Trophoblastic invasion and the development of uteroplacental arteries in the macaque: immunohistochemical localization of cytokeratins, desmin, type IV collagen, laminin, and fibronectin

Uteroplacental circulation development: Doppler assessment and clinical importance

In the first weeks of pregnancy, columns of endovascular cytotrophoblastic plugs develop in the lumen of spiral arteries. Morphologic data show that these plugs become loosened as soon as the end of the second month and the intervillous circulation of maternal blood is likely to be established progressively between the 8th and 12th weeks. The disorganization of the musculo-elastic layers of these vessels provokes a dramatic decrease in vascular tone in the uteroplacental circulation. These modifications appear to govern the establishment of a low-pressure blood flow in the placenta, and hence determine the quality of uteroplacental circulation and normal fetal growth. Placental bed biopsies in women with pre-eclampsia and in a proportion of pregnancies with intrauterine growth retardation have shown that these physiologic changes were absent in the myometrial segments of spiral arteries. Recently, colour Doppler was used to assess intervillous and spiral artery flow in early pregnancy, confirming in vivo free intervillous flow at 12 weeks and a progressive significant decrease in spiral artery resistance with advancing gestation during the first trimester. However, certain data at an earlier gestational age are still contradictory. Particularly, the exact nature of the contents of the intervillous space before 8 weeks, and whether or not this fluid can be considered maternal blood, remains controversial.

Coexpression of cytokeratin and vimentin in mice trophoblastic giant cells

Trophoblastic giant cells reach their maximum size and exhibit a conspicuous synthetic and invasive activity during mouse placentation. The cytoskeleton, given the complex functions of the cells, shows a well-developed network of intermediate filament proteins. Immunohistochemistry combined with confocal and conventional immunofluorescence studies of intermediate filaments proteins cytokeratin and vimentin were performed in mice trophoblastic giant cells on days 9-11 of pregnancy. Specimens were fixed in phosphate-buffered formaldehyde and tissues were processed for routine paraffin embedding. Trophoblastic giant cells from antimesometrial, lateral or mesometrial uterine regions, through days 9-11 of pregnancy, expressed the same staining with both immunoperoxidase and immunofluorescent techniques. Cytokeratin filamentous structures were intensely immunoreactive and were detected throughout the cells cytoplasm; a few cells exhibited strongest fluorescence in the peripheral cytoplasm. Vimentin-positive staining was often distributed throughout the cells cytoplasm, most frequently and more intensely in the peripheral region; in some cells, it was present only in the peripheral regions. It is probable that expression of vimentin in midpregnancy trophoblastic giant cells may be associated with the rapid and conspicuous increase in size and synthetic activity of the cells and also with phagocytosis of degraded materials and invasion of decidual tissue.

Altered trophoblastic differentiation and increased trophoblastic invasiveness during delayed development in the short-tailed fruit bat, Carollia perspicillata

During pregnancy in the short-tailed fruit bat, lengthy post-implantational delays in conceptus development can occur in response to stress in captivity and seasonally in the wild. When comparisons were made between uteri carrying embryos in delay at the primitive streak stage and those growing more rapidly, many differences were noted. During delay the developing chorioallantoic placenta was generally smaller, contained a higher ratio of cytotrophoblast to syncytiotrophoblast, and had been invaded only to a limited extent on its embryonic side by mesoderm. Furthermore, much of the cytotrophoblast appeared relatively undifferentiated, randomly-oriented, linked primarily by primitive junctions, and lacked a basal lamina. In contrast, in placentae serving somite and limb-bud stage embryos, sizeable areas were noted that consisted only of more highly differentiated syncytiotrophoblast perforated by maternal vascular spaces (trophospongium). The first contact of the allantois with the developing placenta was also noted at the somite stage, and this initiated widespread invasion of the placenta by mesenchyme and allantoic blood vessels. Wherever this invasion had occurred, the cytotrophoblast between the mesenchyme and syncytiotrophoblast of the interhaemal barrier consisted of a single, polarized layer of more differentiated cells with an associated basal lamina. Eventually, all of the trophospongium was invaded by cytotrophoblast and vascularized fetal mesenchyme. These observations suggest that in addition to its germinal function, cytotrophoblast in this bat may play a major role in controlling mesenchymal invasion and angiogenesis on the embryonic side of the placenta. During the period of delay, highly invasive trophoblast is also released by the placenta. This invades the myometrium and sometimes extrauterine tissues via interstitial migration along maternal capillaries and veins.

Assessment of hormonally active agents in the reproductive tract of female nonhuman primates

Using the ovariectomized macaque model of postmenopausal women's health, we investigated the effects of long-term treatments (5 weeks-3 years) with estradiol, conjugated equine estrogens (CEE), esterified estrogens, progestins such as medroxyprogesterone acetate (MPA) and nomegestrol acetate, CEE + MPA, tamoxifen, soybean phytoestrogens (SPEs), a variety of putative selective estrogen receptor modulators (SERMs), and androgens. Agents tested were selected on the basis of beneficial effects on arteries and/or bone. Doses were scaled on a caloric or serum-concentration basis to approximate human clinical doses. We evaluated endometrial and mammary gland histopathology and morphometry and used immunohistochemistry to evaluate cell proliferation and expression of estrogen receptor alpha and progesterone receptor (PR). Both estradiol and CEE induced endometrial hyperplasia. MPA antagonized epithelial proliferation induced by CEE in endometrium and induced pseudodecidual stromal hyperplasia in some animals. Tamoxifen induced endometrial polyps, cystic hyperplasia, stromal fibrosis, and PR expression but not Ki-67 expression. SPEs were not estrogenic at dietary doses and antagonized estrogen-induced proliferation in the endometrium and breast. Nandrolone induced mucometra and an adenomyosis-like change. The potential SERM 17 alpha dihydroequilenin did not have uterotrophic or mammotrophic effects. In general, experimental findings in macaques have been predictive of outcomes in human clinical trials of the same agents.

