Basement membranes (amniotic, trophoblastic, capillary) and adjacent tissue in term placenta. An electron microscopic study

Influence of inflammatory infiltrate and alterations of the blood flow in the etiopathogenesis of the basement membrane of the amniotic epithelium (BMAE) thickening in patients with hypertensive syndromes in pregnancy

PURPOSE

To evaluate the mechanisms involved in the etiology of the basement membrane of the amniotic epithelium (BMAE) thickening in patients with hypertensive syndromes in pregnancy (HSP).

METHODS

Eighty placentas from patients presenting HSP were morphologically examined in staining through hematoxylin-eosin and periodic acid Schiff method. Placental morphological changes were classified into: diagnostic of low placental blood flow, characterized by a larger number of syncytial knots, fibrin deposits, and a larger number of vessels in terminal villi; and placentas with inflammation that presented inflammatory infiltrate in membranes or placental villi. Measurements of thickness were made with an automatic image analyzing software.

RESULTS

BMAE thickness was higher in the group with HSP, particularly in cases with gestational hypertension and pre-eclampsia superimposed on chronic hypertension (PSCH). In the placentas of the HSP group, the thickness of the BMAE was higher in cases with inflammatory infiltrate. There was a positive and significant correlation between the BMAE thickness and the thickness of the amniotic epithelium. The BMAE thickening areas were associated with hyperplasia and edema of the amniotic epithelium.

CONCLUSIONS

BMAE thickening in cases with HSP is more evident when there is an interaction between the severe effects of uteroplacental hypoxia, with consequent death and remodelling of the amniotic epithelium cells, as in PSCH, with local inflammatory processes that make this thickening much more evident.

Relation between mechanical properties and microstructure of human fetal membranes: an attempt towards a quantitative analysis

OBJECTIVE

We sought to measure the mechanical baseline behavior of fetal membranes in order to determine constitutive mechanical model parameters for fetal membranes, and to examine their relation to molecular correlates for mechanical function, i.e. collagen and elastin.

STUDY DESIGN

The uniaxial stress-strain response of nine human term fetal membranes was measured. Methods of nonlinear continuum mechanics were applied for the analysis of the stress-strain curves. Thickness of amnion and chorion were determined from histologic sections for each fetal membrane sample. Complementary biochemical analysis was performed to quantify the soluble collagen and soluble elastin components for each sample.

RESULTS

We report a straightforward histologic modality for measurements of amnion and chorion thickness. Average thickness of the amnion and chorion layers were 111+/-78 microm, and 431+/-113 microm, respectively, which are about twice larger than previously reported. The average content of acid-soluble elastin was 2.1% of wet weight and the one of pepsin/acetic acid-soluble collagen was 10.5% of dry weight. Our data show an inverse proportionality between soluble elastin and soluble collagen content. The low strain elastic modulus ranged between 10 and 25 kPa. Correlations were found between biochemical data and mechanical parameters: there is clearly a direct proportionality between small strain elastic modulus and elastin content. Further, a (less pronounced) direct correlation was observed also between soluble collagen content and the parameter governing the increase in stiffness at larger strains in the nonlinear mechanical model. The mechanical tests revealed a relatively low variability for samples from the same membrane but a large variation between donors. The proposed nonlinear model provides a good fit of the experimental data, with a coefficient of determination, R(2), typically in the range of 0.94. Membranes failure originated at the clamping points thus impairing the quantification of ultimate stress and strain. Thus, no correlation was found between maximum stress and collagen or elastin content.

CONCLUSIONS

This study provides a starting point for comprehensive quantitative analysis of the relationship between fetal membranes microstructure and their nonlinear deformation behavior. These insights could become useful in identifying potential medical interventions to prevent membranes rupture.

The etiology of premature rupture of the membranes

The etiology of PROM is multifactorial. It is clear that maternal enzymes, maturational and mechanical forces, chorionicamniotic membrane phospholipid content, collagen disruption, amniotic cell cytokines induced by fetal signals, and bacterial phospholipases and collagenases all play major and interrelated roles. It is also clear that the production of oxytocic prostaglandins is a major, if not exclusive, common pathway leading to PROM and preterm delivery. The increasing awareness of the fetal role, i.e., fetal interleukins, fetal polymorphonuclear leukocytes and type V collagenase, make this area of research ripe for further investigation. The complex host defense mechanisms and biologic variability make any universal treatment impossible. Even with a specific etiology determined, the reduced availability of pharmacologic interventions for the fetal compartment portend suboptimal success. Therefore, it appears that continued research and aggressive measures to optimize the quality and availability of prenatal care are the best foci of our efforts.

