Ovine amniotic and allantoic epithelia across gestation

Amniotic Membrane and Amniotic Epithelial Cell Culture

Amniotic membrane (AM) is considered an important medical device for applications in regenerative medicine. The therapeutic properties of AM are due to its resistant extracellular matrix and to the large number of bioactive molecules released by its cells. To this regard, ovine amniotic epithelial cells (AECs) are a subset of placental stem cells with great regenerative and immunomodulatory properties. Indeed, either oAEC or AM have been object of intense study for regenerative medicine, thanks to several advantages in developing preclinical studies on a high value translational animal model, such as sheep. For this reason, a critical standardization of cultural practices is fundamental in order to maintain, on one hand, AM integrity and structure and, on the other hand, oAEC native properties, thus improving their in vivo therapeutic potential and clinical outcomes.In addition, freshly isolated AECs or AM can be exploited to produce enriched immunomodulatory secretomes that had been used with success into cell-free regenerative medicine procedures.To this aim, here is described an improved oAEC cultural protocol able to preserve their native epithelial phenotype also after the in vitro amplification and an innovative AM in vitro cultural protocol design to prolong the integrity and the biological properties of this tissue in order to collect stable conditioned media enriched with immunomodulatory factors.

Morphological study of equine amniotic compartment

Exfoliative cytology of human amniotic fluid (AF) has been extensively studied since 1940s, but no data exist in equine species. The AF compartment represents the environment in which the foetus grows and matures, and its composition changes, reflecting foetal well-being and development. The aim of this study was to describe for the first time the morphology of equine AF cells and amniotic membrane (AM) with light microscopy (LM) and transmission electron microscopy (TEM). AF was collected at parturition within 5 min after the appearance of the AM with a 60 mL syringe from 34 mares and samples of AM were collected from a subset of 7 mares with normal pregnancy hospitalized for attended parturition. For LM observation, a sample of cytocentrifuged fresh AF was stained with May-Grünwald Giemsa and AM sections were stained with H-E. For TEM observation, AF and AM were fixed, embedded in epoxy resins, then sectioned and stained with uranyl acetate and lead citrate solutions. Nucleated and anucleated squamous cells with basophilic cytoplasm, intensely basophilic cornified cells, polymorphonuclear cells, and clusters of eosinophilic amorphous substance were observed. Cells presumably derived from tracheal epithelium and small round nucleated cells with eosinophilic cytoplasm presumably derived from amniotic or urinary epithelium were occasionally found. Lamellar body-like structures (LBs) were present in some epithelial cells. In AM, epithelial, basal and mesenchymal layers were clearly visible with both techniques as previously described. Epithelial cells had several cytoplasmic vacuolization and microvilli were present on apical surface. The connective tissue presented fibroblasts, mesenchymal and rare polymorphonuclear cells, surrounded by abundant extracellular matrix, with distribution of collagen fibres. This is the first report about equine amniotic compartment description by LM and TEM. As recently reported in human medicine, the AM could be a second potential source of pulmonary surfactant, given the finding of LBs inside the cells which could have the same function as in humans. Further studies in samples collected at different gestational ages could increase the knowledge of AF cells and their modification during pregnancy, as well as a better comprehension of the role of AM as a secondary source of pulmonary surfactant in the horse. The diagnostic evaluation of AF cellular composition in high-risk pregnancies may also be investigated.

Multiomics analyses of vesicular transport pathway-specific transcripts and proteins in ovine amnion: responses to altered intramembranous transport