Unusual patterns of intermediate filament protein expression by the trophoblast and decidual cells of the short-tailed fruit bat, Carollia perspicillata

In the short-tailed fruit bat (Carollia perspicillata) pregnancy can be prolonged by the occurrence of lengthy delays after implantation. This is associated with the development of highly invasive trophoblast that can penetrate the myometrium, mesenteries of the reproductive tract and the oviducts via perivascular (interstitial) routes. In order to confirm the identity and distribution of this trophoblast, intermediate filament protein immunocytochemistry was utilized. In some respects the expression of these proteins differed from what has been reported for more commonly-studied species. Cytotrophoblast in the placenta, its cytotrophoblastic shell and the highly invasive trophoblast strongly expressed cytokeratins. As pregnancy progressed, however, cytokeratin expression by syncytiotrophoblast lining much of the placental labyrinth became very weak. The cytotrophoblastic shell and highly invasive trophoblast also expressed vimentin. The highly invasive trophoblast was unusual in that it developed dendritic processes that sometimes extended out into adjacent tissues in great profusion. Decidual cells generally expressed desmin and vimentin; however, some also coexpressed cytokeratins. These observations indicate that some of the trophoblast in Carollia undergoes a significant epithelial-mesenchymal transformation. They also suggest that caution should be exercised in relying upon intermediate filament proteins as markers for cell identification purposes in exotic species, or when the patterns of protein expression by fetal and maternal cells might be altered in pathological or experimental situations.

The vitronectin receptor plays a role in the adhesion of human cytotrophoblast cells to endothelial cells

During placental development in higher primates trophoblast cells invade maternal blood vessels and migrate along the luminal surface of endothelium. In the present study, the adherence of human cytotrophoblast cells to endothelial cells has been characterized to test the hypothesis that vitronectin receptors (alpha(v) integrins) play a role in intra-luminal trophoblast migration. Adherence was measured using a quantitative fluorescence-based assay and was found to increase in a time-dependent fashion up to about 2 h after which it leveled off. Adhesion was detectable at 4 degrees C but was greatly reduced compared to that seen at 37 degrees C. Adhesion was partially blocked by antibodies against alpha(v)beta3/beta5 integrin, beta1 integrin and by antibodies against P-selectin. Antibodies against beta3 integrin subunits had no effect. Adhesion was reduced by galactose-6-phosphate and fructose-6-phosphate. Flow cytometric analysis revealed alpha(v) integrin on the surface of cytotrophoblast and endothelial cells. Beta1 integrin was detected on the surface of endothelial cells and on cytokine-stimulated cytotrophoblast cells. Beta3 and beta5 integrins were not detected on the surface of either cell type, although beta3 was detected using permeabilized endothelial cells. These results raise the possibility that alpha(v) integrins expressed by both cytotrophoblast cells and endothelial cells, and P-selectin expressed by endothelial cells, may be important in facilitating trophoblast adhesion and migration along the uterine microvasculature.

Vitronectin receptors are expressed by macaque trophoblast cells and play a role in migration and adhesion to endothelium

The objective of this work was to develop an in vitro system that would extend the usefulness of the macaque as a model for studying trophoblast invasion and spiral artery modification. We sought to determine whether trophoblast cells isolated from early gestation macaque placentas expressed vitronectin receptors and tested the idea that these receptors play a role in trophoblast migration and adhesion. Cytotrophoblast cells were isolated from 40-100 day macaque placentas, cultured, and characterized by immunofluorescence microscopy and flow cytometry. The cells expressed alphaV, beta3, and beta1 integrins on their surfaces. Immunohistochemical analysis of early gestation placentas and decidua basalis confirmed that intravascular trophoblast cells express alphaVbeta3/beta5. Using migration chambers we found that the trophoblast cells migrated towards vitronectin but not towards bovine serum albumin. This specific migration was blocked by preincubating the trophoblast cells with anti-vitronectin receptor (alphaVbeta3/beta5) antibodies. In other experiments, macaque trophoblast cells adhered to myometrial endothelial cells in a time-dependent manner and adhesion was significantly blocked by antibodies against alphaVbeta3/beta5 integrin. The results suggest that vitronectin receptors expressed by macaque trophoblast cells play a role in the migratory activity of these cells and may also be important in mediating attachment to endothelium.

First trimester human endovascular trophoblast cells express both HLA-C and HLA-G

PROBLEM

In human pregnancies, trophoblasts, in contrast to placental connective tissue and the fetus itself, come into direct contact with the maternal allorecognizing system at special sites. Villous syncytiotrophoblasts washed around by maternal blood lack HLA class I proteins, whereas extravillous trophoblasts, which deeply invade maternal uterine tissues, express high amounts of HLA-G and also HLA-C, the latter to a lesser degree, however. A subpopulation of extravillous trophoblasts, the endovascular trophoblast, enters maternal spiral artery lumen and, like syncytiotrophoblast, comes into direct contact with maternal blood. Less is known about HLA class I distribution on this endovascular trophoblast subpopulation.

METHOD OF STUDY

A comparative immununohistochemical analysis was done on decidual cryo-sections containing trophoblast-invaded spiral arteries using different anti-HLA class I monoclonal antibodies (mAbs) and a peroxidase-labeled streptavidinbiotin detection system.

RESULTS

MAbs W6/32 (anti-HLA-A, -B, -C, -G), HCA2 (anti-HLA-A, -G) G233 and 87G (both anti-HLA-G) resulted in strong positivity on endovascular trophoblasts. L31 (anti-HLA-C) and HC10 (anti-HLA-B, -C) revealed clear positivity, whereas TU149 (anti-HLA-B, -C, some -A) produced a heterogeneous staining pattern, faintly positive on some endovascular trophoblastic cells and negative on others. MAb LA45 (anti-HLA-A, -B) did not bind to any endovascular trophoblast, neither did BFL.1 (anti-HLA-G) nor 16G1 (anti-HLA-G, soluble).

CONCLUSION

This study shows that trophoblastic cells belonging to the endovascular subpopulation express considerable amounts of HLA-G and slightly less HLA-C.

