Developmental changes in the fine structure of rhesus monkey amnion

Dissecting primate early post-implantation development using long-term in vitro embryo culture

The transition from peri-implantation to gastrulation in mammals entails the specification and organization of the lineage progenitors into a body plan. Technical and ethical challenges have limited understanding of the cellular and molecular mechanisms that underlie this transition. We established a culture system that enabled the development of cynomolgus monkey embryos in vitro for up to 20 days. Cultured embryos underwent key primate developmental stages, including lineage segregation, bilaminar disc formation, amniotic and yolk sac cavitation, and primordial germ cell–like cell (PGCLC) differentiation. Single-cell RNA-sequencing analysis revealed development trajectories of primitive endoderm, trophectoderm, epiblast lineages, and PGCLCs. Analysis of single-cell chromatin accessibility identified transcription factors specifying each cell type. Our results reveal critical developmental events and complex molecular mechanisms underlying nonhuman primate embryogenesis in the early postimplantation period, with possible relevance to human development.

Gestational stage affects amniotic epithelial cells phenotype, methylation status, immunomodulatory and stemness properties

Stem cells isolated from amniotic epithelium (AECs) have shown great potential in cell-based regenerative therapies. Because of their fetal origin, these cells exhibit elevated proliferation rates and plasticity, as well as, immune tolerance and anti-inflammatory properties. These inherent attitudes make AECs well-suited for both allogenic and xenogenic cellular transplants in animal models. Since in human only at term amnion is easily obtainable after childbirth, limited information are so far available concerning the phenotypic and functional difference between AECs isolated from early and late amnia. To this regard, the sheep animal model offers an undoubted advantage in allowing the easy collection of both types of AECs in large quantity. The aim of this study was to determine the effect of gestational age on ovine AECs (oAECs) phenotype, immunomodulatory properties, global DNA methylation status and pluripotent differentiation ability towards mesodermic and ectodermic lineages. The immunomodulatory property of oAECs in inhibiting lymphocyte proliferation was mainly unaffected by gestational age. Conversely, gestation considerably affected the expression of surface markers, as well the expression and localization of pluripotency markers. In detail, with progression of gestation the mRNA expression of NANOG and SOX2 markers was reduced, while the ones of TERT and OCT4A was unaltered; but at the end of gestation NANOG, SOX2 and TERT proteins mainly localized outside the nuclear compartment. Regarding the differentiation ability, LPL (adipogenic-specific gene) mRNA content significantly increased in oAECs isolated from early amnia, while OCN (osteogenic-specific gene) and NEFM (neurogenic-specific gene) mRNA content significantly increased in oAECs isolated from late amnia, suggesting that gestational stage affected cell plasticity. Finally, the degree of global DNA methylation increased with gestational age. All these results indicate that gestational age is a key factor capable of influencing morphological and functional properties of oAECs, and thus probably affecting the outcome of cell transplantation therapies.

Association of PAT proteins with lipid storage droplets in term fetal membranes

As depots for neutral lipids, lipid storage droplets (LDs) accumulate with advancing gestation within the fetal membranes. Little is currently known about the proteins associated with the LDs of these cells. The PAT family [perilipin, adipose differentiation-related protein (ADRP), and tail-interacting protein of 47 kilodaltons (TIP47)] represents a unique group of proteins thought to contribute to LD formation and function. We examined the association of each of the PAT proteins with LDs of term fetal membranes. We found that large LDs of amnion epithelial cells were reactive for neutral lipid stains and simultaneously encoated with ADRP and TIP47, but not perilipin. Within the remaining cell types, LDs were frequently co-labeled with antibodies recognizing ADRP and TIP47; however, in cells harboring only small LDs, the majority of TIP47 labeling was cytoplasmic. Structures labeled with perilipin antibodies were present only in chorion laeve trophoblasts. Gene and protein expression analyses suggested this to be a small molecular weight perilipin isoform, such as that seen in steroidogenic cells. We conclude that LDs are heterogeneous among differing cell types of the fetal membranes. Subclassification of LDs based on associated proteins suggests that these organelles may serve specialized functions within individual cells.

Role of amniotic membrane transplantation for conjunctival reconstruction in ocular-cicatricial pemphigoid

PURPOSE

To evaluate the role and the effectiveness over time of amniotic membrane transplantation (AMT) as a first-step procedure to treat conjunctival reconstruction in late-stage ocular-cicatricial pemphigoid (OCP).

