FINE STRUCTURAL CHANGES IN THE JUNCTIONAL ZONE OF THE RAT PLACENTA WITH INCREASING GESTATIONAL AGE

Modeling Trophoblast Cell-Guided Uterine Spiral Artery Transformation in the Rat

The rat possesses hemochorial placentation with deep intrauterine trophoblast cell invasion and trophoblast-guided uterine spiral artery remodeling, which resembles human placentation. Uterine spiral arteries are extensively remodeled to deliver sufficient supply of maternal blood and nutrients to the developing fetus. Inadequacies in these key processes negatively impact fetal growth and development. Recent innovations in genome editing combined with effective phenotyping strategies have provided new insights into placental development. Application of these research approaches has highlighted both conserved and species-specific features of hemochorial placentation. The review provides foundational information on rat hemochorial placental development and function during physiological and pathological states, especially as related to the invasive trophoblast cell-guided transformation of uterine spiral arteries. Our goal is to showcase the utility of the rat as a model for in vivo mechanistic investigations targeting regulatory events within the uterine-placental interface.

Morphology and physiology of rat placenta for toxicological evaluation

The placenta plays a pivotal role in fetal growth, and placental dysfunction and injury are associated with embryo/fetal toxicity. Histological examination of the rat placenta for safety evaluation provides valuable clues to the mechanisms of this toxicity. However, the placenta has specific and complex biological features unlike those of other organs, and placental structure dramatically changes depending on the time during the gestation period. Thus, time-dependent histopathological examination of the rat placenta should be performed based on the understanding of normal developmental changes in morphology and function. The placentas of rats and humans are both anatomically classified as discoid and hemochorial types. However, there are differences between rats and humans in terms of placental histological structure, the fetal-maternal interface, and the function of the yolk sac. Therefore, extrapolation of placental toxicity from rats to humans should be done cautiously in the evaluation of risk factors. This review describes the development, morphology, physiology, and toxicological features of the rat placenta and the differences between the rat and human placenta to enable accurate evaluation of reproductive and developmental toxicity in studies.

Differential patterning of genes involved in serotonin metabolism and transport in extra-embryonic tissues of the mouse

INTRODUCTION

Serotonin (5-HT) is an important neuromodulator, but recently has been shown to be involved in neurodevelopment. Although previous studies have demonstrated that the placenta is a major source of forebrain 5-HT during early forebrain development, the processes of how 5-HT production, metabolism, and transport from placenta to fetus are regulated are unknown. As an initial step in determining the mechanisms involved, we investigated the expression patterns of genes critical for 5-HT system function in mouse extraembryonic tissues.

METHODS

Mid-through late gestation expression of 5-HT system-related enzymes, Tph1, Ddc, Maoa, and 5-HT transporters, Sert/Slc6a4, Oct3/Slc22a3, Vmat2/Slc18a2, and 5-HT in placenta and yolk sac were examined, with cell type-specific resolution, using multiplex fluorescent in situ hybridization to co-localize transcripts and immunocytochemistry to co-localize the corresponding proteins and neurotransmitter.

RESULTS

Tph1 and Ddc are found in the syncytiotrophoblast I (SynT-I) and sinusoidal trophoblast giant cells (S-TGC), whereas Maoa is expressed in SynT-I, syncytiotrophoblast II (SynT-II) and S-TGC. Oct3 expression is observed in the SynT-II only, while Vmat2 is mainly expressed in S-TGC. Surprisingly, there were comparatively high expression of Tph1, Ddc, and Maoa in the yolk sac visceral endoderm.

DISCUSSION

In addition to trophoblast cells, visceral endoderm cells in the yolk sac may contribute to fetal 5-HT production. The findings raise the possibility of a more complex regulation of 5-HT access to the fetus through the differential roles of trophoblasts that surround maternal and fetal blood space and of yolk sac endoderm prior to normal degeneration.