Comparative placental structure

The primary function of all placentas is to act as an interface between the mother and fetus that allows, and even promotes, appropriate metabolic exchanges. This function is accomplished by bringing maternal and fetal blood into close apposition while maintaining separation of the maternal and fetal circulatory systems. Despite the common physiological functions shared by placentas, however, examination of placental morphology from different animal groups reveals a remarkable diversity of species-specific structural organization.The separation of fetal and maternal blood is always maintained by an elaboration of extraembryonic fetal tissues that cover fetal blood vessels. In some species the outermost layer of this fetal tissue, the trophoblast, is in direct contact with maternal blood. In many other species uterine tissues also contribute to the selective barrier separating the two blood systems. In addition to morphological variation among placentas of different animal groups, placentas undergo substantial structural modifications during pregnancy in a single species. In some animals different types of placentas function successively, or concurrently during a single pregnancy.As a result of these myriad details of placental structure, effective evaluation of fetal-maternal transfer of drugs must consider not only the components of the interhemal barrier of the fully developed placenta characteristic for each species, but also the placental structures functioning at each gestational stage of the fetus.

Atherosis revisited: current concepts on the pathophysiology of implantation site disorders

There are two distinct histological manifestations of impaired placental implantation in humans--incomplete trophoblastic vascular invasion and atherosis. Both have been described to occur in pregnancies affected by a variety of disorders such as preeclampsia, fetal growth restriction, systemic lupus erythematosus, and diabetes. Our purpose was to integrate recent developments in the understanding of implantation site disorders into a pathophysiological scenario that interrelates these placentation disorders and associated pregnancy complications. Sources were identified from a MEDLINE search of English-language articles published from 1966 to 1997. Additional sources were identified from references cited in relevant reports. We selected articles relating to the following topics: atherosis, implantation site disorders, trophoblastic invasion, preeclampsia, fetal growth restriction, implantation site development, atherosclerosis, and endothelial activation-damage. A contemporary version of normal placentation, including vascular adaptation, was reviewed with comments on normal trophoblastic differentiation and vascular invasion. Specific abnormalities of the implantation site, including atherosis and incomplete trophoblastic invasion, were discussed in the context of placental site hypoperfusion and the association with pregnancy complications. It was concluded that atherosis and incomplete trophoblastic invasion may be both a consequence and a cause of placental site hypoperfusion resulting in the development of preeclampsia and a variety of other pregnancy disorders.

Immunohistochemical localization of receptors for progesterone and oestradiol-17 beta in the implantation site of the rhesus monkey

The aim of the present study was to examine the cellular basis of the involvement of oestradiol and progesterone in blastocyst implantation in the primate. To this end, the cellular distribution of receptors for oestradiol (ER) and progesterone (PR) in fetal trophoblast cells and in endometrial compartments of timed lacunar (pre-villous) and villous stages of placentation in primary implantation sites collected on days 13-22 of gestation were investigated in rhesus monkeys. Both in pre-villous stage tissue and in villous stage tissue, cytotrophoblast cells and syncytiotrophoblast cells and other trophoblast derived cells were PR positive, while they were generally ER negative. Maternal endometrial cells were ER negative, while epithelial cells, stromal cells and vascular endothelial cells in maternal endometrium showed heterogeneous staining patterns for PR depending on their relative location; these patterns, however, correlated well with glandular hyperplasia and differentiation, stromal-decidual transformation and vascular response seen during blastocyst implantation.

Trophoblast pseudo-vasculogenesis: faking it with endothelial adhesion receptors

During early development, a subset of fetal (placental) cytotrophoblasts exhibits tumor-like behavior and invades the uterus. To access a supply of maternal blood, they invade arterioles and form heterotypic interactions with, and replace, resident maternal endothelium, creating a hybrid uterine vasculature. Recently, it has become clear that invading cytotrophoblasts transform their adhesion receptor phenotype to resemble the endothelial cells they replace. Furthermore, they express vasculogenic factors and receptors. Is this a form of vasculogenesis?

Cytokines and embryo implantation

The implantation process is currently considered the most relevant limiting factor for successful pregnancy. It is evident that molecular interactions at the embryo-maternal interface at the time of implantation are crucial in order to understand the mechanisms of embryonic implantation. The principal aim of this study is to demonstrate the existence of specific communication pathways between the human embryo and endometrium. The molecular interactions appears to be initiated by the endometrium in the presence of an implanting blastocyst and is mediated through embryonic cytokines (specifically the IL-1 system) and the target is the endometrial epithelial beta3 integrin subunit. If the role of beta3 is accepted as a marker of uterine receptivity these results may indicate that the normal hormonally regulated human endometrium could be the trigger for molecular events preparing the blastocyst to communicate effectively and regulate endometrial adhesion molecules in order to implant.

Evidence for basic fibroblast growth factor as a crucial angiogenic growth factor, released from human trophoblasts during early gestation

The objective of this study was to clarify the possible angiogenesis-promoting factors from human trophoblasts in early stage gestation. The existence of angiogenic growth factors such as basic fibroblast growth factor (bFGF) and vascular endothelial growth factor (VEGF) in the condition medium from human villous trophoblasts was determined. Biological activity of angiogenic growth factors released by trophoblasts was examined using vascular endothelial cell lines. The condition medium from trophoblasts enhanced the growth of endothelial cells. Although cultured trophoblasts exhibited immunoreactive products for both bFGF and VEGF in the cytoplasm, only bFGF was detected in the condition medium by ELISA. The growth-enhancing activity of the condition medium was eliminated completely by the addition of anti-bFGF antibody but not with anti-VEGF antibody. Thus, trophoblastic cells seem to play an important role in extensive angiogenesis occurring in early gestation, mainly by releasing bFGF but not VEGF.