DESIGN

Prospective interventional noncomparative case series.

PARTICIPANTS

Nine eyes (9 patients) with advanced OCP.

METHODS

Preoperatively, the ocular surface conditions were evaluated by immunohistochemistry of conjunctival biopsy and impression cytology specimens. The amniotic membrane was obtained during cesarean section from women who were 39 weeks pregnant and seronegative for human immunodeficiency virus, hepatitis B and C, and syphilis; it was processed, histologically tested, and stored at -80 degrees C. After scar tissue was removed, the preserved amniotic membrane was placed over the cornea, the bulbar, and tarsal conjunctiva, and was secured with 8-0 Vicryl sutures to the conjunctival edges and the deep fornices with double-armed 6-0 silk sutures. In 2 cases a double layer of amniotic membrane was transplanted. All patients received immunosuppressive systemic therapy and preservative-free tear substitutes and steroids topically for at least 6 months. During follow-up (average, 48 weeks; range, 28-96 weeks), a new standardized method was used to evaluate the fornix depth, and impression cytology testing was performed and conjunctival inflammation recorded and used as parameters for monitoring disease activity.

MAIN OUTCOME MEASURES

Symblepharon, increased inferior fornix depth, presence of conjunctival goblet cells, and the degree of conjunctival inflammation.

RESULTS

The conjunctival surface was free from symblepharon in all subjects for the first 16 weeks. At the week 28 examination, a small area of symblepharon was present in four eyes (44.4%). The depth of the fornix was significantly (P < 0.0001, analysis of variance) improved at weeks 4, 16, and 28. The normal conjunctival epithelium with goblet cells was restored in 6 of 9 eyes (66.7%) at the week 4 examination and in 4 eyes (44.4%) at the week 28 examination. Conjunctival inflammation was clinically but not statistically reduced. The visual acuity improved in 5 subjects.

CONCLUSIONS

AMT can be a first-step procedure for ocular surface reconstruction in OCP, but its effectiveness deteriorates slightly over time.

Transfer of [3H]estrone-[35S]sulfate across guinea pig fetal membranes

The possible role of fetal membrane deconjugating activity in the movement of a charged steroid conjugate between fetal and maternal compartments was investigated. The ability of amnion and chorion laeve to transfer [3H]estrone-[35S]sulfate was assessed in both orientations of guinea pig tissue at 45 days and near parturition. While early amnion was impermeable, late tissue transferred approximately 50% (w/w) of the substrate in a bidirectional process that was non-saturable and independent of either deconjugation or ATP. Transfer across early chorion was similar to late amnion. Saturation curves from each tissue were superimposable, as were those of the time course. Transfer across both early and late chorion proceeded in the absence of deconjugation, with no effect of tissue orientation or ATP depletion. However, late chorion exhibited a decrease in estrone-sulfate transfer, as verified by concentration dependency and time course analyses, though transport across the tissue remained non-saturable. The results in amnion were congruous with the presence and absence of tight junctions in the epithelium of early and late tissue, respectively. However, sulfoconjugate transfer across early chorion proceeded in the presence of a paracellular barrier, suggesting specialized regulation of the transport process which extended late into gestation.

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.

Lipoprotein lipase, LDL receptors and apo-lipoproteins in human fetal membranes at term

Ultrastructurally, all cells of human fetal membranes strongly exhibit a large amount of lipid deposits throughout pregnancy. Their origin and function is still unknown. The aim of this study was to investigate the localization of key components of lipid metabolism in this tissue. Using immunohistochemical techniques, the distribution of lipoprotein lipase (LPL), low density lipoprotein receptors (LDL receptors), and apo-lipoprotein B and E was investigated in 20 human fetal membranes at term. In addition, electron microscopy was used to study the intracellular localization of lipoprotein-sized particles. Amnionic epithelium and trophoblast cells reacted strongly for LPL. LDL receptors and apo-lipoproteins were present in amnionic epithelium and fibroblasts of the amnion. In none of the investigated cells were lipoprotein-sized particles identified. Similar results were obtained in all 20 cases. The findings indicate that lipoprotein from the amniotic fluid or from the maternal circulation may serve as substrate for lipids in human fetal membranes.

Ovine amniotic and allantoic epithelia across gestation

BACKGROUND

Extensive studies on the regulation of the volume and composition of amniotic and allantoic fluid in the sheep have suggested that the amniotic and allantoic membranes must play an active role in these processes. Little is known of the functional morphology of the sheep amnion and allantois beyond the presence of an epithelium overlying connective tissue.