Endopolyploid and proliferating trophoblast cells express different patterns of intracellular cytokeratin and glycogen localization in the rat placenta

The presence of keratin intermediate filaments is a characteristic of trophoblast differentiation. Meantime, their intracellular localization in the functionally different subtypes of placental trophoblast is poorly investigated in rodent, whereas their placentae are being broadly investigated in recent years as a model of the feto-maternal interaction. The purpose was to study the intracellular distribution of cytokeratin filaments in correlation with glycogen deposits, both being important constituents of the trophoblast cells in rat placenta. Different rat trophoblast cell populations exhibited different patterns of cytokeratin immunolocalization. The most intensive immunostaining was observed in the highly endopolyploid SGTCs (secondary giant trophoblast cells) at the border with decidua basalis. The most prominent cytokeratin-positive threads were found at the periphery of cytoplasm and in the extensive system of cytoplasmic sprouts by which the SGTC connect each other. Similar cytokeratin intensity and distribution was detected in the TSC (trabecular spongiotrophoblast cells) of the junctional zone of placenta that line the lacunae with the maternal blood. Clusters of highly proliferative pre-glycogen as well as glycogen cells showed some weaker cytokeratin signals mostly in the perinuclear and peripheral zones of cytoplasm. At the 11.5th to the 13.5th day of gestation, the interstitial and endovascular invasive endopolyploid TGTCs (tertiary giant trophoblast cells) prove the intensive cytokeratin staining throughout the cytoplasm and its sprouts. Meantime, the TGTCs were glycogen negative. By contrast, glycogen was heavily accumulated in the glycogen cells that belong both to the junctional zone of placenta and the cuff of the central arterial channel underlying the monolayer of endovascularly invading TGTCs. Thus, the TGTCs that are first to penetrate into the depth of the uterine wall do not contain glycogen but are accompanied by the glycogen-rich cells. The SGTC also contained the prominent deposits of glycogen at the periphery of cytoplasm and in the cytoplasmic sprouts. At the 16th day of gestation, an extensive interstitial invasion of the cytokeratin-positive glycogen trophoblast cells from the junctional zone was observed. The patterns of cytokeratin and glycogen intracellular localization are specific for each subtype of the rat trophoblast; that is, most probably, accounted for by the functional diversity of different trophoblast populations, i.e. patterns of invasion/phagocytosis and their involvement in a barrier at the feto-maternal interface.

A novel secretory protein produced by rat spongiotrophoblast

The placenta secretes various factors in stage- and cell-specific manners. We have identified a cDNA encoding a novel protein with 124 amino acids, which was named spongiotrophoblast specific protein (SSP). SSP is highly homologous to mouse 4311, showing 81% and 59% similarity at the nucleotide and amino acid levels, respectively. Northern blot analysis showed that SSP mRNA was first detected on Day 14 of pregnancy, peaked on Day 16, and remained elevated until term. In situ hybridization analysis showed that SSP mRNA was specifically expressed in spongiotrophoblast cells of Day 20 placenta but not in Day 12 placenta. No expression was detected from the differentiated or undifferentiated rat choriocarcinoma Rcho-1 cell line. Native SSP was detected as a 19-kDa molecule by Western blotting in cell extracts prepared from the junctional zone. SSP was predicted to be a secretory protein, because 1) a hydropathy test revealed that SSP contained an N-terminal hydrophobic region and 2) native SSP was also detected in the cultured media of junctional zone explants. To further investigate a potential signal peptide of this protein, sets of recombinant SSP were generated using a COS7 transfection system. The N-terminal amino acid sequence of secreted recombinant SSP confirmed that the N-terminal 17 amino acids had been cleaved to produce a secretory protein. Thus, we have identified and cloned a novel secretory protein, SSP, which is specifically expressed by rat spongiotrophoblast cells during the latter half of pregnancy.