Amniotic fluid volume (AFV) is determined by the rate of intramembranous (IM) transport of amniotic fluid (AF) across the amnion. This transport is regulated by fetal urine-derived stimulators and AF inhibitors. Our objective was to utilize a multiomics approach to determine the IM transport pathways and identify the regulators. Four groups of fetal sheep with experimentally induced alterations in IM transport rate were studied: control, urine drainage (UD), urine drainage with fluid replacement (UDR), and intra-amniotic fluid infusion (IA). Amnion, AF, and fetal urine were subjected to transcriptomics (RNA-Seq) and proteomics studies followed by Ingenuity Pathway Analysis. The analysis uncovered nine transport-associated pathways and four groups of differentially expressed transcripts and proteins. These can be categorized into mediators of vesicular uptake and endocytosis, intracellular trafficking, pathway activation and signaling, and energy metabolism. UD decreased IM transport rate and AFV in conjunction with enhanced expression of vesicular endocytosis regulators but reduced expression of intracellular trafficking mediators. With UDR, IM transport rate decreased and AFV increased. Energy metabolism activators increased while trafficking mediators decreased in expression. IA increased IM transport rate and AFV together with enhanced expressions of vesicular endocytosis and trafficking mediators. We conclude that IM transport across the amnion is regulated by multiple vesicular transcytotic and signaling pathways and that the mediators of intracellular trafficking most likely play an important role in determining the rate of IM transport. Furthermore, the motor protein cytoplasmic dynein light chain-1, which coexpressed in AF and fetal urine, may function as a urine-derived IM transport stimulator.

Regulation of amniotic fluid volume: insights derived from amniotic fluid volume function curves

Recent advances in understanding the regulation of amniotic fluid volume (AFV) include that AFV is determined primarily by the rate of intramembranous absorption (IMA) of amniotic fluid across the amnion and into fetal blood. In turn, IMA rate is dependent on the concentrations of yet-to-be identified stimulator(s) and inhibitor(s) that are present in amniotic fluid. To put these concepts in perspective, this review 1) discusses the evolution of discoveries that form the current basis for understanding the regulation of AFV, 2) reviews the contribution of IMA to this regulation, and 3) interprets experimentally induced shifts in AFV function curves and amnioinfusion function curves in terms of the activity of the amniotic fluid stimulator and inhibitor of IMA. In the early 1980s, it was not known whether AFV was regulated. However, by the late 1980s, IMA was discovered to be a "missing link" in understanding the regulation of AFV. Over the next 25 years the concept of IMA evolved from being a passive process to being an active, unidirectional transport of amniotic fluid water and solutes by vesicles within the amnion. In the 2010s, it was demonstrated that a renally derived stimulator and a fetal membrane-derived inhibitor are present in amniotic fluid that regulate IMA rate and hence are the primary determinants of AFV. Furthermore, AFV function curves and amnioinfusion function curves provide new insights into the relative efficacy of the stimulator and inhibitor of IMA.

Transport-associated pathway responses in ovine fetal membranes to changes in amniotic fluid dynamics

Current evidence suggests that amniotic fluid volume (AFV) is actively regulated by vesicular transport of amniotic fluid outward across the amnion and into the underlying fetal vasculature in the placenta. Our objective was to determine whether gene expression profiles of potential stimulators, inhibitors, and mediators of vesicular transport are altered in response to changes in intramembranous absorption (IMA) rate. Samples of ovine amnion and chorion were obtained from fetal sheep with normal, experimentally reduced or increased AFVs and IMA rates. Amnion and chorion levels of target mRNAs were determined by RT‐qPCR. In the amnion, caveolin‐1 and flotillin‐1 mRNA levels were unchanged during alterations in IMA rate. However, levels of both were significantly higher in amnion than in chorion. Tubulin‐α mRNA levels in the amnion but not in chorion were reduced when IMA rate decreased, and amnion levels correlated positively with IMA rate (P < 0.05). Dynamin‐2 mRNA levels were not altered by experimental conditions. Vascular endothelial growth factor (VEGF₁₆₄ and VEGF₁₆₄b) mRNA levels increased during both increases and decreases in IMA rate, whereas soluble Flt‐1 levels did not change. Neither HIF‐1α nor PBEF mRNA levels in the amnion were correlated with VEGF₁₆₄ expression levels and were not related to IMA rate. Collectively, our findings suggest that changes in amnion microtubule expression may be important in the regulation of transcellular vesicular transport of amniotic fluid and thus modulate IMA rate. Further, our results are consistent with the concept that the amnion is the rate‐limiting layer for amniotic fluid transport.