METHODS

The ovine amnion and allantois were characterized at a range of gestational ages (27-140 days of gestation, where term is 145-150 days) by electron microscopy (SEM and TEM) and the presence of transporting ATPases examined by use of immunohistochemistry (Ca++-ATPase) and in situ hybridization (Na,K-ATPase).

RESULTS

With increasing gestational age, the cell height of epithelium of the membranes increased, as did the number of apical microvilli and the length of zonulae occludentes. Epithelial cell cytoplasm increased in complexity, and cell shape changed from flattened to cuboidal. Proliferation of cells occurred until close to term. Immunoreactivity to Ca++-ATPase was present in the basolateral membranes at all stages of gestation examined, but hybridization with the alpha and beta subunits of Na,K-ATPase was present only at or after 100 days of gestation.

CONCLUSIONS

The epithelia of the sheep amnion and allantois display characteristics typical of transporting epithelia. As the epithelia mature, changes related to increased capacity for solute and fluid transport regulation occur.

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.

Microscopic and biochemical analysis of the viability and permeability of guinea pig amnion and chorion leave in vitro

Tissue viability and permeability of guinea pig amnion and chorion leave were analyzed microscopically and biochemically. The vital dyes T1111 and fluorescein diacetate were used to locate and determine the integrity of cell plasma membranes in early and late tissue in vitro using confocal laser scanning microscopy and scanning electron microscopy. Early amnion and chorion laeve were each found to contain a single epithelial cell layer, composed of membrane-intact cells. In contrast, plasma membrane lesions were present throughout the epithelium of late amnion. Late chorion laeve contained both regions of intact and damaged epithelial cells on its maternal side. There was also a layer of membrane-intact squamous cells on the fetal side of late chorion laeve. ATP measurements confirmed that early fetal membranes were viable after incubation in isotonic salt solutions at physiological pH. Late amnion was depleted of ATP stores while late chorion laeve retained its capacity for generating energy. These viability markers indicate that late guinea pig amnion is not a viable tissue in vitro, while late chorion laeve is a viable but probably degenerating tissue. Confocal X-Z scans were used to trace the movement of T1111 through the tissue as an indication of permeability to free solutes. Whereas dye will permeate across the main thickness of early amnion and chorion leave, it did not pass between cells, but was blocked, presumably by a line of tight junctions. Late amnion was characterized by the complete permeability to T1111. Late chorion leave contained regions where solute migration was blocked, but overall was a permeable tissue. These results provide an important context for the interpretation of molecular movement across fetal membranes.

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.

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.

Ultrastructure and development of a thick basement membrane-like layer in the anchoring villi of macaque placentas

BACKGROUND

Anchoring villi and cytotrophoblastic cell columns are important structural components involved in placental morphogenesis. We have previously described the presence of an unusual basement membrane-like layer (BMLL) that separates these placental compartments. The purpose of the present study was to identify developmental changes in the ultrastructure of the BMLL and to assess its changes in extracellular matrix composition over the course of gestation.

METHODS

Conventional techniques were used to examine macaque placental tissue by transmission electron microscopy. Standard immunoperoxidase methods were used to identify type IV collagen, laminin and fibronectin in paraffin sections.

RESULTS

Until day 35 of gestation the BMLL was 70-100 nm thick and appeared similar to basement membranes seen in other regions of the villus, although it usually lacked a lamina lucida along the surface adjacent to the cytotrophoblast cells. Immunohistochemistry revealed the presence of laminin and type IV collagen in the BMLL. By 53 days of gestation the BMLL had hypertrophied at the junction of the anchoring villus and cell column, measuring 2,000-5,000 nm in thickness. The BMLL retained immunoreactivity for laminin and type IV collagen. Ultrastructural examination revealed the presence of a new component in the form of 10 nm microfibrils. By 89 days of gestation the BMLL was not reactive for laminin or type IV collagen but otherwise maintained the structural organization seen at 53 days. No additional changes were observed in the BMLL during late pregnancy.

CONCLUSIONS

The BMLL is a distinct extracellular matrix region that separates the distal aspect of the anchoring villus from the proximal portion of the cell columns. Evidence indicates that adjacent cytotrophoblast plays a prominent role in the production of the BMLL. The BMLL may serve to organize this complex tissue by separating fetal mesenchyme from cytotrophoblast cells that are proliferating, differentiating, and migrating. Modifications to the composition of the BMLL may indicate changes in the role this matrix plays in the development of the placenta.