Analysis of rat placental plasma membrane proteins by two-dimensional gel electrophoresis

The placenta plays an essential role in fetal growth and the maintenance of pregnancy and its functions are strictly controlled in a stage-specific manner. To gain an insight into placental functions and their regulation, we analyzed the plasma membrane proteins of rat placenta by two-dimensional polyacrylamide gel electrophoresis (2D/E). Plasma membrane fractions of the placenta obtained on days 12, 14, 16, 18 and 20 of pregnancy were purified by Percoll gradient centrifugation, and subjected to 2D/E analysis. After the proteins on the 2D/E gels had been visualized by silver staining, the patterns on the gels at different stages of pregnancy were compared using image analysis software. Proteins within an isoelectric point (pI) range of 4.0 to 7.0 and a molecular weight (Mw) range of 20-100 kDa were analyzed in detail, and about 800 proteins on average were recognized on each gel. Of these, the expression of 150 proteins was found to change dramatically according to the stage of pregnancy. According to their expression patterns, these proteins were categorized into two groups, Group I and Group II. The proteins belonging to Group I showed a higher intensity of expression on day 12 and disappeared on day 20. They included 119 plasma membrane proteins and were divided into five subgroups. Group II, which consisted of three subgroups, included 31 proteins showing a low or negligible expression on day 12 and higher expression on day 20. Most of the other membrane proteins (about 600) were expressed constantly during pregnancy. On the basis of our data, we constructed a database for plasma membrane proteins of the rat placenta.

Changes in the pattern of expression of alkaline phosphatase in the mouse uterus and placenta during gestation

The development of the rodent chorio-allantoic placenta is a complicated process that results in the formation of a transport system capable of sustaining embryonic and fetal growth and development. Intimately linked to this process is alkaline phosphatase (AP), a cell-surface glycoprotein that possibly functions as a transport protein. In the present study, we have mapped the location of AP-expressing cells in the mouse utero-placental unit during the development of the chorio-allantoic placenta by use of enzyme histochemistry and in situ hybridization histochemistry. We found that at implantation the expression of the tissue non-specific AP (TNAP) gene is located exclusively in the decidua and that most of this decidual expression ceases as the placenta starts to form. One exception is a mesometrially located marginal zone of the decidua, which continues to express the TNAP gene until day 12 and the active protein until at least day 16. Trophoblasts of the chorion already express AP before the time of fusion with the ectoplacental cone, after which AP is expressed by trophoblasts of the resulting ectoplacental plate. AP expression in the mature chorio-allantoic placenta is localized in the placental labyrinth and spongy zones. In the latter zone, expression ceases on about day 14. Giant trophoblasts start to express AP on about day 10, with some cells still positive for AP at day 16. The yolk sac does not express AP at any developmental stage. The results show that AP expression during placental development is neither restricted to cells known to be involved in transport, nor expressed in all cells thought to be involved in this transport. This may indicate that AP is not merely a transport protein but has additional functions.

Giant and binucleate trophoblast cells of mammals

The cellular origin, structure, and function of trophoblastic giant cells (GC) and binucleate cells (BNC) are reviewed. Mammals in which these cells have received the greatest attention include rodents, rabbits, and humans (GCs), and ruminants and equids (BNCs). In almost all cases these cells arise from the cytotrophoblast. All are large cells and contain either two diploid nuclei (BNCs), multiple nuclei (human placental bed GCs), or single nuclei with amplified DNA content (rodent and rabbit GCs). Giant and binucleate cells typically exhibit the capacity for migration or invasion, although the degree of migratory activity varies between species. While most end up within, or at the interface with, endometrial tissue, in some instances the GCs or BNCs contribute directly to the interhemal membrane of the placenta. Hormone production is a property which most GC-BNC populations have in common. Lactogen or gonadotropin has been documented in almost all cells of this type examined to date, and in some animals they are also steroidogenic (e.g., rats and sheep). In spite of some common features, both structural and functional differences remain and it is suggested that use of terms such as mononuclear giant cells, multinucleate giant cells, and binucleate cells be continued rather than assuming that these cells are all members of a single trophoblastic subtype.

