Comparison of the infectivity of influenza viruses in two host systems: the allantois of intact eggs and surviving allantois-on-shell.
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Investigation into a role for the primitive streak in development of the murine allantois.
Despite its importance as the source of one of three major vascular systems in the mammalian conceptus, little is known about the murine allantois, which will become the umbilical cord of the chorio-allantoic placenta. During gastrulation, the allantois grows into the exocoelomic cavity as a mesodermal extension of the posterior primitive streak. On the basis of morphology, gene expression and/or function, three cell types have been identified in the allantois: an outer layer of mesothelial cells, whose distal portion will become transformed into chorio-adhesive cells, and endothelial cells within the core. Formation of endothelium and chorio-adhesive cells begins in the distal region of the allantois, farthest from the streak. Over time, endothelium spreads to the proximal allantoic region, whilst the distal outer layer of presumptive mesothelium gradually acquires vascular cell adhesion molecule (VCAM1) and mediates chorio-allantoic union. Intriguingly, the VCAM1 domain does not extend into the proximal allantoic region. How these three allantoic cell types are established is not known, although contact with the chorion has been discounted. In this study, we have investigated how the allantois differentiates, with the goal of discriminating between extrinsic mechanisms involving the primitive streak and an intrinsic role for the allantois itself. Exploiting previous observations that the streak contributes mesoderm to the allantois throughout the latter's early development, microsurgery was used to remove allantoises at ten developmental stages. Subsequent whole embryo culture of operated conceptuses resulted in the formation of regenerated allantoises at all time points. Aside from being generally shorter than normal, none of the regenerates exhibited abnormal differentiation or inappropriate cell relationships. Rather, all of them resembled intact allantoises by morphological, molecular and functional criteria. Moreover, fate mapping adjacent yolk sac and amniotic mesoderm revealed that these tissues and their associated bone morphogenetic protein 4 (BMP4) did not contribute to restoration of allantoic outgrowth and differentiation during allantoic regeneration. Thus, on the basis of these observations, we conclude that specification of allantoic endothelium, mesothelium and chorio-adhesive cells does not occur by a streak-related mechanism during the time that proximal epiblast travels through it and is transformed into allantoic mesoderm. Rather, all three cell-types are established by mechanisms intrinsic to the allantois, and possibly include roles for cell age and cell position. However, although chorio-adhesive cells were not specified within the streak, we discovered that the streak nonetheless plays a role in establishing VCAM1's expression domain, which typically began and was thereafter maintained at a defined distance from the primitive streak. When allantoises were removed from contact with the streak, normally VCAM1-negative proximal allantoic regions acquired VCAM1. These results suggested that the streak suppresses formation of chorio-adhesive cells in allantoic mesoderm closest to it. Together with previous results, findings presented here suggest a model of differentiation of allantoic mesoderm that invokes intrinsic and extrinsic mechanisms, all of which appear to be activated once the allantoic bud has formed.
Allantois↗
Developmental potency of the murine allantois.
The murine allantois is the future umbilical component of the placenta. The base of the allantois is also thought to contain the future germ line. We have examined the fate and developmental potency of cells within the murine allantois during gastrulation. lacZ-expressing headfold-stage allantoises (approximately 8.0 days postcoitum; dpc) were subdivided into three proximodistal regions and transplanted into three sites in synchronous non-transgenic host embryos: the primitive streak at the level of prospective paraxial mesoderm, the primitive streak at the level of lateral plate mesoderm, and the base of the allantois. After 23 hours in culture, operated conceptuses were examined histologically for contribution of donor allantoic cells to the conceptus. None of the allantoic regions contributed to paraxial mesoderm when placed into the fetus, but all three colonized the endothelium and adjacent mesenchyme of the dorsal aorta. The mid-region was most efficient at colonizing endothelium, whereas the base was the only allantoic region to exhibit relative pluripotency, colonizing several derivatives of all three primary germ layers. Differences in the state of differentiation along the proximodistal axis of the allantois were further borne out when the three allantoic regions were placed into the base of the allantois of host conceptuses. Striking differences were observed in final position along the proximodistal axis of the host allantois. Most grafted cells translocated distally from the base; however, basal donor allantoic cells translocated typically only as far as the host's mid-region, whereas donor allantoic tip cells typically returned to the tip, often colonizing the chorioallantoic fusion junction. Together, our data reveal that the headfold-stage allantois may contain a proximodistal gradient of differentiation, and raise intriguing questions about how this gradient was established and the role it plays in umbilical vasculogenesis.