11β-HSD1 in Human Fetal Membranes as a Potential Therapeutic Target for Preterm Birth

Human parturition is a complex process involving interactions between the myometrium and signals derived from the placenta, fetal membranes, and fetus. Signals originating from fetal membranes are crucial components that trigger parturition, which is clearly illustrated by the labor-initiating consequence of membrane rupture. It has been recognized for a long time that among fetal tissues in late gestation the fetal membranes possess the highest capacity for cortisol regeneration by 11β-hydroxysteroid dehydrogenase type 1 (11β-HSD1). However, the exact role of this unique feature remains a mystery. Accumulating evidence indicates that this extra-adrenal source of cortisol may serve as an upstream signal for critical events in human parturition, including enhanced prostaglandin and estrogen synthesis as well as extracellular matrix remodeling. This may explain why such high capacity for cortisol regeneration develops in human fetal membranes at late gestation. Therefore, inhibition of 11β-HSD1 may provide a potential therapeutic target for prevention of preterm birth. This review summarizes the current understanding of the functional role of cortisol regeneration by 11β-HSD1 in human fetal membranes.

Effect of maternal metabolism on fetal supply: Glucose, non-esterified fatty acids and beta-hydroxybutyrate concentrations in canine maternal serum and fetal fluids at term pregnancy

The progressive adaptations in carbohydrate and lipid metabolism during canine pregnancy are reflected in the concentrations of glucose, non-esterified fatty acids (NEFA) and β-hydroxybutyrate (BHB). The levels of these metabolites in the bitch likely affect fetal concentrations and the composition of amniotic and allantoic fluids (AMF and ALF, respectively). We studied 31 canine parturitions (Cesarean sections) and found that glucose, NEFA and BHB concentrations were significantly higher in maternal serum than in AMF or ALF. Glucose levels in maternal serum, AMF and ALF were closely related (R² ≥ 0.821, P < 0.0001) as well as serum and AMF BHB levels (R² = 0.661, P < 0.0001). In maternal serum, increases in NEFA were associated with increased BHB, and both were negatively related to glucose (P ≤ 0.010). To estimate the effect of the metabolic burden of pregnancy, we evaluated these variables in relation to the dam's body weight and to the ratio of litter weight to the dam's body weight (LW/BW). Maternal serum glucose was not influenced by LW/BW, but it was lower in small than in large/giant bitches. Small breed dogs and those with >10% LW/BW had significantly higher serum NEFA and BHB concentrations. Glucose in AMF and ALF was independent of LW/BW (P ≥ 0.399). AMF NEFA was lower and BHB higher, if LW/BW was >10% (P ≤ 0.048). In conclusion, the extent of the metabolic load of pregnancy in bitches depends on breed size and on the ratio of litter weight to dam's body weight. Maternal concentrations of glucose, BHB and NEFA determine the concentrations of these metabolites in fetal fluids.

Retinoic Acid Pathway Regulation of Vascular Endothelial Growth Factor in Ovine Amnion

Vascular endothelial growth factor (VEGF) has been proposed as an important regulator of amniotic fluid absorption across the amnion into the fetal vasculature on the surface of the placenta. However, the activators of VEGF expression and action in the amnion have not been identified. Using the pregnant sheep model, we aimed to investigate the presence of the retinoic acid (RA) pathway in ovine amnion and to determine its effect on VEGF expression. Further, we explored relationships between RA receptors and VEGF and tested the hypothesis that RA modulates intramembranous absorption (IMA) through induction of amnion VEGF in sheep fetuses subjected to altered IMA rates. Our study showed that RA receptor isoforms were expressed in sheep amnion, and RA response elements (RAREs) were identified in ovine RARβ and VEGF gene promoters. In ovine amnion cells, RA treatment upregulated RARβ messenger RNA (mRNA) and increased VEGF transcript levels. In sheep fetuses, increases in IMA rate was associated with elevated VEGF mRNA levels in the amnion but not in the chorion. Further, RARβ mRNA was positively correlated with VEGF mRNA levels in the amnion and not chorion. We conclude that an RA pathway is present in ovine fetal membranes and that RA is capable of inducing VEGF. The finding of a positive relationship between amnion VEGF and RARβ during altered IMA rate suggests that the retinoid pathway may play a role through VEGF in regulating intramembranous transport across the amnion.

