Ultrastructural studies on maternal-embryonic cell interaction during experimentally induced implantation of rat blastocysts to the endometrium of the mouse

Establishment of a Wisent (Bison bonasus) Germplasm Bank

Simple Summary

The wisent (European bison) is a protected species. For this reason, we undertook the use of biotechnologies—such as in vitro maturation of oocytes, in vitro fertilization of matured oocytes, in vitro culture of embryos, and embryo vitrification—to establish a wisent embryo bank. The competencies of the vitrified embryos were tested by transferring the warming embryos to cattle (interspecies embryo transfer). The pregnancy was confirmed biochemically and using USG, and although the fetuses were resorbed, the embryos’ competence for development was demonstrated. The results of these studies open the way for the cryoconservation of wisent germplasm.

Abstract

The wisent, or European bison (Bison bonasus), belongs to the same family (Bovidae) as the American bison and domestic cattle. The wisent is the largest mammal in Europe, and is called the “Forest Emperor”. The wisent is listed as “Vulnerable” on the IUCN Red List, and is protected by international law. Achievements in reproductive biotechnology have opened new possibilities for the cryoconservation of the wisent germplasm. Therefore, this research aimed to improve a strategy for the protection and preservation of the European bison through the creation of a wisent germplasm bank, based on the following procedures: isolation and in vitro maturation (IVM) of oocytes, in vitro fertilization (IVF) of matured oocytes, in vitro embryo culture (IVC), and embryo cryopreservation. Wisent ovaries were isolated from females outside the reproductive season, and eliminated from breeding for reasons other than infertility. Cumulus–oocyte complexes (COCs) were isolated from follicles greater than 2 mm in diameter and matured for 24 h and 30 h. After IVM, COCs were fertilized in vitro with wisent sperm. The obtained wisent zygotes, based on oocytes matured for 24 h and 30 h, were cultured for 216 h. Embryos at the morula and early blastocyst stages were vitrified and then warmed and transferred to interspecies recipients (Bos taurus). USG and biochemical tests were used to monitor pregnancies. This study obtained embryos in the morula and early blastocyst stages only after oocytes were fertilized and matured for 30 h. On average, per oocyte donor, 12.33 ± 0.5 COCs were isolated, and only 9.33 ± 0.61 COCs were qualified for in vitro maturation (75.68%), while 9.16 ± 0.48 COCs were matured (84.32%). On average, per donor, 5.5 ± 0.34 embryos were cleaved (59.96%) after 48 h post-fertilization (hpf), and 3.33 ± 0.21 achieved the eight-cell stage (36.52%) after 96 hpf, while 1 ± 0.21 morula and early blastocyst stages (10.71%) were achieved after 216 hpf. A total of six embryos (one morula and five early blastocysts) were obtained and vitrified; after warming, five of them were interspecies transferred to cattle (Bos taurus). On day 41 after fertilization, 3 out of 5 pregnancies were detected based on USG, P4, and PAG tests. However, no pregnancy was observed on day 86 after fertilization, indicating embryo resorption. This study shows that obtaining wisent embryos in vitro, and subsequent cryopreservation to create a wisent embryo bank, can be applied and implemented for the wisent protection program.

Mouse↔rat aggregation chimaeras can develop to adulthood

In order to examine interactions between cells originating from different species during embryonic development we constructed interspecific mouse↔rat chimaeras by aggregation of 8-cell embryos. Embryos of both species expressed different fluorescent markers (eGFP and DsRed), which enabled us to follow the fate of both components from the moment of aggregation until adulthood. We revealed that in majority of embryos the blastocyst cavity appeared inside the group of rat cells, while the mouse component was allocated to the deeper layer of the inner cell mass and to the polar trophectoderm. However, due to rearrangement of all cells and selective elimination of rat cells, shortly before implantation all primary lineages became chimaeric. Moreover, despite the fact that rat cells were always present in the mural trophectoderm, majority of mouse↔rat chimaeric blastocysts implanted in mouse uterus, and out of those 46% developed into foetuses and pups, half of which were chimaeric. In contrast to mural trophectoderm, polar trophectoderm derivatives, i.e. the placentae of all chimaeras were exclusively of mouse origin. This strongly suggests that the successful postimplantation development of chimaeras is enabled by gradual elimination of xenogeneic cells from the nascent placenta. The size of chimaeric newborns was within the limits of control mouse neonates. The rat component located preferentially in the anterior part of the body, where it contributed mainly to the neural tube. Our observations indicate that although chimaeric animals were able to reach adulthood, high contribution of rat cells tended to diminish their viability.