Allantois↗
Vascularization in the murine allantois occurs by vasculogenesis without accompanying erythropoiesis.
The aim of this study was to determine whether the blood vessels of the murine allantois are formed by vasculogenesis or angiogenesis. Morphological analysis revealed that differentiation of allantoic mesoderm into an outer layer of mesothelium and an inner vascular network begins in the distal region of the allantois, which is most remote from other tissues, as early as the late neural plate stage (approximately 7.75 days postcoitum). Nascent blood vessels were not found in the base of the allantois until 4-somite pairs had formed in the fetus (approximately 8.25 days postcoitum), and vascular continuity with the yolk sac and fetus was not present until the 6-somite-pair stage (approximately 8.5 days postcoitum). Immunohistochemical analysis demonstrated that flk-1, a molecular marker of early endothelial cells, is expressed in significantly more distal than basal core cells in the early allantois and never in mesothelium. Furthermore, synchronous grafting of donor yolk sac containing blood islands into blood islands of headfold-stage host conceptuses provided no evidence that the yolk sac contributes endothelial cells to the allantois. Finally, when removed from conceptuses and cultured in isolation, neural plate and headfold-stage allantoises formed a conspicuous vascular network that was positive for Flk-1. Hence, the vasculature of the allantois is formed intrinsically by vasculogenesis rather than extrinsically via angiogenesis from the adjacent yolk sac or fetus. Whether allantoic vasculogenesis is associated with erythropoiesis was also investigated. Benzidine-staining in situ revealed that primitive erythroid cells were not identified in the allantois until 6-somite pairs when continuity between its vasculature and that of the yolk sac was first evident. Nevertheless, a small number of allantoises removed from conceptuses at a considerably earlier stage were found to contain erythroid precursor cells following culture in isolation. To determine whether such erythroid cells could be of allantoic origin, host allantoises were made chimeric with lacZ-expressing donor allantoises that were additionally labeled with [3H]methyl thymidine. Following culture and autoradiography, many lacZ-expressing benzidine-stained cells were observed in donor allantoises, but none contained silver grains above background. Moreover, no cells of donor allantoic origin were found in the fetus or yolk sac. Hence, vasculogenesis seems to be independent of erythropoiesis in the allantois and to involve a distal-to-proximal gradient in differentiation of allantoic mesoderm into the endothelial cell lineage. Furthermore, this gradient is established earlier than reported previously, being present at the neural plate stage.
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Growth in the pre-fusion murine allantois.
Prior to fusion with the chorion, the extraembryonic mesoderm of the murine (Mus musculus) allantois differentiates with distal-to-proximal polarity into at least two cell lineages: a chorio-adhesive cell lineage called mesothelium, and the endothelium of the umbilical vasculature. How the allantois grows is less clear, but cell proliferation and addition of mesoderm from the underlying primitive streak appear to play important roles. The aim of this study was to analyze growth in the murine allantois. Techniques of histology and microsurgery were used to examine pre-fusion allantoises at nine developmental timepoints that differed by approximately 2 h. Cell counts revealed that allantoic size increased over time. Two hours of exposure to colcemid enhanced mitotic figures, which were used to calculate the relative number of proliferating cells (mitotic index, MI) in pre-fusion allantoises at each developmental timepoint. Cell proliferation was highest in nascent allantoises and showed signs of slowing by two somite pairs. By five to six-somite pairs, when most allantoises are attaching to the chorion, the overall MI decreased significantly. No regional differences in the mitotic index were observed at any developmental stage. Total cell numbers and the mitotic index were then used to discover the extent of streak contribution to pre-fusion allantoises. Cell proliferation and streak activity were highest in nascent allantoises, after which growth occurred predominantly by cell proliferation. Formation of allantoic regenerates by microsurgical removal and culture in intact conceptuses provided independent confirmation that, as the allantois matured, the primitive streak ceased to be a major contributor to its growth. Thus, the allantois grows by both mitosis and addition of mesoderm from the streak. That the periods of highest cell proliferation and streak activity coincided raises intriguing questions concerning their interplay in the control of growth in the murine allantois.
Allantois↗
Developing allantois is a primary site of 2'-deoxycoformycin toxicity.