Amniotic Epithelial Cell Culture

Ovine amniotic epithelial cells (oAEC) are a subset of placental stem cells with great regenerative and immunomodulatory properties. Indeed, oAEC are object of intense study for regenerative medicine thanks to the several advantages in developing pre-clinical studies on a high value translational animal model, such as sheep. For this reason, a critical standardization of cultural practices is fundamental to preserve in vitro oAEC native phenotype, thus improving their in vivo therapeutic potential and clinical outcomes. Here is described an improved oAEC cultural protocol able to preserve the native epithelial phenotype also after the in vitro amplification.

Progesterone prevents epithelial-mesenchymal transition of ovine amniotic epithelial cells and enhances their immunomodulatory properties

The in vitro expansion is detrimental to therapeutic applications of amniotic epithelial cells (AEC), an emerging source of fetal stem cells. This study provides molecular evidences of progesterone (P4) role in preventing epithelial-mesenchymal transition (EMT) in ovine AEC (oAEC). oAEC amplified under standard conditions spontaneously acquired mesenchymal properties through the up-regulation of EMT-transcription factors. P4 supplementation prevented phenotype shift by inhibiting the EMT-inducing mechanism such as the autocrine production of TGF-β and the activation of intracellular-related signaling. The effect of P4 still persisted for one passage after steroid removal from culture as well as steroid supplementation promptly reversed mesenchymal phenotype in oAEC which have experienced EMT during amplification. Furthermore, P4 promoted an acute up-regulation of pluripotent genes whereas enhanced basal and LPS-induced oAEC anti-inflammatory response with an increase in anti-inflammatory and a decrease in pro-inflammatory cytokines expression. Altogether, these results indicate that P4 supplementation is crucial to preserve epithelial phenotype and to enhance biological properties in expanded oAEC. Therefore, an innovative cultural approach is proposed in order to improve therapeutic potential of this promising source of epithelial stem cells.

Vascular Endothelial Growth Factor Activation of Intramembranous Absorption: A Critical Pathway for Amniotic Fluid Volume Regulation

OBJECTIVE

The purpose of this review is to propose a critical role for vascular endothelial growth factor (VEGF) in mediating the transfer of amniotic fluid from the amniotic compartment through the fetal membranes and fetal surface of the placenta into fetal blood.

METHODS

Experimental findings in humans and animal models on the action of VEGF in mediating fluid transfer are reviewed and interpreted in order to postulate a proposed mechanism for VEGF regulation of amniotic fluid absorption through the fetal membranes and placenta.

RESULTS

Recent scientific advances suggest that up-regulation of VEGF gene expression in the amnion and chorion is associated with increased transfer of amniotic fluid into fetal blood. The possible mechanisms of action for VEGF appear to involve regulation of intramembranous blood vessel proliferation and membrane transport via passive permeation as well as nonpassive transcytotic vesicular movement of fluid.

CONCLUSION

Currently evolving concepts suggest that amniotic fluid volume is regulated through modulation of the rate of intramembranous absorption of amniotic fluid by both passive and nonpassive mechanisms. The permeability factor VEGF appears to be a critical regulator of amniotic fluid transport in the fetal membranes.

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.

Unidirectional transport across cultured ovine amniotic epithelial cell monolayer

OBJECTIVE

To determine whether ovine amniotic cells in monolayer culture exhibit unidirectional transport.

METHODS

Permeability of (14)C-inulin was measured in two directions across ovine amniotic cells grown to confluence on cell culture inserts.