An ultrastructural study of polyembryonic parasitoid embryo and host embryo cell interactions

The morula-stage embryo of the polyembryonic egg-larval parasitoid Copidosoma floridanum forms outside the host embryo and secondarily invades the host body. Electron microscopic analyses of cellular interactions between the extraembryonic syncytium of the parasitic morula and the host embryonic epithelial cells showed that morula penetration into the host embryo did not cause obvious damage to the host cells, except for the abrasion of the embryonic cuticle. Epithelial cells of the host embryo extended microvilli toward the invading C. floridanum morula and also adjacent host cells in the same way. Shortly after settlement of the morula within the host body cavity, gap junctions and adherens junctions with host cells were formed. The morula was then surrounded by a cyst comprised of host cells into which host tracheoles were invaginated.

Effect of cytoplast on the development of inter-subspecies nuclear transfer reconstructed goat embryo

The aim of this study was to investigate effect of cytoplast on the development competence of reconstructed embryos derived from inter-subspecies somatic cell nucleus transfer (SCNT). First, the development potency of reconstructed embryos produced by transferring Boer goat fibroblast cell nucleus of different ages into enucleated Sannen goat ova was evaluated in order to determine which age of nuclear donor is favorable for the reconstructed embryos development. Secondly, the another component of reconstructed embryos, "cytoplast," was evaluated by comparing the effect of ovum cytoplast derived from Sannen male symbol x Boer female symbol descendant on the reconstructed embryos development to that of Sannen goat ovum cytoplast. The results revealed that the development rate of the reconstructed embryos derived from 2 months old Boer goat somatic cells was the highest, their gestation rate was up to 50%, and one viable male offspring was obtained. The cytoplast derived from the crossbreeding goats improves the development competence of reconstructed embryos, which birth rate was 5.5%. The genetic identification of offspring by using PCR-SSCP analysis confirmed that these cloned kids were derived from the donor. The results above reveal that the cytoplast of Sannen goat ovum could induce the dedifferentiation of somatic cell nuclei derived from Boer goat, but the reprogramming process of these reconstructed embryos seems incomplete, probably due to some incorrect processes happened after implantation. Relatedness components of nucleus donor in cytoplast of the crossbreeding goat may be helpful to induce the dedifferentiation of somatic cell nuclei completely and improve the development competence of the reconstructed embryos.

Vascular endothelial growth factor production by rat granulated metrial gland cells and their morphological features in normal and pathological conditions

Granulated metrial gland (GMG) cells are pregnancy-specific cells that may have many functions in successful placentation and pregnancy. In the present study, changes in the rat GMG cell structure, distribution and vascular endothelial growth factor (VEGF) expression during early pregnancy were evaluated by light microscopy. Implantation sites taken from females with spontaneous abortion were also investigated. On Day 7 of pregnancy, GMG cells were distributed through the implantation and interimplantation sites. They formed metrial glands in the mesometrial triangle on Day 9, and were observed in the decidua basalis on Day 14 of pregnancy. Avidin–biotin complex immunohistochemistry revealed that GMG cells showed moderate staining for VEGF at the beginning of pregnancy and intense staining on Days 9 and 10 of pregnancy. They were localised mostly near the newly formed blood vessels. The implantation sites from spontaneously aborting females showed numerous leucocytes in the lumen of mesometrial blood vessels. In spontaneously aborting females, GMG cells showed a distinct morphology, increased in number and volume, their granules were denser and degranulation was observed. These results suggest that rat GMG cells might be a guide for placental angiogenesis and they might share a role with leucocytes in pathological conditions.