The extent of trophoblast invasion in the preplacental vasculature of the guinea-pig

Pregnancy-induced structural changes in spiral arteries seem to be a prerequisite for successful fetal outcome in humans. It is unknown whether these changes also occur in other preplacental vessels (radial and arcuate arteries) in normal pregnancies. Since the radial and arcuate arteries need to dilate in order to accommodate the increase in placental blood flow during pregnancy, it is expected that they are also invaded by trophoblast and respond with structural changes. The objective of the present study was to evaluate the extent of trophoblast invasion in the guinea-pig preplacental vasculature and its effect on the vascular structure of mesometrial, myometrial and arcade arteries. Under general anaesthesia the vascular system of non- (n = 4), mid- (n = 4) and late- (n = 8) pregnant guinea-pigs was fixed by immersion or perfusion. Cross-sections of immersion-fixed mesometrial and arcade arteries were stained with toluidine blue. Cross-sections of perfusion-fixed mesometrial, myometrial and arcade arteries were stained with haematoxylin-eosin, Elastica van Gieson staining and antibodies against alpha-smooth-muscle-actin (ASMA), cytokeratin and factor VIII, to detect vascular smooth muscle, trophoblastic, and endothelial cells, respectively. In addition, the external and internal vascular circumference of sections from perfusion-fixed tissue was determined. All cross-sections were evaluated by light microscopy. In the course of pregnancy, progressive endothelial swelling, disappearance of the elastic lamina interna and disarrangement of the tunica media were observed in the myometrial and throughout the mesometrial arteries up to the junction with the arcade arteries. These changes coincided the migration of keratin-positive staining giant cells. It is concluded that in normal guinea-pig pregnancy, structural changes occur in the entire mesometrial artery and at least a part of the myometrial artery, although such changes were not seen in the arcade artery.

Immunohistochemical examination of trophoblast populations in human first trimester and term placentae and of first trimester spiral arteries with the monoclonal antibody GZ 112

This paper presents an immunohistochemical study with a monoclonal mouse antibody GZ 112, an IgG1 kappa, which is directed against an antigen expressed in first trimester placenta by all proliferative and invasive extravillous trophoblast populations including a population of Langhans cells that represent extravillous stem cells. Additionally, the GZ 112 antigen is associated with morphological changes of spiral arteries preceding local trophoblast invasion. In term placentae, GZ 112 also strongly reacts with all extravillous trophoblast populations, but additionally recognizes partly villous cytotrophoblast and syncytiotrophoblast too, displaying a heterogeneous staining pattern. GZ 112 is directed against a 42-KDa antigen. Intracytoplasmic network-like staining and cross-reactivity with various human surface and glandular epithelia indicate a cytokeratin intermediate filament or a cytokeratin intermediate filament associated molecule as antigen.

Immunohistochemical localization of fibrillin in developing macaque and term human placentas and fetal membranes

The objective of this study was to examine the developmental appearance of the extracellular matrix glycoprotein fibrillin in macaque placentas and fetal membranes and to compare this distribution to that seen in term human placentas and fetal membranes. Standard immunoperoxidase methods were used on paraformaldehyde-fixed, paraffin-embedded tissues. At early gestational ages (26-30 days), fibrillin was found in cell columns and cytotrophoblastic shell, with weak staining in the villous stroma. Staining was abundant in the shell and columns at 53 days as well, and stronger staining was seen in the stroma of the chorionic plate and stem villi. Staining in the shell and remnants of the cell columns in later gestation continued to be positive, though variable. Generally, the strongest staining was present in the distal cytotrophoblastic shell. Stroma in the tips of anchoring villi was also strongly positive. Later in gestation, fibrillin was observed around the multilayered cytotrophoblast of the chorionic plate. Fibrillin was abundant in the stromal cores of human term placental villi. In early macaque amnion, fibrillin staining was abundant in a layer beneath the amniotic epithelium. Later in gestation, macaque chorioamnion staining was generally similar to human term chorioamnion staining, with the heaviest staining in portions of the compact and reticular layers. Fibrillin was sometimes localized in regions known to be rich in connective tissue microfibrils, but, in other regions known to have abundant microfibrils, fibrillin staining was weak. This suggests that some microfibrils in placenta may be composed predominantly of some other protein(s). The function of fibrillin in the various placental compartments is unknown at present. It may provide attachment points for cells while at the same time providing a strong, yet flexible, matrix to accommodate growth particularly in areas subject to shear stress.

Cytodifferentiation of trophoblast in the anchoring villi and trophoblastic shell in the first half of gestation in the macaque

The structure of cytotrophoblast cells in cell columns of the anchoring villi and trophoblastic shell were studied by light and transmission electron microscopy during days 19-70 of pregnancy in the macaque. Additional placentas were prepared for 3H-thymidine autoradiography to demonstrate DNA synthesis. The cell columns of the anchoring villi consist of proximal, mid, and distal regions, and similarly the trophoblastic shell has a region forming the base of the intervillous space, a central zone, and a junctional zone. The proximal region of the cell column is composed of closely apposed, largely undifferentiated cytotrophoblast cells lacking intercellular space. In the mid region, the cells, which contain segregated glycogen areas, are separated into branching strands by extensive extracellular matrix. The distal zone abuts the trophoblastic shell and has fewer mostly peripheral cytotrophoblast cells and abundant matrix. Where cytotrophoblast of the shell is exposed to intervillous space or underlies syncytial trophoblast bordering the intervillous space, pads of cuboidal cells are seen. The central zone of the shell is arranged in strands of contiguous cells. Near the maternal junctional zone, the cells have fewer junctional regions and contain lipid rather than glycogen. Results of thymidine incorporation studies are consistent with observations of others using Ki67 distribution, indicating that cell replication occurs in the proximal region of the cell column, with subsequent migration of cells into the trophoblastic shell. Changes with age include an increase in extracellular matrix in the proximal region, a more linear organization in the expanded central zone of the shell, and a decrease in necrosis at the junction of the shell with endometrium, resulting in close association of shell cytotrophoblast cells, maternal decidual cells, large granular lymphocytes, and macrophages. It is concluded that the cytotrophoblast of the cell columns and trophoblastic shell is a pleomorphic cell type responding to adjacent constituents including the matrix it forms.

Human cytotrophoblasts adopt a vascular phenotype as they differentiate. A strategy for successful endovascular invasion?

