Ultrastructure of the human amnion, chorion and fetal skin

Efficient Wound Healing Using a Synthetic Nanofibrous Bilayer Skin Substitute in Murine Model

Treatment of full-thickness skin wounds with minimal scarring and complete restoration of native tissue properties still exists as a clinical challenge. A bilayer skin substitute was fabricated by coating human amniotic membrane (AM) with electrospun silk fibroin nanofibers, and its in vivo biological behavior was studied using murine full-thickness skin wound model. Donut-shaped silicon splints were utilized to prevent wound contraction in mouse skin and simulate re-epithelialization, which is the normal path of human wound healing. Skin regeneration using the bilayer scaffold was compared with AM and untreated defect after 30 d. Tissue samples were taken from healed wound areas and investigated through histopathological and immunohistochemical staining to visualize involucrin (IVL), P63, collagen I, CD31, and vascular endothelial growth factor. In addition, mRNA expression of IVL, P63, interleukin-6, and cyclooxygenase-2 was studied. The application of bilayer scaffold resulted in the best epidermal and dermal regeneration, demonstrated by histopathological examination and molecular analysis. In regenerated wounds of the bilayer scaffold group, the mRNA expression levels of inflammatory markers (interleukin-6 and cyclooxygenase-2) were downregulated, and the expression pattern of keratinocyte markers (IVL and P63) at both mRNA and protein levels was more similar to native tissue in comparison with AM and no-treatment groups. There was no significant difference in the expression level of collagen I, CD31, and vascular endothelial growth factor among different groups. Conclusively, these promising results serve as a supporting evidence for proceeding to clinical phase to examine the capacity of this bilayer scaffold for human skin regeneration.

Organotypic culture of human amnion cells in air-liquid interface as a potential substitute for skin regeneration

BACKGROUND AIMS

The aim of the present study was to evaluate the effects of air-liquid interface on the differentiation potential of human amnion epithelial cells (HAECs) to skin-like substitute in organotypic culture.

METHODS

HAECs at passage 1-2 were seeded onto a fibrin layer populated with human amnion mesenchymal cells to form the organotypic cultures. The organotypic HAECs were then cultured for 7, 14 and 21 d in two types of culture system: the submerged culture and the air-liquid interface culture. Cell morphogenesis was examined under the light and electron microscopes (transmission and scanning) and analyzed by immunohistochemistry.

RESULTS

Organotypic HAECs formed a single layer epithelium after 3 wk in submerged as well as air-liquid interface cultures. Ultrastructurally, desmosomes were observed in organotypic HAECs cultured in the air-liquid interface but not in the submerged culture. The presence of desmosomes marked the onset of early epidermal differentiation. Organotypic HAECs were positive against anti-CK18 and anti-CK14 in both the submerged and the air-liquid interface cultures. The co-expression of CK14 and CK18 suggested that differentiation of HAECs into skin may follow the process of embryonic skin development. However, weak expression of CK14 was observed after 2 and 3 wk of culture in air-liquid interface. CK10, involucrin, type IV collagen and laminin-5 expression was absent in organotypic HAECs. This observation reflects the initial process of embryonic epidermal differentiation and stratification.

CONCLUSIONS

Results from the present study suggest that the air-liquid interface could stimulate early differentiation of organotypic HAECs to epidermal cells, with a potential use for skin regeneration.

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.

Vacuolated cytotrophoblast: a subpopulation of trophoblast in the chorion laeve

In summary, immunocytochemical analysis of the placenta and fetal membranes discloses two subpopulations of trophoblastic cells in the chorion laeve with distinct morphologic and immunocytochemical features. Both subpopulations are composed of mononucleate cells but one contains clear vacuolated cytoplasm whereas the other contains non-vacuolated eosinophilic cytoplasm. The vacuolated cells are placental alkaline phosphatase (PLAP) positive and human placental lactogen (hPL) negative, whereas the eosinophilic cells are PLAP negative and hPL positive. Both cell types contained immunoreactive keratin and epithelial membrane antigen, but are negative for human chorionic gonadotropin(beta), pregnancy specific beta I-glycoprotein, and prolactin. Electron microscopic studies of the vacuolated cells show that these cells contain numerous lucent non-membrane bound lipid droplets and pinocytotic vesicles. In addition they contain numerous intracellular filaments and desmosomes corresponding to the immuno-cytochemical localization of keratin. Their precise function is not clear, but the abundance of PLAP, an enzyme associated with absorption, suggests that these cells may be involved with maternal fetal transport. The vacuolated cells appear to be limited to the chorion. Their characteristic morphologic and immunocytochemical features and unique anatomic distribution suggests that they represent a distinctive subpopulation of trophoblastic cells for which we propose the term 'vacuolated cytotrophoblast'.