Trophoblast cell differentiation and organization: role of fetal and ovarian signals

The rat chorioallantoic placenta is organized into two distinct zones (junctional and labyrinth) and expresses six members of the placental prolactin (PRL) family: placental lactogen-I (PL-I), PL-I variant (PL-Iv), PL-II, PRL-like protein-A (PLP-A), PLP-B, and PLP-C. These placental hormones are expressed in distinct cell- and temporal-specific patterns and can be used to monitor the state of differentiation of rat trophoblast cells. This study was initiated to examine the role of the fetus and maternal ovaries in the regulation of trophoblast cell differentiation and organization. Expression of the placental PRL family was monitored by Northern and Western blotting and immunocytochemical analysis. The roles of the fetus and maternal ovaries were examined by chemically induced fetal death (DFX, induced by intraamniotic injection of digoxin) and surgical removal of the ovaries (OVX), respectively. The endocrine differentiation of the placenta was assessed on day 19 of gestation (sperm positive = day 0, parturition = day 21). Day 10 of gestation was the earliest day that DFX could be reliably performed. Day 10 is a time point during pregnancy preceding the onset of expression of all members of the placental PRL family except PL-I. DFX on day 10 of gestation did not affect the endocrine differentiation of the trophoblast cells but did alter the organization of the chorioallantoic placenta. PL-II, PL-Iv, and the PLPs were all expressed in their appropriate molecular forms and cell types in placentas developing in the absence of fetal influence. The maternal ovaries, in the absence of the fetus, had two distinct actions on the placenta: ovarian signals were essential during a period at midgestation for the maintenance of the placenta and exposure of the developing placenta to ovarian signals during the second half of gestation, in the absence of the fetus, arrested labyrinth zone development. The active ovarian signals were progesterone and estrogen. We conclude that trophoblast cell differentiation occurs independent of the fetus and maternal ovaries. However, signals from both the fetus and maternal ovaries are required for normal organization of the chorioallantoic placenta.

Growth of mouse ectoplacental cone cells in subcutaneous tissues. Development of placental-like cells

Ectoplacental cones of mouse embryos collected on day 8 of pregnancy were grafted into the dorsal subcutaneous tissue of host mice. The grafts were collected between days 3 and 8 after transfer and processed for light and electron microscope morphological analysis as well as for cytochemistry of nonspecific alkaline phosphatase. Fragments of normal mouse placentas collected between days 12 and 18 of pregnancy were processed similarly. About 37% of the grafts were nonhemorrhagic nodules formed by different kinds of trophoblastic cells. These cells had many morphological and cytochemical features of cells present in normal mouse placentas. Nonphagocytic giant cells, glycogen cells, as well as cells with a well-developed granular endoplasmic reticulum were similar to cells found in the placenta and were always present in the grafts. Cells showing features intermediate between the above-mentioned cells and those whose cytoplasm was poor in organelles also were found in the grafts. The latter resembled cells of layer 1 of the labyrinth of the placenta. These results suggest that trophoblastic cells of the ectoplacental cones had differentiated into placental cells following their transfer to the subcutaneous tissue.

Chorioallantoic placenta formation in the rat: II. Angiogenesis and maternal blood circulation in the mesometrial region of the implantation chamber prior to placenta formation

Rat gestation sites were examined on days 7 through 9 of pregnancy by light microscopy and transmission and scanning electron microscopy to determine the extent of vascular modifications in the vicinity of the mesometrial part of the implantation chamber (mesometrial chamber). At a later time, the mesometrial chamber is, in conjunction with the uterine lumen, the site of chorioallantoic placenta formation. On day 7, in the vicinity of the mesometrial chamber, vessels derived from a subepithelial capillary plexus and venules draining the plexus were dilating. By early day 8, this network of thin-walled dilated vessels (sinusoids) was further enlarged and consisted primarily of hypertrophied endothelial cells with indistinct basal laminas. Sinusoids were frequently close to the mesometrial chamber's luminal surface which was devoid of epithelial cells but was lined by decidual cell processes and extracellular matrix. By late day 8, cytoplasmic projections of endothelial cells extended between healthy-appearing decidual cells and out onto the mesometrial chamber's luminal surface, and endothelial cells were sometimes found on the luminal surface indicating that endothelial cells were migrating. The presence of maternal blood cells in the mesometrial chamber lumen suggested that there was continuity between the chamber and blood-vessel lumens. On day 9, the mesometrial chamber was completely lined with hypertrophied endothelial cells, and sinusoid lumens were clearly continuous with the lumen of the mesometrial chamber. Mesometrial sinusoids and possibly the mesometrial chamber lumen were continuous with vessels in vicinity of the uterine lumen that were fed by mesometrial arterial vessels. Clearing of the mesometrial chamber lumen during perfusion fixation via the maternal vasculature indicated the patency of this luminal space and its confluence with mesometrial arterial vessels and sinusoids. The conceptus occupied an antimesometrial position in the implantation chamber on days 7 through 9, and it was not in direct contact with uterine tissues in the vicinity of the mesometrial chamber. These observations suggest that angiogenesis, not trophoblast invasion or decidual cell death, plays a major role in the opening of maternal vessels into the mesometrial chamber lumen before the formation of the chorioallantoic placenta.