Establishment of the human placenta requires that fetal cytotrophoblast stem cells in anchoring chorionic villi become invasive. These cytotrophoblasts aggregate into cell columns and invade both the uterine interstitium and vasculature, anchoring the fetus to the mother and establishing blood flow to the placenta. Cytotrophoblasts colonizing spiral arterioles replace maternal endothelium as far as the first third of the myometrium. We show here that differentiating cytotrophoblasts transform their adhesion receptor phenotype so as to resemble the endothelial cells they replace. Cytotrophoblasts in cell columns show reduced E-cadherin staining and express VE-(endothelial) cadherin, platelet-endothelial adhesion molecule-1, vascular endothelial adhesion molecule-1, and alpha-4-integrins. Cytotrophoblasts in the uterine interstitium and maternal vasculature continue to express these receptors, and, like endothelial cells during angiogenesis, also stain for alphaVbeta3. In functional studies, alphaVbeta3 and VE-cadherin enhance, while E-cadherin restrains, cytotrophoblast invasiveness. Cytotrophoblasts expressing alpha4 integrins bound immobilized VCAM-1 in vitro, suggesting that this receptor-pair could mediate cytotrophoblast-endothelium or cytotrophoblast-cytotrophoblast interactions in vivo, during endovascular invasion. In the pregnancy disorder preeclampsia, in which endovascular invasion remains superficial, cytotrophoblasts fail to express most of these endothelial markers (Zhou et al., 1997. J. Clin. Invest. 99:2152-2164.), suggesting that this adhesion phenotype switch is required for successful endovascular invasion and normal placentation.

Expression of platelet-endothelial cell adhesion molecule-1 (PECAM) by macaque trophoblast cells during invasion of the spiral arteries

BACKGROUND

Placental development in higher primates is characterized by the invasion of uterine blood vessels by trophoblast cells. These cells proceed to migrate within uterine spiral arteries, opposite to the direction of normal blood flow. Observations indicate adhesion of intra-arterial trophoblast to endothelium as well as to adjacent trophoblast cells.

METHODS

Macaque placenta and endometrial tissues were collected from day 15 of pregnancy (implantation begins on day 9) to term. Standard indirect immunoperoxidase methods were used to identify platelet-endothelial cell adhesion molecule-1 (PECAM), cytokeratins, and factor VIII-related antigen.

RESULTS

In early specimens arterioles were often nearly occluded by cytokeratin-labeled trophoblast cells. Adjacent sections revealed the presence of PECAM on these trophoblast cells and on the endothelium. After day 30 the invaded arteries usually contained a re-formed lumen, and trophoblast cells were increasingly evident in the modified walls of arteries, where PECAM labeling was often reduced on cells distant from the lumen. Endothelium of both invaded and uninvaded uterine vessels retained PECAM reactivity throughout gestation. Trophoblast cells of the cell columns, cytotrophoblastic shell, and mid- to late-gestation chorionic plate were also reactive for PECAM. Villous cytotrophoblast cells did not express PECAM, but a dense border of PECAM was consistently present on the apical surfaces of syncytial trophoblast.

CONCLUSIONS

Because PECAM functions as a counter-ligand for PECAM via homophilic binding, we conclude that this molecule is directly involved in adhesion of trophoblast cells to arterial endothelium in addition to maintaining cohesion between some subpopulations of cytotrophoblast cells.

The fibrinoids of the human placenta: origin, composition and functional relevance

Placental fibrinoids are extracellularly deposited materials which are histologically glossy and acid staining, and can be found in every normal and pathological placenta at all stages of pregnancy. The amount of fibrinoid is, in general, independent of pregnancy outcome and fetal wellbeing. According to new findings, the classical histological term "fibrinoid" covers two distinctive extracellular matrices which differ as regards structure, composition and function. Fibrin-type fibrinoid is mostly composed of fibrin together with other molecules derived from blood clotting or degenerative processes. It is mainly a maternal blood-clot product which is used (a) to adapt the intervillous space to optimized flow conditions and (b) to control growth of the villous trees by encasing new villous branches which caused intervillous stasis or turbulence of maternal blood. Moreover, fibrin-type fibrinoid replaces degenerative syncytiotrophoblast at the maternofetal exchange surfaces, thus acting as a kind of substitute barrier. Matrix-type fibrinoid is a secretory product of invasive extravillous trophoblast cells. It shares some similarities with basement membranes, however, it is secreted in an apolar fashion, embedding the secreting cells. Like basement membranes, it contains laminins, collagen IV, and heparan sulfate. In addition, oncofetal fibronectins, vitronectin, and i-glycosylated molecules but no collagens I, III, and VII can be found. Matrix-type fibrinoid is thought to regulate trophoblast invasion by specific interactions with cell surface integrins. As a kind of "glue", it anchors the placenta to the uterine wall and seems to play an important role in materno-fetal immune interactions at this particular site. Both types of fibrinoid are usually co-localized, thus indicating close morphogenetic and functional interrelations.

Differential expressions of collagen types IV, III, and I during the development of invasive trophoblasts in rats

We examined the differential expressions of collagen types IV, III and I in the developing feto-maternal placental tissue of pregnant rats by a combination of in situ hybridization and immunohistochemistry. At day 9.5 of gestation, polygonal invasive cytotrophoblasts from the ectoplacental cone, which was modifying the maternal central artery, revealed intensely expressed alpha 1 (IV) and alpha 1 (III) collagen mRNAs. The localization patterns of these translated products, collagen type IV and procollagen type III, were slightly different in the invasive cytotrophoblasts. Collagen type IV densely deposited intracellularly and intercellularly in the maternal central artery and in the thickened basement membranes of the cytotrophoblasts. However, expression of alpha 1 (I) collagen mRnA and procollagen type I was hardly detectable in the cytotrophoblasts. At day 13 of gestation, a high level of alpha 1 (IV) collagen mRNA was expressed in the cytotrophoblastic cell layer (trophospongium) and in the invasive large cytotrophoblasts. A moderate level of alpha 1 (III) collagen mRNA was also expressed mainly in the cytotrophoblasts, while alpha 1 (I) collagen mRNA was expressed at very low levels. Interestingly, procollagen type III failed to show linear immunoreactivity in the subepithelial extracellular matrix beneath the maternal artery with the invasive cytotrophoblasts. Additional quantitative analyses of these type IV, III, and I collagen mRNA levels in in situ hybridization experiments between several cell types also revealed significant differences individually. Electron-microscopic study detected no cross-striated collagen fibers in the thickened basement membrane-like structures adjacent to the invasive cytotrophoblasts. Fibrillar and basement membrane collagen gene expressions, their protein syntheses, and the processing of these procollagens seems to be developmentally regulated in the invasive cytotrophoblasts during the organization of feto-maternal placental tissue. The remodeling of the maternal central artery by the invasive cytotrophoblasts is important for ensuring the adequate blood supply to the developing placenta and fetus.