In this study, an assessment of normal mouse allantoic development and its sensitivity to 2'-(R)-deoxycoformycin (dCF; Pentostatin) exposure were examined. Both dissecting microscopy and scanning electron microscopy were used to describe the normal growth and morphogenesis of the mouse allantois over gestational days 7-10 as a preliminary step in evaluating potential abnormal allantoic ontogeny and its effect on umbilical cord and placental development. Two abnormal allantoic/umbilical cord phenotypes were observed subsequent to injecting pregnant mice with 5 mg dCF/kg, i.p., on gestational day 7 (GD 7) and evaluating litters on GD 10, 11, and 12. Abnormal phenotypes included: (1) an allantois which extended approximately halfway across the exocoelom but failed to establish a functional contact with the chorion; and (2) a phenotype characterized by reduced expansion of the allantois across the chorionic surface, a very thin umbilical cord, and aberrant vascularization throughout the structure. Both abnormal phenotypes exhibited either an agenesis or hypogenesis of the umbilical cord and chorioallantoic plate, respectively. Neither abnormal phenotype, however, exhibited errors in the directionality of allantoic growth toward the chorion nor in the formation of aberrant contacts between allantois and adjacent yolk sac or amnionic mesenchyme. Statistical interpretation of the experimental data strongly suggested that abnormalities in allantoic/umbilical cord development were directly associated with embryolethality as evidenced by a decline in the frequency of abnormal allantoic/umbilical cord phenotypes over GD 10-12 (73, 36, and 4%; respectively) and a concomitant increase in the frequency of implantation site resorptions over the same time period (7, 47, and 78%). These results strongly suggest that the developing allantois is very sensitive to the effects of dCF exposure, and that interference with its development leads to embryolethality by GD 12.
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The allantois of the North American opossum (Didelphis virginiana) with preliminary observations on the yolk sac endoderm and trophectoderm.
During the middle of prenatal day 10, the opossum allantois forms as a ventral outgrowth of the hindgut. By day 11 it appears as a large, fluid filled sac and by the middle of day 12 (just prior to birth) it reaches its maximal development. The simple squamous epithelium lining the allantois consists of only one cell type that often contains numerous filaments in the apical cytoplasm. At the luminal surface, the apices of the cells are united by junctional complexes and desmosomes are present between adjacent cells. The luminal surface is irregular, whereas laterally and basally the cell membranes show few if any infoldings. Mitotic figures and presumptive degenerating cells occasionally occur in the allantoic epithelium which rests on a delicate basal lamina. The allantois is covered by a simple squamous mesothelium that lacks a distinct basal lamina. Between the two epithelial sheets lie mesenchymal cells, collagen fibers, and blood vessels. No specializations of cell membranes were noted in either of the epithelial layers. The yolk sac endoderm consists of a single layer of squamous cells whose cytoplasm contains scattered profiles of rough endoplasmic reticulum and mitochondria. Extensive lateral and basal infoldings of the plasmalemma were not observed in these endodermal cells. The morphology of the trophectodermal (trophoblastic) cells indicates a cell type that is active in the transport of materials. Since the endodermal cells that line the allantois lack morphological features that would suggest the presence of mechanisms for transport or exchange, and because they remain relatively unchanged throughout pregnancy, it is thought that the allantois functions primarily in the storage of urinary wastes during prenatal life.(ABSTRACT TRUNCATED AT 250 WORDS)
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Axial rotation in rat embryos: morphological analysis and microsurgical study on the role of the allantois.
In mouse and rat embryos, the embryonic disc develops within a cup-shaped "egg cylinder" and consists of an inner layer of ectoderm and an outer layer of endoderm. Because of this configuration, the embryo first develops in a dorsally flexed position and then undergoes "axial rotation" to a ventrally flexed position. In the present study, we first analyzed the morphological process of axial rotation in rat embryos using novel reference axes set in the egg cylinder that remained invariant during the process. Our new perspective allowed us to demonstrate that the process consists of three movements which start at different stages of development: twisting of the upper body at stage 12/s7-8, twisting of the middle body at stage 13/s11-12, and twisting of the lower body (so called "tail") at stage 14/s15-16. Axial rotation is an interesting developmental event not only because it is such a dynamic process but also because it is one of the earliest morphological signs of body asymmetry. This asymmetry is strongly biased in that the tail almost always finishes up on the right side of the embryo for reasons that are still unknown. In the second part of the study, we performed microsurgical experiments to extend our previous finding that removal of the allantois results in random determination of tail sidedness. We demonstrated that an allantois transplanted from another embryo can prevent this abnormal sidedness in an embryos whose allantois had been removed and that transecting the allantois did not lead to abnormal tail sidedness. A possible explanation is that the allantois produces a chemical factor that controls tail sidedness.
