Reassortment of cells according to position in mouse morulae

Morphology of 16-cell embryo in bovine: Inside cells, compaction, fragmentation and effects of X-sorted semen

In mouse embryos, inside cells are allocated in 16-cell embryos through a well-orchestrated sequence of events involving compaction and polarization. The emergence of inside cells is of great importance as itl later gives rise to the inner cell mass and epiblast. In this study, we report the sequence of critical events in embryology (compaction, inside cells allocation and fragmentation) in bovine 72 h.p.i. 9-16 cell embryos, while also investigating the effects of X-sorted semen on these events. We found a wide distribution of total cell numbers among embryos, attributed to an asynchronous cleavage pattern and blastomere death. Additionally, 13% of embryos displayed irregular shapes. The establishment of the inside cell compartment increased (p < 0.01) in embryos with more cells. However, only 53.8% of 16-cell embryos presented inside cells. Compaction was present in 32.4% embryos and was positively correlated (p = 0.03, OR 3.02) with the establishment of inside cells, occurring independently of cell number. Fragmentation was present in 36% embryos, being more frequent (p = 0.01) in embryos with lower cell numbers. A possible association between irregular shape and fragmentation was considered (p = 0.06). The use of X-sorted semen had no effect on most evaluated parameters. However, it did have a marked effect on cleavage rate (p < 0.01) and the arrest of 2- and 4- cell embryos. In conclusion, bovine embryos exhibit an asynchronous cleavage pattern, high levels of fragmentation, and demonstrate compaction and inside cell allocation later in development compared to mouse embryos. Semen X-sorting has major effects on cleavage and embryo arrest. Further studies are needed to elucidate the association between irregularly shaped embryos and fragmentation, as well as the effects of sex on inside cell allocation.

Intercellular interactions, position, and polarity in establishing blastocyst cell lineages and embryonic axes

The formation of the three lineages of the mouse blastocyst provides a powerful model system to study interactions among cell behavior, cell signaling, and lineage development. Hippo signaling differences between the inner and outer cells of the early cleavage stages, combined with establishment of a stably polarized outer epithelium, lead to the establishment of the inner cell mass and the trophectoderm, whereas FGF signaling differences among the individual cells of the ICM lead to gradual separation and segregation of the epiblast and primitive endoderm lineages. Events in the late blastocyst lead to the formation of a special subset of cells from the primitive endoderm that are key sources for the signals that establish the subsequent body axis. The slow pace of mouse early development, the ability to culture embryos over this time period, the increasing availability of live cell imaging tools, and the ability to modify gene expression at will are providing increasing insights into the cell biology of early cell fate decisions.

Cell fate regulation in early mammalian development

Preimplantation development in mammals encompasses a period from fertilization to implantation and results in formation of a blastocyst composed of three distinct cell lineages: epiblast, trophectoderm and primitive endoderm. The epiblast gives rise to the organism, while the trophectoderm and the primitive endoderm contribute to extraembryonic tissues that support embryo development after implantation. In many vertebrates, such as frog or fish, maternally supplied lineage determinants are partitioned within the egg. Cell cleavage that follows fertilization results in polarization of these factors between the individual blastomeres, which become restricted in their developmental fate. In contrast, the mouse oocyte and zygote lack clear polarity and, until the eight-cell stage, individual blastomeres retain the potential to form all lineages. How are cell lineages specified in the absence of a maternally supplied blueprint? This is a fundamental question in the field of developmental biology. The answer to this question lies in understanding the cell-cell interactions and gene networks involved in embryonic development prior to implantation and using this knowledge to create testable models of the developmental processes that govern cell fates. We provide an overview of classic and contemporary models of early lineage development in the mouse and discuss the emerging body of work that highlights similarities and differences between blastocyst development in the mouse and other mammalian species.