Immunohistochemistry of carbonic anhydrase in human placenta and fetal membranes

The localization of human carbonic anhydrase (CA) isoenzymes HCA I, HCA II, and rat CA II have been studied in human umbilical cord, chorion laeve including amnion and placenta from first and second trimester and also from term pregnancies. Detection techniques of immunofluorescence and immunoperoxidase were used in cryostat and paraffin sections. Both isoenzymes were found in the villous syncytiotrophoblast throughout pregnancy. HCA I staining patterns in the villous endothelium were highly variable whereas increasing immunoreactivity levels of endothelial HCA II were detected as pregnancy advances. The extravillous cytotrophoblast showed generally weaker levels of immunoreactivity. In amnionic epithelium of membranes, chorionic plate and umbilical cord, higher activities for HCA I, HCA II and rat CA II were found than in all other localizations. Our findings emphasize the importance of enzyme mediated bicarbonate/CO2 removal from the feto-placental unit as opposed to simple bicarbonate diffusion or carrier mediated transport. As effective transfer routes should be considered not only umbilical cord--placental villi--intervillous space, but also fetal kidney--amnionic fluid--amnion--uterine vessels.

Development and structure of the placenta and fetal membranes of nonhuman primates

This survey examines placental and fetal membrane structure, function, and development in nonhuman primates. It will focus on these aspects of fetal membranes in species exemplifying the two main types of primate placentation. The chorioallantoic placentas of most strepsirhine primates (e.g., lorises and lemurs), are diffuse, villous and epitheliochorial in structure. Villi are stout, usually unbranched and interdigitate with indentations in the uterine lining. Early in gestation trophoblast cells are columnar but with advancing gestational age considerable thinning of the interhemal membrane occurs. The villous tips have indented regions that appear to be involved in absorption of histotroph. Accessory placental structures known as areolae or chorionic vesicles occur in many strepsirhines. These are cup-shaped specializations that develop opposite the mouths of uterine glands and appear active in absorption of glandular secretions. In addition, most strepsirhines are characterized by a large yolk sac early in gestation, a transient choriovitelline placenta, and most have a large allantoic sac. In strepsirhines, amniogenesis occurs by folding. In contrast, the chorioallantoic placentas of most haplorhines are discoidal, villous, and hemochorial. The species emphasized in the review are macaque monkeys. The placental villi are covered by a layer of syncytiotrophoblast; the latter is active in maternal-fetal transport and hormone synthesis. In some haplorhines, extravillous trophoblast invades deeply into the endometrium and also into spiral arteries supplying the placenta. Other fetal membranes include 1) nonvillous chorion (chorion laeve); there is evidence supporting both an absorptive and synthetic role for trophoblast of the chorion laeve; 2) yolk sac, which is unusual in that a secondary yolk sac is formed. The yolk sac has both synthetic and absorptive functions in early gestation; and 3) amnion; amniogenesis in haplorhines occurs by cavitation.

Artificial matrix barriers: a diffusion study utilizing dextrans and microspheres

Eight artificial matrices (AMs) were evaluated for the ability to restrict the passage of diffusion probes. Three AMs were composed exclusively of interstitial type I collagen (Col I) and differed from each other in thickness only. Four AMs consisted of reconstituted basement membrane (RBM) -coated polycarbonate filters (containing 10 microns diameter pores) and also only differed in thickness. One AM consisted of an uncoated 10 microns pore polycarbonate filter. The diffusion probes were uncharged fluorescein isothiocyanate-labeled dextrans, having molecular weights of 17,900, 42,000, 71,200, and 148,900 and negatively charged latex microspheres, having diameters of 0.08, 0.30, and 0.95 microns. Probes were applied to the AMs, incubated for 72 hr at 37 degrees C, and then analyzed spectrophotometrically. Dextran passage was increasingly restricted for Col I matrices as either molecular weight or collagen thickness increased (range 7% to 0.7%). Thin RBM-coated filters were more permeable to dextrans (range 100% to 30%) than Col I matrices. The diffusion rate of microspheres for Col I matrices (range 3.5% to 0) was similar to both thick and thin RBM-coated filters (range 4% to 0). The uncoated filter permitted the most diffusion for both dextrans and microspheres (range 100% to 7%). These data demonstrate that the AMs presented in this study will allow direct observation of the degradative and migratory potential of cells in vitro as they interact with various extracellular matrices.