The extracellular matrix of the human fetal membranes: structure and function

The human fetal membranes are genetically identical to the fetus and form a highly specialized interface between mother and fetus, of considerable significance to the successful maintenance and termination of pregnancy in the higher vertebrates. Additionally, the upright posture of women presents these tissues with a greater mechanical challenge than in other species. The major extracellular matrix components providing tensile strength and elastic recoil are reviewed, as well as the key enzyme, activator/inhibitor system responsible for their remodelling and breakdown. However, this fails to convey the important concept that the matrix components are bound to each other and to the cells involved in their formation and organization. These matrix components are collectively responsible for the biomechanical properties of the tissue, but they must also be considered as dynamic elements of a broader signalling system, which include hormonal autocrine/paracrine systems. A unifying hypothesis is presented, which attempts for the first time to bring these two facets of the matrix together, which permits a potential coordination of local events at the maternal-fetal interface leading to parturition. In order to understand fully both the normal biology and the pathobiology of these tissues, such integration of the cellular and extracellular signalling pathways must be achieved.

Detection of elastin in the human fetal membranes: proposed molecular basis for elasticity

The human fetal membranes provide a sterile biomechanical container which adjust by growth to mid-pregnancy to the increase in fetal size, and by elasticity to the forceful movements of the fetus. The molecular basis for this elasticity is not known, yet reduced elasticity may lead to their premature rupture and preterm birth, a major problem in perinatal medicine. Classically, elastin confers the property of elastic recoil to elastic fibres which are assembled from a family of tropoelastin precursors. These are covalently cross-linked to form insoluble elastin by formation of desmosine and isodesmosine, catalysed by the enzyme lysyl oxidase. The amnion, chorion and decidua were shown by Northern analysis and RT-PCR to contain detectable levels of tropoelastin mRNA and the mRNA encoding lysyl oxidase. The proteins encoded by these mRNAs were also identified by Western blotting and immunolocalization. Further, insoluble elastin was extracted from the human fetal membranes and shown by comparison to elastin preparations from other elastic tissues to have a reasonable desmosine content. Finally, scanning electron microscopy confirmed the presence of multiple layers of an apparently very thin elastic system in this tissue. This biochemical and histopathologic study has demonstrated therefore that the human fetal membranes synthesize and deposit a novel elastic fibre. The presence of such an elastic system in these tissues provides, for the first time, a probable molecular basis for the elastic properties of this tissue.

Ultrastructure of the materno-embryonic interface in the first trimester of pregnancy

During early pregnancy, the absence of fully developed internal organs means that the embryo is dependent on highly differentiated adnexal structures such as the secondary yolk sac and free-floating amniotic membrane as well as the placental trophoblast. In this review, we describe and illustrate the ultrastructural characteristics of these different cellular barriers which separate maternal and embryonic tissues during the first trimester of pregnancy. Samples of chorionic plate, umbilical cord, secondary yolk sac and amniotic membrane have been obtained from intact gestational sacs of pregnancies between 6 and 11 weeks and examined at the ultrastructural level. Features indicating intense biosynthetic activity were found in the syncytiotrophoblast of the chorionic plate, the endoderm of the secondary yolk sac and mesenchymal cells of the amniotic membrane. Barriers in the form of a well-developed basal lamina were present between the trophoblast and mesenchyme of the chorionic plate and beneath the epithelium of the amniotic membrane and umbilical cord, but were incomplete between the mesenchymal tissues of the yolk sac and mesothelial and endodermal layers, and also around the capillaries of the chorionic plate. Basement membrane thickening and interactions with the underlying stroma were observed with increasing gestational age in connection with amniotic epithelial differentiation and development of basal foot processes. After 9 weeks, the yolk sac showed a marked degeneration of surface cells, accompanied by increased fibrosis of the mesenchyme. These findings are discussed with reference to the biological functions of the adnexal structures in the development of the growing embryo, and their possible role is assessed in the physiology of exchange during the first trimester of pregnancy.