Preimplantation mouse embryo: developmental fate and potency of blastomeres

During the past decade we have witnessed great progress in the understanding of cellular, molecular, and epigenetic aspects of preimplantation mouse development. However, some of the issues, especially those regarding the nature and regulation of mouse development, are still unresolved and controversial and raise heated discussion among mammalian embryologists. This chapter presents different standpoints and various research approaches aimed at examining the fate and potency of cells (blastomeres) of mouse preimplantation embryo. In dealing with this subject, it is important to recognize the difference between the fate of blastomere and the prospective potency of blastomere, with the first being its contribution to distinct tissues during normal development, and the second being a full range of its developmental capabilities, which can be unveiled only by experimental perturbation of the embryo. Studies of the developmental potential and the fate of blastomeres are of the utmost importance as they may lead to future clinical application in reproductive and regenerative medicine.

From mouse egg to mouse embryo: polarities, axes, and tissues

This review describes the three classical models (mosaic, positional, and polarization) proposed to explain blastocyst formation and summarizes the evidence concerning them. It concludes that the polarization model incorporates elements of the other two models and best explains most known information. I discuss key requirements of a molecular basis for the generation and stabilization of polarity and identify ezrin/E-cadherin, PAR proteins, and Cdx2 as plausible key molecular players. I also discuss the idea of a network process operating to build cell allocations progressively into committed differences. Finally, this review critically considers the possibility of developmental information being encoded within the oocyte and zygote. No final decision can be reached on a mechanism of action underlying any encoded information, but a cell interaction process model is preferred over one that relies solely on differential inheritance.

Cleavage pattern and emerging asymmetry of the mouse embryo

Early mammalian development is regulative - it is flexible and responsive to experimental intervention. This flexibility could be explained if embryogenesis were originally completely unbiased and disordered; order and determination of cells only arising later. Alternatively, regulative behaviour could be consistent with the embryo having some order or bias from the very beginning, with inflexibility and cell determination increasing steadily over time. Recent evidence supports the second view and indicates that the sequence and the orientations of cell divisions help to build the first asymmetries.

Lineage allocation and cell polarity during mouse embryogenesis

The first developmental lineage allocation during the generation of the mouse blastocyst is to outer trophoblast or to inner pluriblast (inner cell mass; ICM) cells. This allocation seems to be initiated at the 8-cell stage, when blastomeres polarise. Polarisation is followed by differentiative divisions at the subsequent two cleavage divisions to generate polar outer and non-polar inner 16- and 32-cells. The key events in polarisation are regulated post-translationally through a cell contact-mediated pathway, which imposes a heritable determinant-like organisation on the blastomere cortex. Two proteins in particular, E-cadherin and ezrin, are intimately involved in the generation and stabilisation of developmentally significant information. Transcriptional differences between lineages appear to follow and may coincide with the lineage commitment of cells.

Fate of the inner cell mass in mouse embryos as studied by microinjection of lineage tracers

We microinjected horseradish peroxidase and rhodamine-conjugated dextran into single inner cell mass (ICM) cells of preimplantation mouse embryos to study their fate in culture. Simultaneous iontophoresis of both lineage markers allowed immediate localization of the injected cell by epifluorescence, followed by microdrop culture of individual embryos. After 24 hr in culture, labeled descendants were found in the polar trophectoderm, ICM, and parietal endoderm, providing direct evidence that the ICM contributes descendants to the trophectoderm and the endoderm in the intact mouse embryo. Our results substantiate the totipotency of the ICM during the expanding blastocyst stage and further demonstrate that the ICM is a stem cell population from which cells are recruited into these tissue lineages during growth of the blastocyst.

An embryogenic model to explain cytogenetic inconsistencies observed in chorionic villus versus fetal tissue

While the fetus and placenta have a common ancestry, chorionic villus tissue does not always reflect fetal genotype. Data are presented from 15 CVS subjects in whom cytogenetic inconsistencies were observed when comparing (1) cultured chorionic villi, (2) direct chromosome preparations of intact villi, and (3) cultured fetal tissue. Embryogenic models are presented to explain these discrepancies. Mosaicism confined to direct chromosome preparations was the most commonly observed inconsistency. This can be explained by postzygotic non-disjunction limited to cytotrophoblast. In all but one instance, the abnormal cell line was limited to the placenta, with the normal cell line reflecting fetal genotype. Analysis of direct chromosome preparations from multiple individually processed villus fragments may be helpful in recognizing mosaicism confined to the placenta. While both direct chromosome preparations and villus cultures can be misleading, the latter are more likely to reflect fetal genetic status since they are derived from the extraembryonic mesoderm.