Compatible invasion of a phylogenetically distant host embryo by a hymenopteran parasitoid embryo

Embryonic invasion into the tissue of genetically different organisms has been known only in mother-embryo interactions of viviparous organisms. Hence, embryonic invasions have been thought to occur only within the same or closely related species. For endoparasitic Hymenoptera, which are oviposited in their host egg but complete their development in the later stages, entry into the host embryo is essential. To date, the entry of these parasitoids is known to be accomplished by either egg deposition directly into the embryo or by the newly hatched larva boring into the embryo. However, Copidosoma floridanum is a polyembryonic parasitoid whose development is characterized by a prolonged embryonic stage, and which lacks a larval form during its host embryogenesis. We have analyzed the behavior and fate of C. floridanum embryos co-cultured with their host embryo in vitro. Here, we show that the morula-stage embryo of C. floridanum actively invades the host embryo. Histological analyses have demonstrated that C. floridanum embryonic invasion is associated with adherent junction to host cells rather than causing an obvious wound on the host cells. These findings provide a novel case of embryonic invasion into a phylogenetically distant host embryo, ensuring cellular compatibility with host tissues.

Distribution patterns of immunocompetent cells in the pregnant mouse uteri carrying allogeneic mouse and xenogeneic vole embryos

Mouse and vole embryos were allogeneically and xenogeneically transferred into pseudopregnant CD-1 and immunodeficient (scid) female mice, and we investigated the distribution of immunocompetent cells, uterine natural killer (uNK) cells, mast cells and macrophages, in the implantation sites on days 6, 7 and 8 of gestation. The survival rate of the vole embryos decreased gradually with increased gestation, but the rate was higher in the scid uteri than in the CD-1 mice. The number of uNK cells increased markedly at the mesometrial triangle and the outer decidual area in the CD-1 uteri containing vole embryos; by contrast, scid uteri having vole embryos showed almost the same number as those having mouse embryos. Mast cells were present in large numbers at the myometrium, but rarely in the decidua in all types of pregnant uteri. Cells at the myometrium were more numerous in xenogeneic than in allogeneic transfer. Many mast cells appeared in the inner decidua where xenogeneically transferred vole embryos were dead and aborted. Macrophages were present in the outer decidua and myometria in all types of pregnant uteri, and their distribution pattern did not change even in aborted uterine sites. These results suggest: (1) the response of macrophages to dead embryos is completely inhibited, (2) uNK cells and mast cells increase near dead and aborted embryos, and (3) the increment in uNK cells responding to xenogeneic embryos is suppressed in scid mice, and the suppression may contribute partly to survival of the embryos.

Polyploidization in the trophoblast and uterine glandular epithelium of the endotheliochorial placenta of silver fox (Vulpes fulvus Desm.), as revealed by the DNA content

Dynamics of genome multiplication during establishment of interrelations between trophoblast and glandular epithelium of the endometrium has been studied in the course of formation of placenta in the silver fox. During formation of the placenta, penetration of the trophoblast into the zone of the endometrial glandular epithelium and of endometrial blood vessels into the zone of expanding trophoblast occurs. The trophoblast, which gradually replaces epithelium and a part of the stroma of the endometrium, closely adjoins endometrial vessels but does not disrupt them, thereby the endotheliochorial placenta is formed. Cytophotometric measurements of the DNA content in trophoblast nuclei have shown that most of them are polyploid: predominantly 4-64c, occasionally 128c and 256c. Polyploidy of the trophoblast may be a consequence of various types of polyploidizing mitoses. Cytophotometric measurements of the DNA content in mitotic figures have revealed the presence of mitoses of diploid cells, i.e. with the DNA amount of 4c (2n), and polyploid cells, i.e. 8c (4n), and 16c (8n), therefore trophoblast cells in the silver fox placenta are able to enter mitosis up to the octaploid level. Higher degrees of polyploidy in the trophoblast cells seem to be achieved by endoreduplication. Polyploidization of the uterine glandular epithelial cells during placentation in the silver fox occurs until the level of 8c. Thus, the tissue-specific response of the uterus to the implanting embryo consists of active proliferation and polyploidization of the glandular epithelium, which may compensate formation of prominent population of decidual cells (i.e., connective tissue cells). In the endotheliochorial placenta of the silver fox the regularity is confirmed that cells of both maternal and fetal origin are, as a rule, polyploid in sites of their contact in placenta, which may be of protective significance in the contact of allogenic organisms.