Localization and distribution of vasoactive neuropeptides in the human placenta

Neuropeptides play an important role in the regional regulation of blood flow and hormone secretion. Few studies report the presence of peptides in the human placenta. Our experiment evaluates neuropeptides in the human placenta using immunocytochemical techniques. Representative tissue sections from full-term placentae were fixed immediately after delivery and processed into paraffin sections or frozen. They were treated with multiple immunofluorescence, streptavidin-biotin-peroxidase complex and immunogold-silver staining techniques in combination with well-established monoclonal and polyclonal antibodies, using appropriate absorption controls to ensure the validity of the staining. Vasoactive intestinal polypeptide (VIP), calcitonin gene-related peptide (CGRP), neuropeptide tyrosine (NPY), galanin, somatostatin, met-enkephaline, helodermin and substance P-like immunoreactivities were demonstrated within decidual cells. Endothelin-1 was found in both trophoblasts and endothelial cells. Peptide immunoreactivities in the human placenta especially at the decidual interface between mother and fetus supports a role for the diffuse neuroendocrine system (DNES) in the regulation of placental blood flow critical for fetal growth and development.

Extracellular matrix components of the placental extravillous trophoblast: immunocytochemistry and ultrastructural distribution

Invasive extravillous trophoblast cells of the human placenta are embedded in a self-secreted extracellular matrix, the matrix-type fibrinoid. The ultrastructure and molecular composition of the matrix-type fibrinoid of the term human placenta were studied by transmission electron microscopy and immunogold labelling. We used antibodies directed against different matrix proteins such as collagen type IV, laminin, vitronectin, heparan sulfate, various fibronectin isoforms, and against the oncofetal blood group antigen, "i". Immunogold labelling patterns of matrix proteins are the basis for the subdivision of the trophoblast-derived matrix-type fibrinoid into mosaic-like patches of structurally and immunocytochemically different compartments. Firstly, fine granular patches with structural similarities to basal lamina material are composed solely of collagen type IV and laminin. Secondly, an ultrastructurally amorphous glossy substance shows reactivity with antibodies against heparan sulfate and vitronectin. A third type of patches, fine fibrillar networks embedded in the above-mentioned glossy matrix, are reactive with antibodies against normal fibronectin isoforms (IST-4, IST-6, IST-9) and oncofetal isoforms (BC-1, FDC-6). The blood group precursor antigen "i" was not only expressed on the surfaces of the extravillous trophoblast cells but was associated with the fibronectin-positive fibrils. In conclusion, within this extracellular matrix, clear compartments of different composition can be distinguished from each other. Glycosylation with "i" in this matrix may be involved in immunological masking, thus preventing rejection of placenta and fetus.

Evaluation of trophoblast invasion in placental bed biopsies of the baboon, with immunohistochemical localisation of cytokeratin, fibronectin, and laminin

Biopsies of placentas (n = 21), placental bed (n = 17) and decidua (n = 26) of various gestation periods (30 to 140 days) were used to study trophoblast invasion in the baboon. Application of immunohistochemical staining for cytokeratin allowed proper identification of trophoblast. Earlier reports showing restricted trophoblast invasion in this species were confirmed by the finding that endovascular trophoblast was present in only one third of biopsies containing spiral arteries. Moreover, immunostaining for cytokeratin revealed that in several arteries only a few isolated trophoblastic cells were present, while the vessel had not undergone the normal physiological change. Trophoblast invasion could only be detected within decidual, but not in myometrial, segments of spiral arteries. Interstitial trophoblast invasion was very limited and multinuclear giant cells were absent. Immunohistochemical staining suggested a contribution of laminin to the fibrinoid deposition within the physiologically changed spiral arteries, while fibronectin was present intracellularly in the invaded trophoblast. Because of differences in the trophoblast invasion pattern, the baboon cannot be regarded as a satisfactory experimental model to explore results of inadequate endovascular trophoblast invasion which, in the human, leads to pregnancy complications such a preeclampsia.

Macaque intra-arterial trophoblast and extravillous trophoblast of the cell columns and cytotrophoblastic shell express neural cell adhesion molecule (NCAM)

BACKGROUND

During placental development in higher primates trophoblast cells originating in the cell columns migrate along endometrial surfaces to form the cytotrophoblastic shell. A subpopulation of these cells invades uterine arteries, where they migrate on the surface of endothelium, against the flow of blood. These intra-arterial cells become sequestered in the walls of the arteries where they are referred to as intramural trophoblast. Because migration depends upon binding the cell surface to other cells or to extracellular matrix, we investigated the potential role of neural cell adhesion molecule (NCAM, CD56) in arterial invasion by trophoblast cells.

METHODS

Tissues from macaque placentas and endometrium were fixed and embedded in paraffin. Standard immunoperoxidase methods were used to identify NCAM.

RESULTS

NCAM labeled cells were present within spiral arteries of the decidua basalis. Trophoblast cells within or adjacent to the arterial lumen were distinctly NCAM positive, whereas most intramural trophoblast cells revealed reduced or no reactivity for NCAM. Maternal endothelial cells and villous trophoblast cells were negative. Uterine veins were tapped by trophoblast but did not contain migratory cells. The cell columns of early pregnancy contained cells outlined by rims of reactivity for NCAM. Labeled cells were also seen in the cytotrophoblastic shell, arranged as discontinuous groups. These groups variably occupied the entire thickness of the shell, or only the proximal (adjacent to the intervillous space) or distal layers of the shell. Later in gestation, the cytotrophoblastic shell developed a different pattern of staining, such that only cells located nearer to the intervillous space or near the tips of the anchoring villi were positive.