Developmental fate in chimeras derived from highly asynchronous murine blastomeres

The developmental fate of single blastomeres from eight-cell murine embryos reaggregated with intact two-cell embryos was evaluated after culture. Fluorescein isothiocyanate was used to follow developmental fate in preblastocyst chimeric embryos. Expression of stage-specific embryonic antigen 3 was used to assay developmental fate at the blastocyst stage, and glucosephosphate isomerase variants were used to assay at the blastocyst stage and after implantation. The results suggest that the descendents of the 1/8 component stay in a patch area and do not selectively migrate to the inner cell mass (ICM). This is in contrast to many studies that indicate that smaller blastomeres, which are more advanced in development, migrate to the ICM. The differences in experimental designs are discussed. Possible mechanisms for this phenomena are that the eight-cell blastomere is physically excluded from the ICM by position or polarization, or that it is differentiating ahead of the two-cell component and becomes trophectoderm.

A quantitative analysis of cell allocation to trophectoderm and inner cell mass in the mouse blastocyst

The allocation of cells to the trophectoderm and inner cell mass (ICM) in the mouse blastocyst has been examined by labelling early morulae (16-cell stage) with the short-term cell lineage marker yellow-green fluorescent latex (FL) microparticles. FL is endocytosed exclusively into the outside polar cell population and remains autonomous to the progeny of these blastomeres. Rhodamine-concanavalin A was used as a contemporary marker for outside cells in FL-labelled control (16-cell stage) and cultured (approximately 32- to 64-cell stage) embryos, immediately prior to the disaggregation and analysis of cell labelling patterns. By this technique, the ratio of outside to inside cell numbers in 16-cell embryos was shown to vary considerably between embryos (mean 10.8:5.2; range 9:7 to 14:2). In cultured embryos, the trophectoderm was derived almost exclusively (over 99% cells) from outside polar 16-cell blastomeres. The origin of the ICM varied between embryos; on average, most cells (75%) were descended from inside nonpolar blastomeres with the remainder derived from the outside polar lineage, presumably by differentiative cleavage. In blastocysts examined by serial sectioning, polar-derived ICM cells were localised mainly in association with trophectoderm and were absent from the ICM core. In nascent blastocysts with exactly 32 cells an inverse relationship was found between the proportion of the ICM descended from the polar lineage and the deduced size of the inside 16-cell population. From these results, it is concluded that interembryonic variation in the outside to inside cell number ratio in 16-cell morulae is compensated by the extent of polar 16-cell allocation to the ICM at the next division, thereby regulating the trophectoderm to ICM cell number ratio in early blastocysts.

Origin of the inner cell mass in mouse embryos: cell lineage analysis by microinjection

The mouse inner cell mass is established by cells that are allocated to internal positions after the 8-cell stage. We analyzed the timing of this allocation by microinjecting two cell lineage markers, horseradish peroxidase and rhodamine-conjugated dextran, into mouse blastomeres at the 8- to 32-cell stage. Prospective analysis was performed by coinjection of peroxidase and dextran, followed by 12-22 hr of culture and staining for peroxidase activity; retrospective analysis was performed by injection of peroxidase alone and localization of sister cells without further culture. Both approaches indicated that cells are allocated to internal positions during the fourth and fifth cleavage divisions, but not the sixth cleavage division, of the mouse embryo. Thus, outer cells can have inner descendants until the late morula/early blastocyst (32-cell) stage, but cells remaining outside after the fifth cleavage division are restricted to a trophectoderm fate. This information about cell lineage indicates that the previously observed totipotency of the cleaving mammalian embryo's cells is a regulative attribute that is used in normal development.