The concentration of collagen and the collagenolytic activity in the amnion and the chorion

Fetal membranes obtained from second-trimester abortions, elective Caesarean sections and after normal deliveries were studied. The hydrolytic activity against a DNP-peptide expressing the collagen turn-over was found to be very high in the second trimester and relatively high at term of pregnancy. The activity in the chorion was twice that in the amnion. The concentration of another collagen-degrading enzyme, leukocyte elastase, which is present during inflammatory reactions was high at the site of rupture. The collagen concentration given as micrograms hydroxyproline per mg dry weight was unchanged throughout pregnancy and labour in both fetal membranes. The concentration in amnion was twice that in the chorion. After delivery, the collagen content given as microgram/cm2 in the chorion was decreased at the rupture line. This was due to decreased thickness of the membrane, whereas the hydroxyproline concentration (microgram/mg dry weight) was unchanged. The amnion was thinner than the chorion, but the decreasing collagen content (micrograms/cm2) at the rupture line was less pronounced. The thinning of especially the chorionic membrane might be secondary to the mechanical stress of pregnancy and labour and/or, as our results suggest, due to enhanced catabolism.

An immunofluorescence study of extracellular matrix associated with cytotrophoblast of the chorion laeve

Using immunofluorescence we have studied the distribution within and beneath the cytotrophoblast of chorion laeve of five extracellular matrix components: types I, III and IV collagen, fibronectin and laminin. Fibronectin, laminin and types I and IV collagen are located in a network of extracellular matrix which encapsulates cells of the cytotrophoblast multilayer. The results suggest that cytotrophoblast in all layers is active in matrix biosynthesis. The pseudo-basement membrane, which links this intercellular matrix to the underlying reticular layer, contains rich deposits of fibronectin and type III collagen along with more restricted foci of laminin and type IV collagen. The reticular layer contains fibronectin, type III collagen and small amounts of type I collagen, but no laminin or type IV collagen. Type III collagen is also found in the maternal decidua. Extracellular matrix markers may be useful in defining trophoblast differentiation pathways. Possible reasons are discussed for the unusual architecture of extracellular matrix of cytotrophoblast of the chorion laeve. Cytoplasmic actin occurs in cells throughout the chorion laeve, while cytokeratin appears only in cytotrophoblast. Plasminogen also occurs throughout cytotrophoblast in pericellular locations.

Bacterial attachment to the chorioamniotic membranes

Intra-amniotic infections are believed to result from bacteria of cervical and vaginal origin which gain access to the amniotic sac. The logical sequence in this process would be bacterial attachment to the maternal surface, followed by migration through the chorioamniotic membranes to the fetal surface. Fresh sterile chorioamniotic membranes were interposed between two arms of specially constructed incubation vessels. Bacteria (Escherichia coli, group B streptococci, or Neisseria gonorrhoeae) were inoculated into the arm (containing a basal salt medium) contiguous with the maternal surface. The arm contiguous with the fetal surface of the membrane contained pseudoamniotic fluid. At intervals up to 24 hours after inoculation, the membranes were removed, washed, fixed in glutaraldehyde, and examined by means of scanning and transmission electron microscopes. The ability of group B streptococci and E. coli to attach to and invade the chorioamniotic membranes was demonstrated by this technique. It appeared that group B streptococci had a greater capacity to attach and invade than did E. coli, whereas N. gonorrhoeae predictably failed to attach.

Epithelium of the human chorion laeve in diabetes mellitus. Light and electron microscopic examination

The epithelium of human chorion laeve from pregnancies complicated by diabetes mellitus was examined under the light and electron microscopy. In comparison with normal chorion laeve, the epithelium of chorion laeve in diabetes mellitus shows the following morphological changes: The trophoblast cells display more microvilli and cell processes on their surface. Trophoblast cells with different cytoplasmic characteristics can be seen. Some display few organelles and large glycogen depositions; other are rich in organelles. Furthermore, some cells with electron dense filaments and few cell organelles can be found. Necrotic cells are frequently present in the outer epithelial layers. The intercellular spaces in the epithelial layers near the basement membrane are narrower. Since vasculopathy of the decidual arteries with disturbance of blood circulation is often found in pathological pregnancies, we assume that the morphological changes found are due to deficiency in nutrient and oxygen supply.

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.

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.

Scanning electron microscope appearances of normal human amnion and umbilical cord at term

The surface morphology of normal term amnion was studied by scanning electron microscopy. Four patterns were observed. Two of the patterns showed distinctive minority populations of cells. In one, there were cells surrounded by intracellular channels; in the other, there were large flat cells. The microvilli in all four patterns showed variable morphology in different samples. Possible functional implications of the findings are discussed.