RESULTS

Permeability was 0.69 + 0.17 (SE) microl/min/cm(2) from the top of the insert where the cells were attached to the bottom and 0.11 + 0.02 microl/min/cm(2) in the reverse direction (P < 0.001). Permeability was similar in the two directions for chorionic cells and umbilical vein endothelial cells. Addition of vascular endothelial growth factor did not alter permeability.

CONCLUSIONS

Transcellular transport in ovine amniotic monolayers is directionally specific. The transport ratio of 6.2:1 for (14)C-inulin is comparable to the in vivo ratio of 4.7:1 for the ovine intramembranous pathway and is consistent with passive diffusion in both directions and vesicular transport in one direction across the monolayer at 5.2 times the rate of passive diffusion.

Initial development of bovine placentation (Bos indicus) from the point of view of the allantois and amnion

The aim of this study was to perform a morphological characterization of the initial bovine placental development, between 20 and 70 days post-insemination (p.i.), with emphasis on the differentiation of the allantois and amnion. After collection, the conceptuses were dissected, macroscopically measured and photographically documented. The extraembryonic membranes were cut into fragments measuring 5 cm(2), and then fixed in 4% paraformaldehyde for analysis using light microscopy, and in 2.5% glutaraldehyde for use in scanning and transmission electron microscopy. The extraembryonic and fetal membranes presented variable degrees of development throughout the periods analysed. The macroscopic appearance of vascularization of the allantois, its attempt to merge with the chorium and the effective appearance of the first cotyledons in development were the events observed from 30 to 40 days of pregnancy. The measurements of the amnion increased gradually as gestation developed. The allantoic epithelia presented cellular dimorphism from 20 to 25 days of pregnancy, but was shown to be immature from 60 to 70 days of pregnancy.

The amniotic plaques in sheep of the Karagouniko breed

The structure of amniotic plaques and adjacent epithelium of full term ewes of the Karagouniko breed were studied using scanning electron microscopy (SEM) and light microcopy. The amniotic plaques appeared as cauliflower-like structures mainly trifurcate or as single papillae. The wall of their stems possessed numerous foldings and round openings. Of interest to note was the abundant vascularization observed in sections of the amniotic plaques. The adjacent amniotic epithelium to the plaques revealed a heterogenous surface which was composed of cells of various forms.

Cloning and cellular expression of aquaporin 9 in ovine fetal membranes

OBJECTIVE

Amniotic fluid (AF) absorption across fetal membranes is essential for AF volume homeostasis, balancing fetal swallowing, urine flow, and lung liquid production. In sheep, AF is absorbed primarily across the amniotic membrane into fetal vasculature situated between the amnion and chorion. Aquaporins (AQPs) are cell membrane proteins that serve as water channels. Recent studies have demonstrated the expression of AQP 1, 3, 8, and 9 in human chorioamniotic membranes and placenta. As AF dynamics continued to be explored primarily in the ovine model, we sought to clone and characterize the expression of ovine AQP9 in fetal membranes.

METHODS

Ovine AQP9 gene was cloned with the use of homology reverse transcriptase-polymerase chain reaction (RT-PCR). RT-PCR and Northern analysis were used to determine AQP9 gene expression, and immunohistochemistry (IHC) used to localize AQP9 protein expression in ovine fetal membranes.

RESULTS

A 2085-base pair (bp) full-length complementary DNA (cDNA) sequence of ovine AQP9 was cloned. The ovine AQP9 cDNA is 86%, 82%, and 82%, and the predicted amino acid sequence (295 amino acids) is 77%, 71%, and 69% identical to human, rat, and mouse AQP9, respectively. RT-PCR and Northern analysis detected AQP9 messenger RNA expression in ovine amnion and allantois, but not in placenta, chorion, or umbilical cord. Immunohistochemistry localized AQP9 protein in epithelia of amnion and allantois.