Cell fate in the polar trophectoderm of mouse blastocysts as studied by microinjection of cell lineage tracers

Horseradish peroxidase (HRP), together with Fast Green or rhodamine-conjugated dextran (RDX), was used as an intracellular lineage tracer to determine cell fate in the polar trophectoderm of 3.5-day-old mouse embryos. In HRP-injected midstage (approximately 39-cell) and expanded (approximately 65-cell) blastocysts incubated for 24 hr, the central polar trophectoderm cell was displaced from the embryonic pole an average of 20 micron (5% of blastocyst circumference) and 29 micron (6% of blastocyst circumference), respectively. Expanded blastocysts injected with HRP + Fast Green and incubated for 24 hr or with HRP + RDX and incubated for 48 hr showed a displacement of 24 micron (4% of blastocyst circumference) and 88 micron (14% of blastocyst circumference), respectively. Up to 10 HRP-positive trophectoderm cells were observed among embryos incubated for 48 hr, indicating that in those cases, the labeled progenitor cells had divided at least three times. Our observations show that the central polar trophectoderm cell divides in the plane of the trophectoderm in expanded blastocysts and, along with its descendants, is displaced toward the mural trophectoderm. The systematic tandem displacement of labeled cells and their descendants toward the abembryonic pole suggests the presence of a proliferative area at the embryonic pole of the blastocyst. Large shifts in inner cell mass (ICM) position in relation to the trophectoderm do not occur during blastocyst expansion. Furthermore, random movements within the polar trophectoderm population do not account for the replacement of labeled cells by unlabeled polar trophectoderm cells. Rather, we propose the hypothesis that the ICM contributes these replacement cells to the polar trophectoderm during blastocyst expansion.

Myosin rings and spreading in mouse blastomeres

The relationship between myosin organization and cell spreading in the preimplantation mouse embryo was studied by indirect immunofluorescence in embryos cultured on lectin-coated substrates. Binding of cell surface polysaccharides to substrate-bound concanavalin A and wheat germ agglutinin induced changes in myosin distribution that resembled those which occur during cell-cell contact interaction. This involved an initial loss of myosin from the contact region that was associated with the development of stable cell-substrate attachments. In addition, a ring of myosin was formed along the edge of the cells' contact to the substrate. The presence of such a ring may be related to the potential for subsequent cell spreading. A myosin ring was also identified in the apical junctional region of the outer morula cells where it similarly separated the cell periphery into contacted and free peripheral domains. Following these changes in myosin organization the embryos spread on the substrate by extension of lamellipodia. These movements were coupled to the dissolution of the myosin ring and the reorganization of myosin into filament bundles. The sequence of changes in the pattern of myosin distribution suggests that contact regulation of myosin organization plays an important role in controlling the spreading behavior of blastomeres and perhaps more generally in the organization of cells into epithelia.

Spatial distribution of blastomeres is dependent on cell division order and interactions in mouse morulae

Spatial distribution of blastomeres was examined in 16- to 30-cell morulae obtained from aggregates of 1/2----1/2 and 1/2----2/4 blastomeres. The advanced blastomeres (2/4) contributed disproportionately more inner cells while there was a corresponding decline in the contribution from the delayed blastomere (1/2) so that a balance between the total number of inner and outer cells was retained. There was, however, no marked change in the relative number of outer cells. It is suggested that once formed, the inner more adhesive cells divide relatively faster than the outer cells whose behaviour is dictated by the inner cells. The outer less adhesive cells spread over the inner cells; cell spreading is incompatible with division. The degree to which cell spreading and retardation of division of outer cells occurs may be dictated by the number of inner cells present at any one time and this partly determines the entry of further cells inside. The suggested mechanism for cell allocation is highly flexible and, indeed, essential to encompass the wide variety of patterns of cell interactions and distribution observed in morulae. It is also proposed that the retardation of division of outer cells may trigger differentiation of trophectoderm by inducing endoreduplication and the blastomeres delayed from dividing for the longest period of time may mark down the abembryonic pole and establish the embryonic-abembryonic axis.