CONCLUSIONS

We conclude that NCAM is one of a group of cell adhesion molecules that participates in trophoblast cell adhesion during migration within maternal arteries. NCAM appears to be active in trophoblast-trophoblast cell interactions. It may also contribute to binding trophoblast cells to the surfaces of arterial endothelium as well as extracellular matrix molecules of the arterial wall. After secondary invasion into the arterial walls NCAM is down-regulated, possibly in response to the appearance of extracellular matrix capsules, where other cell adhesion molecules may appear.

Invasive processes in the normal Harderian gland of Syrian hamster

In this contribution we will pay special attention to several morphological findings that we can observe, under some circumstances, in the normal Harderian gland of the Syrian hamster. The accumulation of porphyrins in this gland results in mitochondrial damage and extensive cell death. Many damaged cells are secreted into the lumen of the tubule-alveoli, but most of them seem to produce an invasive process that even affects the vascular components of the gland. In this way, many blood vessels are invaded and appear partially filled with the invasive mass, which sometimes totally occludes the lumen of the vessels. We have also observed other surprising features related to a special kind of activity in certain secretory cells. Such activity results in a peculiar "segregation" of a cytoplasmic fragment, containing the nucleus. The affected cells seem to gather up their cytoplasm and nucleus towards the basal zone, while the rest of the cell, including practically the whole amount of lipid droplets, is relegated to the vicinity of the lumen. All these phenomena seem finally to result in the detachment of some clusters, composed of a limited number of cells, which display a basophilic cytoplasm practically free of lipid droplets.

Assessment of early placental development in the cynomolgus monkey (Macaca fascicularis) using colour and pulsed wave Doppler sonography

Colour flow mapping and pulsed wave Doppler were used to assess the process of placental growth and development in the cynomolgus monkey from 32 to 71 days gestational age. Fetal and maternal vessels were reliably visualised and insonated. Accurate longitudinal non-invasive assessment of placentation is possible using this technique.

Keratinocyte growth factor and its receptor in the rhesus macaque placenta during the course of gestation

Keratinocyte growth factor (KGF) is synthesized and secreted exclusively by mesenchymal cells, and acts through its receptor (KGFR) to stimulate epithelial proliferation. In vivo, KGF and KGFR comprise a mesenchymal-epithelial cell paracrine system that can mediate epithelial cell mitosis. In preliminary work, we noted that KGF was expressed in the rhesus monkey placenta, and we now report on the expression of placental KGF and KGFR mRNAs during the course of gestation in this species. In-situ hybridization revealed that during early gestation, KGF mRNA was strongly expressed in placental mesenchymal cells. These cells, which were also immunoreactive for vimentin, were mainly located on the periphery of the mesenchymal cores of both anchoring and floating villi. KGFR mRNA was expressed in the adjacent trophoblastic epithelium, which was immunoreactive for cytokeratin. In-situ hybridization revealed that KGF mRNA expression was very high in the youngest placentae (34-days gestation) and decreased gradually to minimal levels by late gestation (157 days). Northern blot analysis indicated also that the KGF MRNA signal was strongest in early gestation samples and weakest by late gestation. Analysis for KGFR mRNA by a reverse transcriptase-polymerase chain reaction technique showed that KGFR mRNA expression could be detected at all stages. However, in-situ hybridization indicated that KGFR mRNA expression was highest in early gestation placentae and least in the oldest placentae. Autoradiographs of frozen sections of placenta that had been incubated with [125I]KGF to detect receptor binding showed that grain density over the trophoblast was highest in the youngest and least in the oldest placentae. PCNA and Ki-67 expression followed this same temporal trend. We conclude that the KGF/KGFR system may be important in proliferation of the placental trophoblast during early- to mid-pregnancy in rhesus monkeys.

Physiological dilation of uteroplacental arteries in the guinea pig depends on nitric oxide synthase activity of extravillous trophoblast

The trophoblast invasion of uteroplacental arteries in the guinea pig has been studied by means of electron microscopy and immunohistochemisty. To identify trophoblast cells, smooth muscle cells, and endothelial cells, antibodies against cytokeratins, smooth muscle myosin, desmin, and vimentin were employed. Furthermore, the immunohistochemical expression patterns of nitric oxide synthase isoforms (eNOS, mNOS and bNOS) were studied and were compared with the enzyme histochemical staining for NADPH-diaphorase. Dilation of uteroplacental arteries begins prior to day 30, when trophoblast cells that coexpress endothelial and macrophage nitric oxide synthase can be found in the vicinity of the vessels and replace the surrounding peritoneal mesothelium. Trophoblast invasion of the arterial walls and the subsequent wall destruction are only secondary effects. Starting around day 50, the final steps of pregnancy-dependent vessel modifications involve intraarterial trophoblast adhesion to the endothelium and subsequent replacement of the endothelium by the trophoblast cells. These may centrifugally invade the vessel media eventually forming intraluminal plugs. These findings led us to the conclusion that in the guinea pig pregnancy-induced physiological dilation of the uteroplacental arteries is due to the effect of nitric oxide rather than being caused by trophoblast-induced media destruction.

