Ultrastructure of the epithelium of the chorionic villi of the human placenta

Distribution of Notch family proteins in intrauterine growth restriction and hypertension complicated human term placentas

Members of the Notch family have been detected in many developmental and cell specification processes during placental development. However, Notch protein expression in Intrauterine Growth Restriction (IUGR) and Pregnancy Induced Hypertension (PIH) is not clear. In this study we aimed to clarify the immunolocalization of Notch proteins in full-term placentas after IUGR and PIH in comparison with normal placentas. Formalin-fixed, paraffin-embedded term placentas obtained by caesarean operations were processed for immunohistochemical localization of Notch 1, 2, 4 and Jagged 2. Transmission electron microscopy was also performed. In normal term placentas, all Notch proteins were intensely immunostained in the brush border of cells of the syncytiotrophoblast layer of the basal (maternal) side and the chorionic plate (fetal) side. The endothelial cells were also intensely immunostained in both sides for Notch 1. However, in IUGR and PIH placentas, the immunoreactivities of all Notch proteins were decreased significantly in the brush border of cells of the syncytiotrophoblast layer and the reaction was generally observed in the cytoplasm of syncytiotrophoblast cells in the basal and chorionic plate sides. The reactivity in endothelial cells was also significantly decreased. Our results have shown that the immunoreactivity and localization of Notch proteins is altered in pathologic placentas. Therefore, we propose that deregulated expression of Notch proteins may contribute to the disruption of trophoblast differentiation, endothelial cell function and/or feto-maternal traffic down-regulation during pregnancy or vice versa in such pathologic conditions.

Apoptosis contributes to vascular lumen formation and vascular branching in human placental vasculogenesis

Placental vasculogenesis consists of several stages, including appearance of hemangioblasts and angiogenic cell islands, setting up a primitive vascular network, and transition from vasculogenesis to sprouting and nonsprouting angiogenesis. In the present study, we hypothesized that placental vasculogenesis and angiogenesis require apoptosis during the formation of primitive vascular pattern, vessel elongation, and angiogenic branching. Vasculogenesis and apoptotic cells were identified using CD31 immunohistochemistry, hematoxylin-eosin (H-E) staining, CD31-TUNEL double-labeling, and transmission-electron microscopy (TEM). No TUNEL-positive cell was detected in angiogenic cell islands; however, several TUNEL-positive cells were observed during the primitive lumen formation. Interestingly, some of the stromal cells located between vasculogenic areas during the endothelial tube elongation and angiogenic branching also were TUNEL-positive. The presence of morphological aspects of apoptosis, such as nuclear shrinkage and nuclear bodies (apoptotic bodies), also was confirmed in H-E-stained and TEM-depicted sections. Quantitative analysis showed that higher ratios for apoptotic cells were found in the core stroma of villi among the vascular branching areas and in the primitive capillary lumen compared to angiogenic cell cords and vasculatures with advanced lumens (P < 0.05). In conclusion, our results suggest that apoptosis likely is involved in the physiologic mechanisms of placental vasculogenesis and angiogenesis, such as lumen formation and angiogenic branching.

Sequential Expression of VEGF and its Receptors in Human Placental Villi During Very Early Pregnancy: Differences Between Placental Vasculogenesis and Angiogenesis

Vascularization within the human placenta is the result of the de novo formation of vessels derived from pluripotent precursor cells in the mesenchymal core of the villi. Vascularization of placental villi starts at around day 21 post conception (p.c.) with a four somite embryo. At this stage progenitors of haemangiogenic cells differentiate to form first vessels. These progenitor cells are thought to be directly derived from mesenchymal cells rather than originating from fetal blood cells. We investigated the relation between differentiation of stromal cells towards endothelial cells and vascular structures and the expression pattern of the respective growth factors. Using transmission electron microscopy and immunohistochemistry (for VEGF, Flt-1, Flk-1, CD14, CD34, and CD68) the development of placental vasculogenesis during very early stages of pregnancy (days 22-48 p.c.) was studied. We found that VEGF is strongly expressed in villous cytotrophoblast cells and subsequently in Hofbauer cells while its receptors Flt-1 and Flk-1 are found on vasculogenic and angiogenic precursor cells. The developmental expression and secretion of VEGF suggests its involvement in recruitment, maintenance and formation of first angiogenic cells and vessels. Interactions between VEGF and Flk-1 and Flt-1 may regulate placental vasculogenesis and angiogenesis in a paracrine and autocrine manner. The sequential expression of growth factors in different cell types may point to the fact that placental vasculogenesis and angiogenesis are clearly distinct events.