Trophinin expression in the mouse uterus coincides with implantation and is hormonally regulated but not induced by implanting blastocysts

Trophinin mediates apical cell adhesion between two human cell lines, trophoblastic teratocarcinoma and endometrial adenocarcinoma. In humans, trophinin is specifically expressed in cells involved in implantation and early placentation. The present study was undertaken to establish trophinin expression by the mouse uterus. In the pregnant mouse uterus, trophinin transcripts are expressed during the time which coincides with the timing of blastocyst implantation. Trophinin is also expressed in the nonpregnant mouse uterus at estrus stage. Uteri from ovariectomized mice did not express trophinin, whereas strong expression was induced by estrogen but not by progesterone. Trophinin transcripts and protein were found in the pseudopregnant mouse uterus. No differences were detected in trophinin expression by the uteri in the pregnant, pseudopregnant, and pseudopregnant received blastocysts. In delayed implantation model, trophinin proteins were found in both luminal and glandular epithelium, whereas dormant blastocysts were negative for trophinin. Upon activation with estrogen, however, no significant changes were detected either in the blastocyst or in the uterus. These results indicate that ovarian hormones regulate trophinin expression by the mouse uterus, and that an implanting blastocyst has no effect on trophinin expression in the surrounding endometrial luminal epithelial cells.

Polytene chromosomes in mammalian cells

This article deals with the structural and functional organization of polytene chromosomes in mammals. Based on cytophotometric, autoradiographic, and electron microscopic data, the authors put forward a concept of nonclassic polytene chromosomes, with special reference to polytene chromosomes in the mammalian placenta. In cells with nonclassic polytene chromosomes, two phases of the polytene nucleus cycle are described, such as the endointerphase (S phase) and endoprophase (G phase). The authors generalize that the main feature of nonclassic polytene chromosomes is that forces binding the sister chromatids are much weaker than in the Diptera classic polytene chromosomes. This concept is confirmed by comparative studies of human, mink, and fox polytene chromosomes. The final step of the trophoblast giant cell differentiation is characterized by a transition from polyteny to polyploidy, with subsequent fragmentation of the highly polyploid nucleus into fragments of low ploidy. Similarities and dissimilarities of pathways of formation and rearrangement of nonclassic polytene chromosomes in mammals, insects, plants, and protozoans are compared. The authors discuss the significance of polyteny as one of the intrinsic conditions for performance of the fixed genetic program of trophoblast giant cell development, a program that provides for the possibility of a long coexistence between maternal and fetal allogenic organisms during pregnancy.

Changes in histochemical distribution of cell surface heparan sulfate proteoglycan in mouse uterus during the estrous cycle and early pregnancy

We have investigated the changes in immunolocalization of a cell surface heparan sulfate proteoglycan (HSPG) in the mouse uterus during the estrous cycle and at the time of implantation in early pregnancy. A monoclonal antibody prepared against syndecan, a cell surface HSPG from mouse mammary epithelium (gift of Dr. M. Bernfield), was reacted with unfixed and fixed frozen sections of uteri from normally cycling, 3.5 and 4.5 days pregnant, and estradiol-treated immature and ovariectomized mature mice. A polyclonal antibody prepared against basal lamina HSPG from Engelbreth-Holm-Swarm (EHS) tumor cells (gift of Dr. John Hassell) was used as a positive control. The latter showed no variation during the estrous cycle or early pregnancy. Localization of syndecan in uterine epithelium changed from basolateral to predominantly basal as the cycle progressed from metestrus toward estrus. A similar pattern was seen in immature and ovariectomized mature females that had received estradiol injections. With the onset of pregnancy, the basolateral localization became progressively less intense from 3.5 days through 4.5 days of pregnancy. Thus, cell surface HSPG distribution is modulated by hormonally dependent changes in cycling and pregnant mice, supporting previous suggestions that early pregnancy in mice is accompanied by a turnover and rearrangement of uterine epithelial cell surface.