CONCLUSION

The presence of significant AQP9 messenger RNA and protein expression in ovine fetal membranes suggests that AQP9 may be a major water channel for intramembranous AF resorption in sheep. The cloning of ovine AQP9 and the demonstration of AQP9 expression in amnion and allantois significantly enhances our understanding of ovine AF regulation and offers the potential for therapeutic approaches for the treatment of oligohydramnios and polyhydramnios.

Comparison of amniotic and intramembranous unidirectional permeabilities in late-gestation sheep

OBJECTIVE

Amniotic fluid volume is regulated by the intrinsic modulation of intramembranous absorption. However, neither the mechanisms nor the rate-limiting barriers of this transport are known. We tested the hypothesis that the amnion is the rate-limiting barrier of intramembranous absorption by comparing unidirectional permeabilities of the amnion in vitro and the intramembranous pathway in vivo.

STUDY DESIGN

Unidirectional permeabilities to 99m technetium pertechnate or [14 C]inulin of fresh ovine amnion were measured in vitro in a Ussing chamber; the permeability-surface area products were calculated by the multiplication of the permeabilities by gestational age-specific amniotic surface areas. Unidirectional permeabilities of the intramembranous pathway of the 2 tracers were calculated from solute fluxes between amniotic fluid and fetal blood in chronically catheterized late-gestation fetal sheep. Statistical comparisons included t -tests, least squares regression, analysis of variance, and analysis of covariance.

RESULTS

In the isolated amnion in vitro, the ratio of permeabilities in the amniotic fluid to chorionic direction and the reverse direction was not significantly different from unity for 99m technetium pertechnate (1.03+/-0.10 [SE]; n=7) or [14 C]inulin (1.10+/-0.17; n=7). In contrast, in the in vivo preparation, the ratio of intramembranous permeabilities outward from the amniotic fluid and the reverse direction was greater than unity for 99m technetium pertechnate (2.10+/-0.34; n=8; P=.014) and [14 C]inulin (4.68+/-1.24; n=7; P=.025). The permeability-surface area product of 99m technetium pertechnate (2.18+/-0.79 mL/min) of the isolated amnion was similar to the in vivo intramembranous permeability (n=8) in the amniotic fluid to fetal blood direction (1.42+/-0.34 mL/min) and greater than that in the reverse direction (0.84+/-0.25 mL/min; P=.046). The permeability-surface area product of [14 C]inulin of the amnion (0.53+/-0.20 mL/min) was similar to intramembranous permeability (n=7) in the amniotic fluid to fetal blood (0.68+/-0.15 mL/min) direction and greater than that in the reverse direction (0.22+/-0.06 mL/min; P=.0097).

CONCLUSION

Solute transport across the ovine amnion depends on solute size and appears to be limited only by the amnion's passive diffusional properties. In vivo intramembranous transport similarly depends on solute size but is not exclusively a passive diffusional process because it is primarily unidirectional outward from the amniotic fluid. Although it is a major barrier, the amnion is not the only barrier and does not appear to be responsible for the unidirectional nature of intramembranous absorption. Thus, unidirectionality appears to be imparted by nonpassive mechanisms in non-amnion tissues, which most likely includes vesicular transport within the endothelial cells of the intramembranous microvessels.

Ontogeny of aquaporins 1 and 3 in ovine placenta and fetal membranes

A sensitive and highly reproducible method has been used to show that Aquaporin 3 (AQP(3)) mRNA is present in the ovine placenta and chorion from at least 60 days of gestation (term=145-150d) with levels increasing substantially (>16 fold) at 100 days, and remaining constant thereafter. By immuno- and hybridization histochemistry, the epithelial cells expressing AQP(3)were found to be the trophoblast cells. Some AQP(3)was expressed in fibroblasts of the amnion and allantois but none was expressed in the epithelia of these membranes. AQP(1)was expressed in endothelial cells of fetal and maternal blood vessels but not in any epithelial cell of the ovine placenta and fetal membranes. The level of AQP(3)expression is consistent with known ovine placental permeabilities to water, glycerol and urea.