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Hemangioblast commitment in the avian allantois: cellular and molecular aspects.
We recently identified the allantois as a site producing hemopoietic and endothelial cells capable of colonizing the bone marrow of an engrafted host. Here, we report a detailed investigation of some early cytological and molecular processes occurring in the allantoic bud, which are probably involved in the production of angioblasts and hemopoietic cells. We show that the allantois undergoes a program characterized by the prominent expression of several "hemangioblastic" genes in the mesoderm accompanied by other gene patterns in the associated endoderm. VEGF-R2, at least from stage HH17 onward, is expressed and is shortly followed by transcription factors GATA-2, SCL/tal-1, and GATA-1. Blood island-like structures differentiate that contain both CD45(+) cells and cells accumulating hemoglobin; these structures look exactly like blood islands in the yolk sac. This hemopoietic process takes place before the establishment of a vascular network connecting the allantois to the embryo. As far as the endoderm is concerned, GATA-3 mRNA is found in the region where allantois will differentiate before the posterior instestinal portal becomes anatomically distinct. Shortly before the bud grows out, GATA-2 was expressed in the endoderm and, at the same time, the hemangioblastic program became initiated in the mesoderm. GATA-3 is detected at least until E8 and GATA-2 until E3 the latest stage examined for this factor. Using in vitro cultures, we show that allantoic buds, dissected out before the establishment of circulation between the bud and the rest of the embryo, produced erythrocytes of the definitive lineage. Moreover, using heterospecific grafts between chick and quail embryos, we demonstrate that the allantoic vascular network develops from intrinsic progenitors. Taken together, these results extend our earlier findings about the commitment of mesoderm to the endothelial and hemopoietic lineages in the allantois. The detection of a prominent GATA-3 expression restricted to the endoderm of the preallantoic region and allantoic bud, followed by that of GATA-2, is an interesting and novel information, in the context of organ formation and endoderm specification in the emergence of hemopoietic cells.
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Allantois as the bursa-equivalent in man.
This article proposes the hypothesis that allantois is the anatomical homologue of bursa of Fabricius in mammalia. This hypothesis is based on immunohistochemical study of sections from hematopoietic organs of three fetuses with cloacal exstrophy. In the complex maldevelopment of cloacal exstrophy the common features are absence of distal midgut and exstrophy of cloaca with an exstropic intervening intestinal band. It has been suggested that gut is of dual origin and in addition to the yolk sac, allantois, absent in this maldevelopment, is involved in the embryogenesis of intestines by developing into distal midgut. There was absence of B-lymphocyte in two of three fetuses with rudimentary midgut and aplastic allantois. Meanwhile, B-cells were identified in the hematopoietic tissues and the prominent Peyer's patches of the more developed exstrophic allantois in the third case presented here. Therefore, it is concluded that allantois as the origin of distal midgut and lymphatic-rich ileocecal portion of digestive system is directly involved in the B-cell formation and represents the bursa of Fabricius in man.
Abnormalities, Multiple↗
Bone morphogenetic protein 4 in the extraembryonic mesoderm is required for allantois development and the localization and survival of primordial germ cells in the mouse.
Evidence suggests that the specification of primordial germ cells (PGCs) in the mammalian embryo does not depend on maternal determinants. Rather, previous genetic analysis in the mouse has shown that bone morphogenetic protein 4 (Bmp4) is required for the formation of both PGCs and allantois. Bmp4 is expressed in both the trophoblast-derived extraembryonic ectoderm (ExE) and in the epiblast-derived extraembryonic mesoderm (ExM), in which the PGCs, allantois primordium, and angioblasts are first detected. We have shown that Bmp4 made in the ExE functions to induce precursors of PGCs and allantois in the adjacent epiblast, resulting in complete lack of both cell types in homozygous null mutants. However, the function of Bmp4 in the ExM is totally unknown. To address this question, we generated tetraploid (4N) chimeras by aggregating Bmp4 null ES cells with wild-type tetraploid embryos. In this combination, wild-type tetraploid cells contribute to the extraembryonic trophoblast and primitive endoderm lineages but are excluded from the epiblast and its derivatives, including the ExM. Our results clearly demonstrate that Bmp4 made in the ExM does not affect the establishment of either PGC or allantois lineages, but is required for PGC localization and survival and for the differentiation of the allantois. These findings suggest that Bmp4 expressed in epiblast-derived tissues plays vital roles in reproduction by regulating both the development of the germ line and the vascular connection between the embryo and the placenta.