Chorioallantoic placenta formation in the rat: I. Luminal epithelial cell death and extracellular matrix modifications in the mesometrial region of implantation chambers

On days 7 and 8 of pregnancy, mesometrial regions of rat gestation sites were examined by light microscopy and transmission electron microscopy to determine what changes occur before the chorioallantoic placenta forms in that region. By day 7, gestation sites contained a uterine lumen mesometrially and an antimesometrial extension of the uterine lumen, the implantation chamber. The implantation chamber consisted of a mesometrial chamber between the uterine lumen and the conceptus, an antimesometrial chamber that contained the conceptus, and a decidual crypt antimesometrial to the conceptus. Stromal cells that formed the walls of the implantation chamber were closely packed decidual cells, while those that surrounded the uterine lumen were loosely arranged. Late on day 7, a portion of the epithelium lining the mesometrial chamber was degenerating, but this area of initial degeneration was never adjacent to the antimesometrial chamber. By early day 8, most of the epithelial cells lining the mesometrial chamber were degenerating and were being sloughed into the chamber lumen. Although degeneration of these epithelial cells morphologically resembled necrosis, it was precisely controlled, since adjacent epithelial cells lining the uterine lumen remained healthy. The space that separated the denuded luminal surface of the mesometrial chamber from underlying decidual cells became wider and was occupied by an extracellular matrix rich in cross-banded collagen fibrils. Decidual cell processes, that earlier had penetrated the basal lamina beneath healthy epithelial cells, protruded into this matrix and penetrated the basal lamina at the luminal surface. By late day 8, large areas of denuded chamber wall were covered with decidual cell processes, little remained of the basal lamina, and cross-banded collagen fibrils were scarce in the area occupied by decidual cell processes. During the times studied, uterine tissues that formed the walls of the mesometrial chamber were not in direct contact with the conceptus. This study indicates that trophoblast does not play a direct role in epithelial degeneration, basal lamina penetration, or extracellular matrix modifications in the mesometrial region of implantation chambers where part of the chorioallantoic placenta forms, although trophoblast may be required to trigger or modulate some of the changes.

Altered placental morphology associated with murine trisomy 16 and murine trisomy 19

The morphology of placentas from trisomy 16 and trisomy 19 mouse conceptuses aged 12 to 18 gestational days was studied at the light microscopic level. Comparisons were made with placentas from normal littermate animals. Trisomy 16 placentas showed marked changes from normal: 1) the junctional zone showed little indication of normal morphologic differentiation throughout gestation; 2) clusters of germinal trophoblast cells persisted in the labyrinth throughout gestation, whereas these cells disappeared by gestational day 16 in the normal littermate placentas; 3) the labyrinth was reduced in size in the trisomic placentas, and the differentiation of the interhemal membranes was delayed. The size of the labyrinths from trisomy 19 placentas appeared to be decreased, but otherwise the placentas appeared to have normal morphology. These observations and others from the literature show that placental development is affected by the presence of a trisomic genome, and that different trisomies influence the development of the placenta differently. For trisomy 16, we propose that the striking changes of the junctional zone may be associated with the trisomy 16-related gene dosage effect for alpha- and beta-interferon cell surface receptors. Because of the homology for this and other genes on mouse chromosome 16 with genes on human chromosome 21, findings related to the altered development of the trisomy 16 mouse may be relevant to understanding some of the phenotypic variations associated with human trisomy 21, the Down syndrome.

