact areas of perikarya or at processes of adjacent fibroblasts. They often exhibited a peculiar pattern of their particles on the P-face of the membrane. Small rows of junctional particles were found on the P-faces of interconnected smooth muscle cells and gap junctions frequently bridged myoendothelial and interendothelial contact zones. The significance of the junctional complexes is discussed in relation to functional systems within the villous stroma of the human full term placenta.

Endothelial contractility - an undecided problem in vascular research

The rather old conception that endothelial cells possess an autonomous contractile capability has been reevaluated by several suthors during the past ten years on the basis of three different arguments of various validity: (1) After the topical application of inflammatory mediators tmajno et al. regularly found the endothelial nuclei furnished with numerous identations together with many interendothelial "gaps"; Both findings are assumed to be the morphological correlate of an endothelial shortening due to the contraction of the cells. While nuclear indentations seemed to be a rather weak argument to substantiate contractile capabilities, a mechanism other than contraction is outlined for the formation of "gaps"; (2) The second argument in favour of endothelial contractility is the occurrence of cytoplasmic filaments that occasionally form cross striated bundles and/or show a "thick" and "thin" variety. If all these data are assumed to be the morphological evidence for the contractile capability of cells then the conclusion: the more filaments the higher the contractile activity, must be valid. But when compiling those endothelia that are particularly rich in filaments this conclusion does not make sense, because e.g. the endothelium covering the venous valves is crowded with filaments yet an especially high "contractile activity" does not seem very probabble. On the other hand, the supposition that endothelial conttractility is entirely independent of the existence of cytoplasmic filaments leaves the question unanswered what then are the filaments for if not serving mechanical purposes. This line of reasoning is supported by both the localization of the filaments predominantly in those endothelia that have to sustain higher degrees of various mechanical stresses and the fact that filamentous structures significatnly increase in number under the influence of hypertension. (3) The final argument brought forward to substantiate endothelial contractility is the demonstration of actin and tropomyosin in the endothelium of various types of blood vessels that also occur under the influence of hypertension; tbut the significance of these findings as a proof for endothelial contractility is curtailed by the fact that the occurrence of actin alone is not conclusive for any contractile capabilities; Furthermore, a convincing demonstration of myosin in endothelial cells is still lacking and the "thick" filaments are believed to be noncontractile. Hence we suggest that the endothelial filaments together with the myoid proteins do not serve as a means for "contractility" in a true sense but simply act as a design to originate tensile strength.