Developmental expression of Ki-67 antigen and proliferating cell nuclear antigen in macaque placentas

Placental growth is largely determined by the proliferation of cytotrophoblast cells. However, the distribution of cytotrophoblast cells engaged in the cell cycle during placental development is poorly understood. Recently, antibodies have been developed that identify two proteins directly involved with DNA synthesis: Ki-67 protein and proliferating cell nuclear antigen (PCNA). Immunolocalization of Ki-67 and PCNA provides a measure of the proliferating cells in tissues. We examined, in macaque placentas, the spatiotemporal pattern of expression of these proteins during gestation. Tissues from 24 macaque placentas collected from 22-153 days of pregnancy were prepared for paraffin sections. Standard immunoperoxidase techniques were used to identify Ki-67 and PCNA. The proteins generally co-localized, although PCNA was usually represented in more cells than Ki-67. Early in gestation the cell columns contained many labeled cells. The cytotrophoblastic shell was occupied by numerous cells with PCNA positive nuclei, but few were reactive for Ki-67. By 45 days of pregnancy the immunolabeled cells in the cell columns were concentrated in the proximal regions, adjacent to the anchoring villus tips. The number of positive cells decreased by 100 days when the cell columns were diminished, leaving the anchoring villus tips buried in the shell. Labeled cells were rarely present in the shell at late pregnancy. The single layer of cytotrophoblast cells in the chorionic plate contained numerous reactive cells throughout early and mid-gestation. After approximately 100 days the cytotrophoblast layer of the chorionic plate was stratified over large areas. Soon thereafter few cells of the chorionic plate were labeled. The chorionic villi contained reactive cytotrophoblastic cells throughout gestation. Extravillous cytotrophoblast cells invading spiral arteries were sometimes labeled for PCNA but not Ki-67. We conclude that compartments of the placenta are distinguished by specific patterns of cytotrophoblast cell proliferation. Moreover, these patterns correspond to macroscopic growth parameters of the placenta. Evidence suggests that the macaque placenta slows its rate of diametrical growth at approximately 100 days of gestation. It is at about this time that the cell columns are absorbed into the trophoblastic shell and this pool of proliferating cells is diminished. The growth in diameter of the chorionic plate matches that of the shell. In this compartment also the architecture changes at about 100 days as the cytotrophoblast layer stratifies. This stratification may result from continued proliferation of cytotrophoblast cells when the diametrical rate of growth is decreasing. Soon thereafter, proliferation decreases in this compartment also. By contrast, labeled cells were found in chorionic villi throughout gestation.(ABSTRACT TRUNCATED AT 400 WORDS)

Differentiation of the chorionic plate of the placenta: cellular and extracellular matrix changes during development in the macaque

BACKGROUND

The chorionic plate forms the fetal side of the placental disc, and its proper growth and development is important to the formation of a normal placenta. The development and structure of the chorionic plate has received little attention. Therefore, we have conducted a developmental and immunohistochemical study of the chorionic plate of the macaque placenta.

METHODS

Conventional light and transmission electron microscopy techniques were used to study macaque placental tissues collected from 22 days of gestation to near term. Standard immunoperoxidase methods were used to identify type IV collagen, laminin, and fibronectin in paraffin sections.

RESULTS

Early in gestation the chorionic plate trophoblast consisted of an outer layer of syncytiotrophoblast and a single underlying layer of cytotrophoblast. Beginning at about 100 days of gestation, the cytotrophoblast layer became stratified. The cytotrophoblast cells also became surrounded by variable amounts of extracellular matrix containing type IV collagen, laminin, and fibronectin. Ultrastructurally, the matrix contained abundant 10-12 nm diameter microfibrils. During later gestation the syncytiotrophoblast had a tendency to separate from the cytotrophoblast.

CONCLUSIONS

The chorionic plate of the macaque placenta undergoes several distinctive morphological changes over the course of gestation. During the period of rapid diametrical growth of the disc, the chorionic plate trophoblast consists of a layer of syncytiotrophoblast and a single layer of cytotrophoblast. During later gestation the cytotrophoblast layer stratifies at a time coincident with that at which diametrical growth of the disc slows. The cytotrophoblast cells of later gestation appear synthetically active and at least some of their products are extracellular matrix components that encapsulate many of these cells. These components include type IV collagen, laminin, fibronectin, and microfibrils.

Identification of 72-kilodalton type IV collagenase at sites of trophoblastic invasion of macaque spiral arteries

The walls of uterine spiral arteries are invaded by extravillous trophoblast cells and are thereby converted to the uteroplacental arteries of pregnancy. The mechanisms by which this invasion occurs are not understood, but the extracellular matrices that are breached suggest participation by specific proteinases. In this report we describe the immunohistochemical localization of 72-kd type IV collagenase (gelatinase A or MMP-2) among intra-arterial trophoblast cells and endometrial cells during development of macaque uteroplacental arteries. Cytokeratin-positive trophoblast cells were identified within arteries at each stage studied (between days 22-128 of gestation). Many of these cells, whether located in the arterial lumen or within the vessel wall, were immunoreactive for the proteinase. Early in the invasive process these trophoblast cells were associated with discontinuities of the endothelial basement membrane and later became interspersed with smooth muscle cells of the tunica media. While trophoblast cells comprised the entire thickness of the arterial wall in many locations, typically only a subset of these cells expressed the proteinase. Many endometrial stromal cells were also immunoreactive for the proteinase, as were some arterial endothelial and smooth muscle cells. It is concluded that this, and probably other, proteinases are active throughout gestation in the restructuring of uterine spiral arteries and other endometrial tissues as necessary to accommodate the development of the fetus.

Trophoblastic invasion and modification of uterine veins during placental development in macaques

Trophoblast cells invade and modify the uterine vasculature to provide circulation of maternal blood through the placenta. Although evidence indicates fundamental differences between trophoblast modification of arteries and veins, interactions between trophoblast cells and uterine veins have not been addressed. In this report we describe the processes by which trophoblast cells invade and restructure uterine veins during placentation in the macaque. Antibodies were used to identify trophoblast, endothelium, and basement membranes. During early gestation, trophoblast migrated from the trophoblastic shell and, by intravasation, replaced portions of the wall and endothelium of veins in the vicinity of the shell; this is in contrast to invasion by extravasation reported for the arteries in this species. These areas had discontinuous endothelial basement membranes and the endothelial cells were variably hypertrophied. Deeper portions of veins were not invaded; this too is in contradistinction to the spiral arteries where trophoblastic modification extends to the myometrial segments. Later in gestation, those portions of veins interacting with trophoblast were contained within the trophoblastic shell or situated such that one side abutted the shell. These regions of the veins were lined by endothelium, but it could not be determined whether this represented re-endothelialization of regions formerly lined by trophoblast or if these endothelial cells were never displaced.