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Expression of genes associated with allantois emergence in ovine and bovine conceptuses.
In the development of ruminant embryos, the emergence and growth of the allantois is critical for the establishment of the chorioallantoic placenta. The allantoic membrane contributes to all the vasculature that perfuses the placental tissues and the fetal membranes. Using suppressive subtractive hybridization to compare mRNA from Day 13 ovine preimplantation conceptuses (prior to allantoic emergence) with Day 17 allantoic membrane, we identified nine genes whose expression was associated with the emergence of the allantoic sac. Collagen alpha 1 type XII, collagen alpha 2 type I, collagen alpha 2 type V, epsilon 4 beta-globin, osteonectin, and uroplakin were expressed at significantly greater levels in ovine Day 17 allantois compared to Day 13 conceptuses. These genes are associated with the extracellular matrix and most likely are involved in establishing and strengthening the structural integrity of the allantoic sac and in the development of the blood vessels. RalB expression increased with development although at significantly greater levels in the allantois only at Day 19. Hoxa-10 and RhoA showed no differential expression during this period. All these genes showed a similar temporal pattern of expression in bovine conceptuses at equivalent stages of development with significantly greater expression of all these genes, except for Hoxa-10, found in Day 24 allantois compared to Day 14 conceptuses. This suggests that the role they play in allantoic emergence, growth and function is conserved in both ruminant species and that their expression is regulated in a similar manner. The interactions and regulation of this process remains to be fully explained.
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Hematopoietic potential of the pre-fusion allantois.
We previously showed that the fetal component of the placenta has a vigorous hematopoietic activity. Whether this organ is an environmental niche where hematopoietic stem cells (HSC) proliferate and become committed to various lineages, or whether it is also a site for HSC emergence, was left open. This issue can be addressed only if the components that will give rise to the placenta are tested prior to vascularization. The fetal part of the placenta forms through the fusion of the allantois and the chorionic plate around the stage of 7 somite pairs. The allantois, a mesodermal rudiment that provides fetal blood vessels to the placenta, was retrieved before fusion. We found in this rudiment expression of CD41, a known marker of early embryonic hematopoietic progenitors. c-Kit encoding a progenitor specific receptor was also expressed. Significantly, as early as the 1-2 somite stage, the allantois yielded erythroid, myeloid and multipotent clonogenic progenitors, when pre-cultured in toto prior to seeding in a semisolid medium. These results provide evidence that the allantois has hematopoietic potential per se. Whether this potential also involves the ability to produce HSC is still to be determined.
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Study of the murine allantois by allantoic explants.
The murine allantois will become the umbilical artery and vein of the chorioallantoic placenta. In previous studies, growth and differentiation of the allantois had been elucidated in whole embryos. In this study, the extent to which explanted allantoises grow and differentiate outside of the conceptus was investigated. The explant model was then used to elucidate cell and growth factor requirements in allantoic development. Early headfold-stage murine allantoises were explanted directly onto tissue culture plastic or suspended in test tubes. Explanted allantoises vascularized with distal-to-proximal polarity, they exhibited many of the same signaling factors used by the vitelline and cardiovascular systems, and they contained at least three cell types whose identity, gene expression profiles, topographical associations, and behavior resembled those of intact allantoises. DiI labeling further revealed that isolated allantoises grew and vascularized in the absence of significant cell mingling, thereby supporting a model of mesodermal differentiation in the allantois that is position- and possibly age-dependent. Manipulation of allantoic explants by varying growth media demonstrated that the allantoic endothelial cell lineage, like that of other embryonic vasculatures, is responsive to VEGF(164). Although VEGF(164) was required for both survival and proliferation of allantoic angioblasts, it was not sufficient to induce appropriate epithelialization of these cells. Rather, other VEGF isoforms and/or the outer sheath of mesothelium, whose maintenance did not appear to be dependent upon endothelium, may also play important roles. On the basis of these findings, we propose murine allantoic explants as a new tool for shedding light not only on allantoic development, but for elucidating universal mechanisms of blood vessel formation, including vascular supporting cells, either in the intact organism or in existing in vitro systems.
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Differences in sialic acid-galactose linkages in the chicken egg amnion and allantois influence human influenza virus receptor specificity and variant selection.