Localization and synthesis of the tumor protein oncomodulin in extraembryonic tissues of the fetal rat

The calcium-binding protein oncomodulin, previously found only in tumors, has been detected during rat development. Specific antisera to purified rat hepatoma oncomodulin (MW 11,500) were used to detect oncomodulin by radioimmunoassay (RIA) and by avidin-biotin-peroxidase complex (ABC) immunohistochemistry. Using RIA, oncomodulin was found to increase in placenta from below the limits of detection (2 ng/mg protein) on Day 13 to approximately 25 ng/mg on Day 16 of pregnancy, and to remain high through to the end of gestation. Determinations on separated inner and outer placenta showed the increase to be greater in the outer placenta (basal zone and decidua) than in the inner placenta (labyrinth). The ABC technique on paraffin sections produced positive staining for oncomodulin throughout the placenta, with the most intense staining occurring in the outer placenta (cytotrophoblast and giant cells of the basal zone). Parietal and visceral yolk sac, and amnion also stained positively, while fetal organs did not. Oncomodulin synthesis measured by [35S]methionine incorporation into immunoprecipitates occurred in isolated inner and outer placenta, whole placenta, the separated trophectoderm and endoderm of the parietal yolk sac, and amnion. No oncomodulin synthesis could be measured in visceral yolk sac, fetal liver, or 16-day embryo. This occurrence in developing and transformed tissues demonstrates that oncomodulin is an oncodevelopmental protein.

Lack of heparan sulfate proteoglycan in a discontinuous and irregular placental basement membrane

A discontinuous basement membrane of variable width that surrounds spongiotrophoblast cells of rat placenta was examined for the presence of type IV collagen, laminin, a heparan sulfate proteoglycan, entactin, and fibronectin using monospecific antibodies or antisera and the indirect peroxidase technique. At the level of the light microscope, the basement membrane was immunostained for type IV collagen, laminin, entactin, and fibronectin. Heparan sulfate proteoglycan immunostaining, however, was virtually absent even after pretreatment of sections with 0.1 N acetic acid, pepsin (0.1 microgram/ml) or 0.13 M sodium borohydride. Examination in the electron microscope confirmed the lack of immunostaining for heparan sulfate proteoglycan, whereas the other substances were mainly localized to the lamina densa part of the basement membrane. The absence of heparan sulfate proteoglycan in this discontinuous and irregular basement membrane even though type IV collagen, laminin, entactin, and fibronectin are present, suggests that heparan sulfate proteoglycan may have a structural role in the formation of basement membrane.

Review article: trophoblast invasion and the establishment of haemochorial placentation in man and laboratory animals

Trophoblast invasion is an essential component of haemochorial placentation and has to be considered to relation to reactive changes in the maternal uterine tissues. Some comparative aspects of human and laboratory rodents are discussed and, although there is an obvious phylogenetic gap between the two, many characteristics of placental development are found to be analogous. Trophoblast growth into the uterus is different in different species: localized trophoblast growth forming a bulky tissue (mouse, rat, hamster) contrast with a dispersion of independent trophoblastic elements, forming an interstitial invasion (guinea pig, man). In the rat, mouse, hamster and man retrograde intra-arterial trophoblast migration occurs in maternal vessels supplying blood to the developing placenta. Early changes in maternal tissue might influence trophoblast behaviour. Decidualization probably is a key phenomenon, and the relation of decidual necrosis to trophoblast invasion is considered. Some kind of controlled immune response by the mother also may be involved. These considerations apply to stromal or interstitial invasion as well as to intravascular trophoblast migration but, for the latter, haemodynamic factors probably influence tissue reactions.

Experimental rat choriocarcinoma: an electron microscopic study

Experimental choriocarcinomas of the rat were studied with electron microscopy and compared with the ultrastructure of rat trophoblasts at mid-and late-gestational stage. The choriocarcinoma numbered m-803 contained abundant cytoplasmic organellae and was quite similar to the trophospongial cell at late-gestational stage. The M-801 closely resembled the trophospongial cell at mid-gestational stage. The m-673, which has 3-beta-ol dehydrogenase enzyme system, could not be studied sufficiently inspite of the examination of more than 30 blocks of epon embedded specimen, because of the degeneration and necrosis occurring in the specimens.