Diffusion of dextrans and microspheres in the human amniotic basement membrane model

The human amniotic basement membrane model was utilized to determine diffusion ratios of dextrans and beads according to size selectivity. Diffusion through both intact and denuded amnions was determined after 24 and 72 h. Four neutrally charged fluorescein isothiocyanate labeled dextrans, having molecular weights of 17,900, 42,000, 71,200 and 156,000, diffused through the amnion. The amnion functioned as a sieve in that the passage of dextrans was increasingly restricted as molecular weight increased. In contrast, uncharged latex microspheres (1.05 micron +/- 0.07 micron (SD] and fluorescent carboxylated microspheres (1.57 micron +/- 0.13 micron (SD] failed to pass through the amnion. Light and electron microscopy revealed no preformed channels through which the 1.05 micron microspheres could pass through the amnion. Statistical analysis of cross-sectional thickness of individual and similarly treated amnions (intact or denuded) showed a difference in thickness (P = 0.05).

Importance of vitamin C in maintenance of the normal amnion: an experimental study

Collagenous matrix in amnion accounts for most of the dry weight of the tissue and provides its mechanical strength and resistance to rupture. Cell and organ culture techniques have been utilized to study the influence of vitamin C upon the synthesis and deposition of extracellular matrix by cells of normal amnion at term. The cultures have been examined using light and electron microscopy and metabolic labelling. These studies show that both epithelial cells and fibroblasts of the deeper stromal layer are active in the production of fibrillar matrix at this time. Matrix deposition by epithelial cells in culture increases several-fold when the vitamin C concentration in the supernatant medium is increased from zero to 50 micrograms/ml, and, at the latter concentration, a continuous anastomosing fibrillar collagenous meshwork appears beneath the cells. This study thus provides new evidence that an adequate dietary supply of vitamin C is probably needed to maintain the strength of the chorioamnion.

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.

Transport of horseradish peroxidase across monkey trophoblastic epithelium in coated and uncoated vesicles

This study used membranous chorion of the macaque monkey placenta to examine uptake and processing of exogenous proteins. Tissue was incubated with either cationic or anionic horseradish peroxidase. Incubation time was varied between 5-25 min to follow the endocytic pathways. In spite of some differences in binding, uptake and processing of the isozymes was similar. In the presence of tracers at 37 degrees C both horseradish peroxidases were taken up in large (150-175) nm diameter) coated vesicles. In addition, coated tubules 300-400 nm in length and 50-100 nm in diameter were seen in the apical cytoplasm. Studies using ruthenium red indicated that the coated tubules were derived from long coated invaginations of the free surface that pinch off into the apical cytoplasm. Often, the tubules bud off small (85-105 nm diameter) protein-filled coated vesicles which traversed the cytoplasm and fused with the basal-lateral plasma membrane. In other cases, the tubules or vesicles lost their clathrin coats and fused to form larger endocytic vesicles which later fused with phagolysosomes. After long incubation, larger uncoated vesicles (endosomes) were seen releasing their contents at the basal-lateral membrane. These results suggest that multiple transport pathways exist in this epithelium. The first, involving only coated structures, may function to sort and concentrate specific ligands important for embryonic development. The second, involving the formation and translocation of large uncoated vesicles to the basal-lateral membrane, may also provide nutrients to the embryo. A third pathway directs the protein to phagolysosomes where it is presumably degraded.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Fine structure of human chorionic membrane. Ultrastructural and histochemical examinations

Normal human term chorionic membrane was examined by electron microscopy and histochemistry. In comparison with previous studies of the human chorion, our findings showed the following new morphological aspects: (1) The cellular junctions between adjacent trophoblastic cells were made up not only of desmosomes but also of gap junctions and occasional tight junctions. (2) The epithelial basement membrane was stratified and discontinuous. (3) Besides the fibroblasts and macrophages (Hofbauer cells), the connective tissue of the human chorion also contained myofibroblasts. Histochemically, the surface coat of the plasma membrane of the chorionic epithelial cells, the microfibrils, and the proteoglycans associated with the collagen fibrils showed a positive reaction to ruthenium red. This was not so for the basement membrane of the chorionic epithelium. Since the tight junctions are seldom present in the chorionic membrane, they are apparently not significant for the paracellular protein transfer. Similar to the amnion epithelium, it is assumed that the intercellular spaces and the discontinous basement membrane of the chorionic epithelium represent a paracellular route for non-placental protein transfer. As the myofibroblasts in human amnion, we suggest that the myofibroblasts in the chorionic membrane may contribute to the protection of the fetal membranes from overdistension.