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.

Amniotic basement membrane: a barrier to neutrophil invasion

Polymorphonuclear neutrophils activated by N-formylmethionyl-leucylphenylalanine are able to cross the human amnion as shown by Russo et al. (1981). With the same technique, we added polymorphonuclear neutrophils to either the epithelial or stromal surface of the amnion. The lower compartment of the incubation chamber contained 10(-8) mol/L of N-formylmethionyl-leucylphenylalanine. Of 24 amnion tested, only one showed polymorphonuclear neutrophil permeation. Ultrastructural observations indicated that the basal lamina and the zona reticularis of the basement membrane acted as barriers to polymorphonuclear neutrophil invasion. Recent studies indicate that the basal lamina and the zona reticularis of the basement membrane contain collagen type V. No data support the existence of collagenase of polymorphonuclear neutrophil origin that cleaves collagen type V. If enzymatic degradation of extracellular matrix components play a role in polymorphonuclear neutrophil invasion, it is possible that type V collagen acts as a barrier to polymorphonuclear neutrophil invasion in the human amnion.

Ultrastructure of the full-term shark yolk sac placenta. II. The smooth, proximal segment

The smooth, proximal portion of the yolk sac placenta of the sandbar shark, Carcharhinus plumbeus is comprised of: (1) An outermost epithelial ectoderm; (2) an intervening collagenous stroma; and (3) an inner mesothelium. The surface epithelium may be one to three cell layers thick. The surface epithelium comprises two cell types. A cuboidal cell that has a dome-like apical surface covered with microvilli and an ovoid nucleus predominate. These cells contain lipid inclusions, many cytoplasmic filaments, and are joined by desmosomes. The second cell type has a convoluted nucleus and a flattened cell apex with microvilli, cilia, and paddle cilia. Golgi complexes and elements of the endoplasmic reticulum are relatively uncommon in the cytoplasm of both cell types. Microplicae also occur on the surface of some cells. The smooth, proximal portion of the placenta is sparsely vascularized. The innermost cellular elements of the surface epithelium rest on a prominent basal lamina. A collagenous zone separates the epithelial basal lamina from the basal lamina of the mesothelium. The mesothelial cells are squamous with a fusiform nucleus, many pinocytotic pits and vesicles, and a large number of cytoplasmic filaments. The endoplasmic reticulum, except for occasional patches of the rough type, and the Golgi complex are poorly developed. Ultrastructural tracer studies show that this portion of the placenta does not absorb horseradish peroxidase (HRP) and trypan blue.

Distribution and characterization of anionic sites in the basal lamina of developing human amniotic epithelium

We have studied the distribution of anionic sites in the basal lamina of developing human amniotic epithelium by using the cationic stain ruthenium red. Amnions at 7-12 weeks of gestation and at term contained ruthenium red-positive granules in a quasi-regular array on both the cellular and interstitial sides of the lamina densa. In order to characterize the anionic sites, small pieces of amnion were incubated in the presence or absence of either chondroitinase ABC, neuraminidase, Streptomyces hyaluronidase, or heparitinase in appropriate buffer systems. Incubation in the presence of heparitinase resulted in the complete disappearance of the basal lamina-associated granules, but other enzymes tested had no demonstrable effect on these granules. We conclude that the anionic sites associated with amnion basal lamina, and demonstrable with ruthenium red, consist of glycosaminoglycans rich in heparan sulfate, probably present as heparan sulfate proteoglycan. Because amniotic fluid has a low protein content and amniotic epithelium (at least at term) lacks tight junctions, we postulate that the heparan sulfate proteoglycan associated with the amnion basal lamina may have an important function as a permeability barrier to anionic macromolecules.

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.

The permeability of human amniotic epithelium: studies using lanthanum as a tracer

To study the permeability of human amniotic epithelium to small molecular weight substances, pieces of nonplacental amnion, with attached chorion laeve and decidua, were exposed to solutions containing lanthanum salts and processed for electron microscopy. Lanthanum penetrated the intercellular spaces and often reached the basal lamina region. In addition, some lanthanum was bound to the glycocalyx of the microvilli on the apical surfaces of the cells. Little lanthanum was found deep to the basal lamina. The results suggest the intercellular pathway is of major importance in the movement of small molecules across amniotic epithelium.