The fine structure of the chinchilla placenta

The structure of the placental labyrinth, interlobular or "coarse" syncytium, visceral (splanchnopleuric) yolk sac, giant cells and subplacenta of the chinchilla was studied with the electron microscope. The fine structure of the interhemal membrane of the placental labyrinth was found to be hemomonochorial, consisting of a single layer of syncytial trophoblast. In this respect, the placental labyrinth was similar to that of another caviomorph rodent, the guinea pig. The labyrinthine trophoblast had pinocytotic vesicles as well as larger vaculoes and multivesicular bodies. The interlobular syncytium contained granular endoplasmic reticulum, and in one case from early in gestation there were intracisternal granules in the ER. The visceral endodermal cells of the inverted yolk sac placenta had a well-developed system of apical vesicles and tubules as well as larger cytoplasmic vacuoles. Their appearance was similar to that of endodermal cells found in other rodents which are known to absorb proteins and other substances from the uterine lumen. Towards term the giant cells were often vacuolated and contained large deposits of glycogen as well as lipid droplets. The syncytial trophoblast of the subplacenta contained numerous moderately electron-dense granules which may be secretory in function; cytotrophoblastic cells lacked these granules. The subplacental syncytium often surrounded spaces or lacunae which contained an electron-dense granular material.

The fine structure of the trophospongial layer of the kangaroo rat placenta

The trophospongial layer of the near-term kangaroo rat placenta was examined in the electron microscope. Two ultrastructurally different cellular zones were distinguishable - an inner zone adjacent to the labyrinth and a basal zone located mesometrial to the inner zone. Inner zone cells contained a well developed granular ER and Golgi apparatus as well as polymorphous membrane-limited granules. The cells also contained modest amounts of glycogen and lipid droplets. Basal zone cells were also rich in ER; some cells had dilated ER cisternae containing a highly structured material which appears as a sheet of hexagons when viewed en face. Basal zone cells may, among other things, function as glycogen storage cells, since they had large cytoplasmic accumulations of glycogen. Unlike the situation in some other rodents, maternal blood draining from the trophospongial layer was always contained in channels lined by a layer of squamous cells which, in turm, was separated from the trophospongial cells by a basal lamina. The trophospongial zones are compared with the trophospongial regions of other rodent placentas and possible functions are considered.

Ultrastructural observations on the maturation of the placental labyrinth of the golden hamster (days 10 to 16 of gestation)

Morphogenesis of the labyrinthine part of the chorioallantoic placenta of the golden hamster between day 10 of gestation and term (day 16) was studied by light and electron microscopy. During this period the labyrinth increases greatly in both size and complexity. Trabeculae of the labyrinth, thin partitions composed of trophoblastic tissue and fetal capillaries which delimit the maternal blood spaces, apparently proliferate both by appositional and interstitial growth. From the time of its formation (day 9 of gestation) until term the labyrinth is hemotrichorial in organization (i.e. three layers of trophoblast separate maternal blood from fetal capillaries). Both the inner and intermediate layers of trophoblast (layers III and II, respectively) are syncytial. The outer trophoblastic layer (III), which is in direct contact with maternal blood, remains cellular, although many of its component cells grow to giant cell dimensions ("labyrinthine giant cells"). Between the tenth and fourteenth days of gestation the anatomical barrier to diffusion between maternal and fetal blood is progressively reduced. This is accomplished both by gradual attenuation of the trophoblastic layers and fetal capillary endothelium and by the formation of discontinuities (gaps) in layer I, and diaphragmed fenestrae in fetal capillary endothelium. The labyrinthine placental barrier is fully developed and probably attains maximal functional efficiency by the fourteenth day of gestation. Late in the fifteenth day of gestation, a few hours before parturition, distinct degenerative changes are apparent in the trophoblastic layers and fetal capillaries of the trabeculae. The factors responsible for initiation these degenerative changes and the onset of parturition are still controversial.