Intercellular junctions in the human fetal membranes. A freeze-fracture study

Freeze-fracture replicas of the human reflected and placental amnion and chorion laeve at term were studied in order to give a systematic survey of the nature and extension of the intercellular junctions in the fetal membranes. No differences could be detected between the reflected and placental amniotic epithelium. In both the replicas never displayed plasma membrane differentiations typical of occluding junctions, while communicating junctions were occasionally and desmosomes frequently seen. In the chorionic trophoblast maculae occludentes, communicating junctions and desmosomes were regularly encountered. It is assumed that the maculae occludentes are remnants of occluding junctions which early in gestation possibly seal off the chorionic cavity; it appears improbable that they contribute significantly to the permeability properties of the chorionic trophoblast, since it is known from previous ultrastructural studies that large open intercellular channels cross the chorionic trophoblast. Thus the absence of occluding junctions, which could act as effective permeability barriers, in both epithelial components of the fetal membranes suggests that the factors able to influence the amniotic fluid turnover or the paraplacental protein exchange are the geometrical relationships and physico-chemical properties of the intercellular channels in the amniotic epithelium and chorionic trophoblast. In addition, communicating junctions were present between fibroblasts in the chorion laeve but not in the amnion, possibly indicating differences in the functional state of these cells and/or their extracellular microenvironments.

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.

The bovine allantoic and amniotic epithelia. SEM and TEM studies

The ectodermal and endodermal coverings of the allanto-amniotic membrane in cattle fetuses of various gestational ages have been studied by scanning and transmission electron microscopy. We have observed that the allantoic and amniotic epithelia have rather similar cells which are rich in filaments but poor in organelles. Neither epithelium reflects its different origin nor the differences in the composition of the two fetal fluids. Maturation changes occur in the pattern of the various surface specializations until midterm. Coral-like luminal outgrowths, which also contain organelles, were observed in both epithelia, whereas microplicae had formed only on fully differentiated amniotic cells. Interspersed smooth-surface cells in the allantoic epithelium that differ also in nuclear shape, glycocalyx, and mitochondria are regarded as a second cell type. Besides fluid-filled blisters and cornified pustules, the most typical feature of the amniotic epithelium is a rather regular lateral cell interdigitation with tongue-like lamellae of even thickness. Micropinocytotic vesicles are more frequent than in the allantois. Both epithelia possess similar tight junctions; no morphological or histochemical indications of an active sodium transport were evidenced.

Developmental changes in the fine structure of the chorion laeve (smooth chorion) of the rhesus monkey placenta

Developmental changes in the fine structure of the chorion laeve (smooth chorion) of the rhesus monkey were studied at two time periods during gestation: 1) Early (19-60 days of gestation), before the chorionic epithelium fuses with the parietal decidua, and 2) near term, when the fused chorioamnion has also fused with the parietal decidua. Early in gestation the chorionic epithelium consisted of columnar and cuboidal cells one or two layers thick. The apical border of the cells had microvilli and coated pits, and adjacent cells were joined by tight junctions and desmosomes. The chorionic epithelial cells during this early period contained numerous large vesicles and vacuoles of varying electron-density. The apical cytoplasm contained various small coated vesicles and tubules. Taken together these observations were interpreted as indicating a possible role for these cells in endocytosis or phagocytosis of substances from the uterine lumen; i.e., a potential role in histiotrophic nutrition during this early period. Late in gestation the trophoblastic cells were more irregular in shape. The cells contained abundant granular endoplasmic reticulum (ER) and mitochondria, and a well-developed Golgi complex, suggesting the cells were actively synthetic late in gestation. The numerous cytoplasmic vacuoles characteristic of the trophoblastic cells of early gestation were absent near term. Glycogen deposits and lipid droplets were moderately well-developed near term. Most of the cells were joined by desmosomes but wide intercellular spaces, unobstructed by any cell junctions, were frequently observed. This observation provides at least one explanation for the increase in permeability of the chorion laeve later in gestation. Cells of the parietal decidua associated with the chorion laeve were also examined. These cells generally had a well-developed granular ER and Golgi apparatus, and numerous mitochondria. Limited numbers of membrane-bounded secretory bodies, similar to those in human decidual cells, were also present.