Rat granulated metrial gland cells differentiate in pregnant chimeric mice and may be cytotoxic for mouse trophoblast

Shielding an ovary and part of the uterus of mice during irradiation allowed chimeric mice, reconstituted with rat or mouse bone marrow (respectively, RBM and MBM chimeras), to be studied in a subsequent pregnancy. Some of the granulated metrial gland (GMG) cells which differentiated in the RBM chimeric mice were rat GMG cells, showing the contribution of donor bone marrow precursors to the uterine response to pregnancy. Rat and mouse GMG cells were found in the maternal blood spaces of the placental labyrinth adjacent to the degenerate layer 1 trophoblast. This suggests that both rat and mouse GMG cells exhibit cytotoxic activity against the mouse trophoblast.

Interspecific melanocyte chimaeras made by introducing rat cells into postimplantation mouse embryos in utero

Dissociated cells of whole midgestation rat embryos were injected into implanted albino mouse embryos on Day 8.5 of gestation in utero. This successfully produced viable interspecific chimaeras which were found to have pigmented hairs. Two of them had many pigmented hairs covering a large area of their bodies, including a forelimb and a hindlimb. The fact that some of the introduced rat cells differentiated into functional melanocytes suggests that embryonic cells of both species were able to interact with each other normally and that the foreign cells were kept from maternal immunological assault.

Changes in the distribution of vinculin during preimplantation mouse development

It has been proposed that vinculin is a microfilament bundle-membrane linking cytoskeletal protein. We used double-fluorescence microscopy to study the distribution of vinculin and F-actin in mouse oocytes and preimplantation embryos. In oocytes and in the cells of cleavage- and blastocyst-stage embryos, vinculin exhibited a diffuse cytoplasmic distribution and was concentrated in a submembranous layer. The presence of vinculin in oocytes was confirmed by immunoblotting. In oocytes, a distinct concentration of actin was observed above the second metaphase spindle. During the 8-cell stage, compacting blastomeres exhibited partial polarization of cortical vinculin and actin toward their outward-facing surfaces. In precompaction-stage blastomeres, the submembranous layer of vinculin contained a ring-like concentration in the most peripheral region of each intercellular contact area. During later development, the amount of vinculin localized in the areas of intercellular contacts became modified. In embryos ranging from the compacted 8-cell stage to the mid-morula stage, the vinculin-specific fluorescence was only intense in some intercellular contacts, being indistinct in most contact areas. In late morulae, the flattened outer cells increasingly exhibited concentration of vinculin in contact areas. In contrast, actin-specific fluorescence was clearly evident in most intercellular contacts throughout the morula stage. At the early blastocyst stage, all contacts of the trophectoderm (TE) cells again regularly exhibited concentration of both components. At the late blastocyst stage, the staining pattern changed once again: the contact-associated concentration of vinculin-specific fluorescence was not observed in polar TE cells, while remaining clear in mural TE cells. In blastocyst outgrowths, TE cells displayed typical vinculin plaques at the peripheries of the cells. The continuous changes in the distribution of vinculin and actin suggest that these components are involved in the control of cellular relationships during early development. Immunoelectron microscopy and experiments using cytochalasin were performed in an attempt to relate the distribution of vinculin to the ultrastructural features of embryo cells.

Changes in the distribution of a spectrin-like protein during development of the preimplantation mouse embryo

The mouse blastocyst expresses a 240,000-mol-wt polypeptide that cross-reacts with antibody to avian erythrocyte alpha-spectrin. Immunofluorescence localization showed striking changes in the distribution of the putative embryonic spectrin during preimplantation and early postimplantation development. There was no detectable spectrin in either the unfertilized or fertilized egg. The first positive reaction was observed in the early 2-cell stage when a bright band of fluorescence delimited the region of cell-cell contact. The blastomeres subsequently developed continuous cortical layers of spectrin and this distribution was maintained throughout the cleavage stages. A significant reduction in fluorescence intensity occurred before implantation in the apical region of the mural trophoblast and the trophoblast outgrowths developed linear arrays of spectrin spots that were oriented in the direction of spreading. In contrast to the alterations that take place in the periphery of the embryo, spectrin was consistently present in the cortical cytoplasm underlying regions of contact between the blastomeres and between cells of the inner cell mass. The results suggest a possible role for spectrin in cell-cell interactions during early development.