Human influenza viruses are more efficiently isolated by inoculating patient samples into the amniotic rather than the allantoic cavity of embryonated chicken eggs. This type of cultivation selects virus variants with mutations around the hemagglutinin (HA) receptor binding site. To understand the molecular basis of these phenomena, we investigated the abundances of sialic acid (SA) linked to galactose (Gal) by the alpha-2,3 linkage (SA alpha2,3Gal) and SA alpha2,6Gal in egg amniotic and allantoic cells and in Madin-Darby canine kidney (MDCK) cells. Using SA-Gal linkage-specific lectins (Maackia amurensis agglutinin specific for SA alpha2,6Gal and Sambucus nigra agglutinin specific for SA alpha2,3Gal), we found SA alpha2,3Gal in both allantoic and amniotic cells and SA alpha2,6Gal in only the amniotic cells. MDCK cells contained both linkages. To investigate how this difference in abundances of SA alpha2,3Gal and SA alpha2,6Gal in allantoic and amniotic cells affects the appearance of host cell variants in eggs, we determined the receptor specificities and HA amino acid sequences of two different patient viruses which were isolated and passaged in the amnion or in the allantois and which were compared with MDCK cell-grown viruses. We found that the viruses maintained high SA alpha2,6Gal specificities when grown in MDCK cells or following up to two amniotic passages; however, further passages in either the amnion or allantois resulted in the acquisition of, or a complete shift to, SA alpha2,3Gal specificity, depending on the virus strain. This change in receptor specificity was accompanied by the appearance of variants in the population with Leu-to-Gln mutations at position 226 in their HA. These findings suggest that lack of SA alpha2,6Gal linkages in the allantois of chicken eggs is a selective pressure for the appearance of host cell variants with altered receptor specificities and amino acid changes at position 226.
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The allantois and chorion, when isolated before circulation or chorio-allantoic fusion, have hematopoietic potential.
The chorio-allantoic placenta forms through the fusion of the allantois (progenitor tissue of the umbilical cord), with the chorionic plate. The murine placenta contains high levels of hematopoietic stem cells, and is therefore a stem cell niche. However, it is not known whether the placenta is a site of hematopoietic cell emergence, or whether hematopoietic cells originate from other sites in the conceptus and then colonize the placenta. Here, we show that the allantois and chorion, isolated prior to the establishment of circulation, have the potential to give rise to myeloid and definitive erythroid cells following explant culture. We further show that the hematopoietic potential of the allantois and chorion does not require their union, indicating that it is an intrinsic property of these tissues. These results suggest that the placenta is not only a niche for, but also a source of, hematopoietic cells.
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Cultivation of excysted metacercariae of Echinostoma caproni to ovigerous adults in the allantois of the chick embryo.
Chemically excysted metacercariae of Echinostoma caproni inoculated into the allantois of domestic chick embryos became ovigerous in that site within 9 days postinoculation. The egg preparation technique of Saville and Irwin was markedly better than that of a modified Zwilling procedure for obtaining large numbers of postinoculation embryos with worm infections. Adults of E. caproni from the allantois were larger and became ovigerous sooner than worms grown on the chorioallantois. Only worms from the allantois produced eggs with fully developed miracidia. Miracidia were released from these eggs, but an insufficient number was available to attempt infections in Biomphalaria glabrata snails.
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Passive permeability of ovine amnion and allantois to chloride--gestational changes.
The passive permeability to 36Cl of isolated pieces of amnion (112), amniochorion (41) and allantois (54) from 55 pregnant ewes was studied in vitro. There was a significant decrease in the permeability coefficient (K) with increasing age in both amnion and allantois when samples from two age groups (72 to 109; 110 to 147 days of gestation) were analysed. There was a highly significant correlation (P less than 0.001) between amnion permeability and amniotic fluid [Cl-], but none between allantoic permeability and allantoic fluid [Cl-]. Amniotic fluid osmolality decreased significantly over the last third of gestation and was significantly correlated with amniotic fluid [Cl-]. Allantoic fluid osmolality increased with age and did not correlate with allantoic fluid [Cl-]. The anion exchange blocker, SITS, significantly decreased K in the amnion in the younger age group, suggesting that anion exchange through cell membranes may be one route of Cl- movement through the young amnion. The results suggest that the permeability of the amnion may play a role in the regulation of the composition of amniotic but not allantoic fluid in sheep.
Allantois↗