Structural differentiation of human uterine luminal and glandular epithelium during early pregnancy: an ultrastructural and immunohistochemical study

The differentiation of human endometrial epithelium is a dynamic event that occurs throughout the menstrual cycle and early pregnancy. The structural transformation and differentiation of human uterine luminal and glandular epithelium of early human pregnancy (n=14) was investigated ultrastructurally and immunohistochemically using antibodies against cytokeratin (CT), endothelial marker CD31, Fas, and proliferating cell nuclear antigen (PCNA). Ultrastructurally, luminal epithelial cells showed distinctive euchromatic nuclei with prominent nucleoli and relatively loose cell membranes in all poles (apical to basal). Subcellular components were easily recognized in luminal epithelium except in degenerating cells. Mainly two cell types, dark and clear cells, formed the glandular epithelium. In the early gestation period, microvilli were abundant on the apical and apico-lateral poles of these cells. Only a few cytoplasmic projections were observed in dark cells. Numerous cilia were observed on the apical pole of some clear cells, located at the adluminal segment. In contrast, dark cells lacked cilia, nuclear channels, or giant mitochondrial profiles. Glycogen synthesis and apocrine secretion were recognizable for several days during early gestation. The apocrine secretory activity differed among dark cells of the glandular epithelium. The immunoreactivity of PCNA and Fas, and ultrastructural observations in the glandular epithelium suggest that, even in different segments of the same gland, epithelial cells do not regress during early gestation, but proliferate, perhaps representing a resistance against trophoblastic invasion. These morphological and molecular changes suggest that both luminal and glandular epithelium may play an important role in cellular defense and limitation for trophoblastic invasion during early pregnancy since plasma membrane alterations of the surface epithelium take place at the apical, basal and lateral poles compared to early secretory phase endometrial cells. Besides glandular epithelium may be consequently responsible for uterine secretions, which may be critical for early embryo development.

Structural changes in placental barrier of smoking mother. A quantitative and ultrastructural study

Human placental villi from all three trimesters of pregnancy, in 90 non-smoking pregnants and 244 cigarette smoking pregnants, were investigated. In smokers, the numbers of syncytial knots and cytotrophoblastic cells increased, depending on the cigarettes per day, syncytial buds and vasculo-syncytial membranes decreased as the pregnancy proceeded. The mean birth weight and placental weight in smokers were decreased, depending on the cigarettes per day at the third trimester. The ultrastructure of placental villi from smokers were compared with that on non-smokers. The villi from smokers had abnormalities of the microvilli, focal syncytial necrosis, decreased syncytial pinocytotic activity, degenerated cytoplasmic organelles. Using a morphometric method, the basement membranes were measured throughout pregnancy. The mean thickness of the basement membranes of trophoblastic layer and fetal capillary were found to not only increase with the maternal smoking during pregnancy but also attain the maximum in the heavy smokers by the third trimester of pregnancy. There was increased collagen in the villous stroma and shrinkage endothelial changes in fetal capillaries, in smokers the deleterious effect of the cigarette smoking on the placental barrier was heavy damage. As a result of impairment of placental barrier, the transports between mother and fetus were hampered.

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.

Fetal membranes and placenta of the African green monkey (Cercopithecus aethiops)

This study examined developmental changes in fetal membranes and placenta of Cercopithecus aethiops from a Carnegie developmental stage 12 embryo to near-term fetuses. Ultrastructurally, yolk sac cells (endoderm and mesothelium) were similar to comparable stages in other primates. Endodermal cells had few apical microvilli, abundant rough-endoplasmic reticulum, electron dense mitochondria and dense bodies. In contrast, mesothelial cells were squamous with numerous microvilli, small mitochondria and a few short strands of rough endoplasmic reticulum. Amnion cells early in gestation were squamous with few microvilli, large glycogen deposits and poorly developed cytoplasmic components. Tight junctions and desmosomes held adjacent cells together. The basal surface was smooth and the basal lamina was distinct. As development proceeded the amniotic cells became cuboidal and possessed numerous microvilli. Cytoplasmic organelles were better developed and glycogen deposits increased by mid-gestation. A thick layer of microfibrils and collagen fibers was prominent below the basal lamina. Near-term, the glycogen had virtually disappeared and the amount of lipid droplets increased. Basal infoldings and podocytic processes and the extracellular matrix had increased. The smooth chorion consisted of pseudostratified columnar cells. Cells had short microvilli, numerous granules and vesicles of variable size and electron density in early gestation. With increasing age, amounts of granules and vesicles decreased, as the endoplasmic reticulum became prominent. The chorionic trophoblast was a continuous layer in mid-pregnancy and its cells had well-developed organelles and inclusions. Late in gestation, the trophoblastic layer became discontinuous and wide intercellular spaces and channels were present. In the placenta, the trophoblastic elements showed features characteristic of primate placenta.