Adhesion of mouse blastocysts to uterine epithelium in culture: a requirement for mutual surface interactions

Blastocysts readily adhered to inert materials in culture, but they resisted adhesion to living cells even after several days under conditions which encouraged cell aggregation. As far as could be determined by observing their spreading behavior on polylysine- and polyglutamate-coated dishes, the mechanism of adhesion of blastocysts to inert surfaces was similar to that of freshly dissociated cells and cell lines. However, their adhesion to vesicles of isolated uterine epithelium, which was encouraged by hanging drop culture, was by a different mechanism that involved microvilli on both the embryonic and maternal surfaces. This interactive step, which was similar to that seen during attachment in vivo, was followed by a brief period of close trophoblast-epithelial contact which led ultimately to phagocytosis of sloughed epithelium. Blastocysts showed a clear preference for adhesion to cultured epithelium in vesicles that had begun to collapse. In this case the cells showed a columnar profile with sharply defined microvillous apexes, unlike the flattened cells in fully expanded vesicles or on culture dishes. We conclude that the preimplantation adhesion of mouse blastocysts requires specific changes on both the embryonic and maternal surfaces to overcome the mutual nonadhesiveness typical of epithelia. The relatively rapid adhesion of blastocysts to a culture dish, on the other hand, is more typical of the well-known spreading behavior of cells on a highly attractive surface.

Histological and immunological studies of post implantation death of Mus caroli embryos in the Mus musculus uterus

Mus caroli blastocysts surgically transferred to the Mus musculus uterus implant and develop successfully during the first 9.5 days of gestation. At this time, the trophoblast giant cell area becomes infiltrated with lymphocytes and, subsequently, massive hemorrhage occurs, associated with embryo death. This process is specific for the xenogeneic embryos and does not affect adjacent allogeneic embryos present in the same uterus. Cells taken from the resorbing tissues contain a population of large Thy 1+, Lyt 1+, Lyt 2+ cytotoxic cells capable of lysing Mus caroli concanavalin A blasts in vitro. It is suggested that failure of the Mus caroli embryos in the Mus musculus uterus is mediated by the maternal immune system and that the xenogeneic blastocyst transfer system provides a suitable animal model system for studies of mechanisms that prevent rejection of the fetal allograft.

Morphodynamics of outgrowths of mouse trophoblast in the presence and absence of a monolayer of uterine epithelium

Mouse blastocysts were cultured in the presence and absence of a confluent uterine, luminal, epithelial monolayer in order to assess (10 comparative developmental abilities and (2) the existence and nature of embryo-uterine intercellular contacts. Embryonic development and mural trophoblast outgrowth were examined by light, transmission and scanning electron microscopy, and by time-lapse cinematography. The results demonstrate that cultured mouse blastocysts are capable of development that is essentially equivalent to the early egg cylinder stage in the presence and absence of a uterine epithelial substratum. Penetration of the endometrium and displacement of uterine luminal epithelial cell in vivo is paralleled in vitro by the displacement of uterine cells from the advancing edge of outgrowing mural trophoblast. The ability of mural trophoblast to continue to outgrow and displace uterine cells persists for a finite period of time. The findings are discussed with respect ot (1) the nature of embryo-uterine contact in vivo and (2) mechanisms proposed to explain trophoblastic displacement of uterine luminal epithelial cells during normal implantation.

Possible involvement of macrophages in embryo--maternal relationships during ovum implantation in the rat

Distribution of phagocytic cells in the rat endometrium during the estrous cycle and early gestation was examined by histological, electron microscopic, and histochemical techniques. The results show that numerous macrophages emerge around the nidus shortly after the onset of ovum implantation. Such macrophages, however, were not present within the decidua, suggesting that this tissue may be a protective barrier against the migration of phagocytic cells towards the implants. Approximately 48 hours after the onset of implantation, the number of endometrial macrophages decreased dramatically.

