Classification of human placental villi. I. Histology

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.

Myofibroblast as a major cellular constituent of villous stroma in human placenta

The human placenta requires contractile structures to generate energy for blood propulsion. Smooth muscle cells are not present in significant numbers in the human placenta while fibroblasts lack effective contractile properties. This study provides the following evidence that the stromal cells of the placental villi, cotyledonary septa and perivascular connective tissue are myofibroblasts. (I) Stromal cells are strongly positive for dipeptidylpeptidase IV (EC 3.4.14.5) which occurs exclusively in myofibroblasts as far as connective tissue cells are concerned. (2) The isoenzyme pattern of the placental dipeptidylpeptidase IV is identical to that of myofibroblasts in palmar fibromatosis on isoelectric focusing. (3) Antibodies raised against isolated placental dipeptidylpeptidase IV cross-react with dipeptidylpeptidase IV from myofibroblasts of palmar fibromatosis as shown by immunohistochemistry. (4) On electron microscopic examination, stromal cells present all the ultrastructural features of myofibroblasts. It is concluded that, except for the vascular component and a negligible number of Hofbauer cells, myofibroblasts make up nearly all the cellular constituents of human placental villous stroma.

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.

The basement membrane proteins laminin and type IV collagen in isolated villi in pre-eclampsia

The distribution and concentrations of the basement membrane proteins laminin and type IV collagen were studied in isolated placental villi in normal pregnancy and in pre-eclampsia. In both cases these proteins could be localized by immunofluorescence in the trophoblast and capillary basement membranes, and occasionally also in the matrix surrounding the capillaries. The basement membrane proteins were quantified by solubilizing the villi with proteolytic enzymes and by subsequently measuring the concentrations of two resistant domains of these proteins (7-S collagen and the fragment PI, representing type IV collagen and laminin, respectively) with specific radioimmunoassays. The ratio type IV collagen:laminin was significantly higher in pre-eclamptic samples than in the controls, most probably reflecting a decrease in laminin concentration in the villi in pre-eclampsia. Such a change in the chemical composition of placental basement membranes could weaken the attachment of trophoblast cells to the underlying basement membrane and also modify the permeability and exchange properties of the villi.

Morphological study of the fetal vasculature of the human term placenta: scanning electron microscopy of corrosion casts

Microvascular corrosion casts of the fetal vessels of the normal human placenta were studied using the scanning electron microscope. All available evidence suggests that when prepared under standard conditions the resultant casts accurately replicate the original luminal morphology of the vessels, and are comparable between placentae. In conjunction with stereo-pair photography this technique allows for the rapid and unequivocal interpretation of the three-dimensional configuration of villous vasculature. Previously described structures such as dilated capillary loops and perivascular capillary networks can be viewed in new and dramatic perspective. It is hoped the technique may thus be of assistance to physiologists in the interpretation of experimental data concerning placental transport. Further work is in progress to apply the technique to the investigation of possible changes in fetal vasculature occurring in abnormal pregnancies.

Developmental changes in rhesus monkey placental villi and cell columns. Scanning electron microscopy

Developing rhesus monkey placentas were analyzed by scanning electron microscopy with special attention directed toward defining stages in the development of the villus branches. The initial phase in formation of villi was the conversion of reticulated trabeculae of syncytial trophoblast into chorionic villi by growth and proliferation of cell columns of cytotrophoblast. These villi were stout and unbranched. The second phase of development appeared to be the longitudinal splitting of the villi and cell columns to form groups of parallel branches but there was a common insertion of these into the basal plate. The third phase in formation of villi, which appeared to begin at about the same time as the longitudinal splitting occurred, was the outgrowth of large-diameter side branches in a zone nearer the chorionic plate. At about 38-40 days of gestation the next stage in villus formation occurred, characterized by the emergence of numerous, small syncytial sprouts. Continued proliferation of villi at later stages of gestation resulted in a decreased diameter of the terminal villi and an increasing complexity in the course of fetal capillaries.

A three-dimensional study of the normal human placental villous core: II. Stromal architecture

The three-dimensional architecture of the villous core of the chorionic villi has been studied in the human placenta throughout pregnancy by scanning electron microscopy, and compared with light microscopic and transmission electron microscopic, findings. Five different types of villi are recognizable: the mesenchymal villi are characterized by a network of tiny bundles of collagen fibrils, enmeshing mesenchymal cells, Hofbauer cells and vessels. The immature intermediate villi display a system of intercommunicating channels, parallel to the major axis of the villi. Within the channels, Hofbauer cells are present. The mature intermediate villi show narrow and short stromal channels, surrounded by a rather tight network of collagen fibrils. Large vessels and a highly compact network of collagen are typical of the stem villi. The terminal villi contain sinusoidally dilated capillaries and a small amount of connective tissue. These findings have numerous functional implications. They are particularly related both to the presence of the stromal channels which could facilitate the macrophagic and immunological task of the Hofbauer cells, and to the different types of villi, playing different roles in placental and fetal life, under normal and pathological conditions.

[Problems of distinction of normal, arteficial and pathological structures in mature human placental villi. III. Morphometric studies in rhesus incompatibility (author's transl)]

Sections of human placental villi which were up to 80 micrometer in diameter were examined light microscopically and morphometrically. The villi were obtained by taking an aspiration biopsy of the attached placenta from 20 normal patients at term and from eight patients delivered at or before 37 weeks because of varying degrees of rhesus incompatibility. The distribution of three histological types of villi was determined. The intermediate villi from patients with erythroblastosis showed an increase in number and volume matching the severity of the disease. The intermediate villi also showed an increase in the amount of cytotrophoblasts, in the amount of materno-fetal diffusion surface area, in the thickness of syncytium, and in the number of Hofbauer cells. The terminal villi were not affected by erythroblastosis and the mature end villi showed a relative decrease in number. To obtain these results an optimal fixation technique was required.

[Problems of distinction of normal, artificial, and pathological structures in mature human placental villi. II. Morphometrical studies on the influence of the mode of fixation (author's transl)]

Morphometrical relations of human placental villi from 14 normal placentas of the 38th to 42nd week of pregnancy terminated by Cesarean section were examined in relation to the method of fixation. Villous tissue was obtained from central areas of the in situ placenta with intact maternal circulation by means of puncture aspiration and punch biopsy. Further punch biopsies were performed 2, 5, 10 and 20 min after manual removal of the placenta. Semithin sections of these villi were photographed and examined morphometrically. The findings demonstrate clear effects on quantitative data as a result of the period of ischemia between manual removal and fixation of tissue. Significant changes of several parameters (mean trophoblastic volume, size of epithelial plates, volume of connective tissue, volume of fetal vessel lumina, etc.) occur as early as 2 min after separation of the placenta from the maternal circulation. They are mostly related to a collapse of intravillous vessels. The villous branches may be classified according to structure and morphometry, if they have been obtained under optimal conditions of fixation and prior to any vascular collapse.