Cell surface specializations and intercellular junctions in human amniotic epithelium: an electron microscopic and freeze-fracture study

Cell surface specializations and intercellular junctions of human term amniotic epithelium were examined by conventional thin-section electron microscopy, after staining with the cationic probes ruthenium red and cationic ferritin, and by freeze-fracture methods. Desmosomes were the predominant type of intercellular junction and often the most apical of the junctional types. In freeze-fracture replicas, desmosomes were characterized by roughly circular areas of large, often irregular, P-face intramembranous particles. Gap junctions were identified in the laterobasal regions between cells. In thin sections they were characterized by a narrow intercellular space, and in freeze-fracture replicas had a typical plaquelike arrangement of P-face intramembranous particles and E-face depressions. Hemidesmosomes at the basal cell surface were characterized by occasional large particles and clusters of particles on both the E and P fracture faces. No evidence of tight junctions was found. The apical cell surface was heavily stained by both ruthenium red and cationic ferritin, indicating the negatively charged nature of this surface. Ruthenium red penetrated between the epithelial cells and bound to anionic materials on the lateral cell surfaces, especially at the location of desmosomes. Below the base of the intercellular cleft, large ruthenium red-positive granules were present in the extracellular matrix. The possibility that the anionic substances in the intercellular region may contribute to the control of permeability in the amniotic epithelium is discussed.

Amniotic epithelium in rhesus incompatibility: light and electron microscopic examination

Human amniotic epithelium from patients with normal pregnancy (normal amniotic epithelium) and from patients with materno-fetal rhesus incompatibility has been examined under the light and electron microscope. In comparison with normal amniotic epithelium, the epithelium in rhesus incompatibility displays the following morphological changes: 1. Unequal distribution and variable shape of the plump microvilli 2. Considerable narrowing of the intercellular spaces 3. Vacuolization of the cytoplasm 4. Absence of glycogen granules 5. Thickening of the basal membrane. There is a direct correlation between the morphological changes in the amniotic epithelium and the severity of the disease. Since rhesus factors are present only in erythrocytes, it is probably that these changes are not based on a direct immune conflict. On the contrary, they are more likely due to fetal hypoxia and to the cytotoxic effect of unbound, unconjugated bilirubin in the amniotic fluid.

Isolation and characterization of human placental chorionic villar extracellular matrix

The cell-free extracellular matrix of human placental chorionic villi has been prepared by a procedure employing extraction of the terminal villar fragments with the detergents Triton X-100 and sodium deoxycholate. The isolated human placental extracellular matrix retains an intact, but collapsed, histoarchitecture, as observed by scanning and transmission electron microscopy. It remains intact, in large part because of the presence of continuous sheets of villar basement membranes and associated interstitial collagen fibers and scattered patches of fibrin. The staining charcteristics and chemical composition of the isolated human placental extracellular matrlix are similar to those reported for basement membranes in several tissues and indicate the presence of collagen-like and glycoprotein components in this preparation. Gel electrophoresis of urea-SDS-mercaptoethanol extracts of the matrix showed that it consists of several polypeptide components of various molecuar weights, some of which are associated into high molecular weight complexes by disulfide bonds.

Presence of fibronectin in basement membranes and acidic structural glycoproteins from human placenta and lung

Cold-insoluble globulin was detected in both trophoblast and alveolar basement membrane preparations, whereas it was not detected in GBM preparations. Acidic structural glycoproteins from both placental villi and lung parenchyma, which were extracted with 0.3 M acetic acid and recovered by adjusting the pH to 4.7, also contained CIg. Fractions of TBM, solubilized by either dilute alkali (0.01 N NaOH), by reduction and alkylation of the disulfide bonds, or by 0.3 M acetic acid extraction, all contained the antigen and possessed properties similar to those of ASG. The ASG fractions also reacted with antifibrinogen, but proof that the two types of determinants occur on a given molecular species is lacking at present. Purified collagenase solubilized CIg from ABM, from lung parenchyma, and from the stroma of placental villi, and this finding is strong evidence for an association of CIg with collagen in these connective tissues.