Effects of ovarian hormones on cell membranes in the rat uterus. II. Freeze-fracture studies on tight junctions of the lateral plasma membrane of the luminal epithelium

Freeze-fracture techniques have been used to study tight junctions on the lateral plasma membrane of cells of the luminal epithelium of the rat uterus under various hormonal regimes. Tight junctions from ovariectomized control rats extended some 0.5 micron down the lateral membrane and the junctional strands often formed a network of closely packed, circular compartments. Following treatment of rats with estrogen for 3 days the tight junctional region still extended 0.5 micron down the lateral membrane, but the strands ran more parallel to the apical surface. They did not enclose circular compartments. After treatment with progesterone, either alone or with estrogen in such a way as to condition the ovariectomized uterus for implantation, a third pattern of junctional organization emerged. In these animals the junctional region extended 1.1 micron down the lateral membrane and the strands frequently crosslinked, enclosing compartments of varying and irregular size and shape. Our observations suggest that ovarian hormones could regulate the contents of the uterine lumen by altering the structure and extent of the tight junctions which connect the epithelial cells enclosing the lumen.

Trophoblastic and uterine luminal epithelial surfaces at the time of blastocyst adhesion in the rat

Fixed uteri from rats on the afternoon of day 6 of pregnancy were split to expose the implantation chambers, their enclosed blastocysts, and the imprints of the blastocyst on the adjacent epithelium of the chamber. Some of the implantation chambers were prepared for scanning electron microscopy; other chambers were treated with colloidal iron hydroxide, with cationized ferritin, or with the tannic acid method, and subsequently were prepared for transmission electron microscopy. In this manner, the disposition of the surface-coat markers on the surface of the blastocysts, surface of the uterus within the chamber, and the surface of the uterus that had been apposed to a blastocyst were compared. Despite the pronounced morphological differences between the microvilli of the uterine luminal epithelium in the imprint and those in the rest of the chamber, the binding of the markers was remarkably similar. No evidence of removal of surface coat could be found in that area of the uterus in contact with the blastocyst. In addition, in two instances in the cationized ferritin-treated material, and in another instance in tannic acid-stained material, regions of the apparently adhering trophoblastic cell membranes and uterine cell membranes had abundant coat materials and, possibly, even secretory materials interposed. When blastocyst-sized glass beads were introduced into uteri from animals made pseudopregnant or unilaterally pregnant, the beads failed to elicit a decidual response and made an imprint that did not resemble the imprint of a blastocyst in an implantation chamber. It was concluded that, at least in the initial stages of adhesion, the blastocyst does not bring about a physical removal of the demonstrable aspects of the surface coat of the uterus. It was concluded further, that glass beads are not a suitable object for mimicking a blastocyst in the rat uterus.

A study of hetero-specific sperm-egg interactions in the rat, mouse, and deer mouse using in vitro fertilization and sperm injection

Hetero-specific fertilization of zone-free eggs is used in these experiments as a tool to analyze the barriers to hybridization and to gain insight into the mechanisms of normal fertilization. When the zonae of rat eggs, which are a barrier to hetero-specific fertilization, are removed with pronase, the eggs can be fertilized by mouse sperm and the zygotes start to develop normally. A rat egg fertilized with mouse sperm completes meiosis and forms both male and female pronuclei. Chromosomes from both parents are found on he spindle at the metaphase stage of the first cleavage division. Under present culture conditions, embryos develop only to the two-cell stage, but this initial development of the hybrid is apparently normal. The question of whether sperm and egg membrane fusion is requisite for normal development is addressed by injecting sperm directly into the cytoplasm of unfertilized eggs. The injection of mouse sperm into rat eggs frequently leads to activation and formation of male and female pronuclei. The first cleavage division is indistinguishable from that following hetero-specific fertilization. Capacitated and uncapacitated sperm react alike when injected into eggs. Egg activation, however, is necessary for male pronucleus formation. Sperm from the deer mouse Peromyscus maniculatus bairdii, which are incapable of fertilizing even zonea-free eggs, respond like mouse sperm when injected into rat eggs. These data indicate that sperm interactions with the egg cytoplasm are less species-specific than interactions at the egg surface. Furthermore, the normal surface interactions of sperm and eggs are not essential for the start of development.