Factors Affecting the Preimplantation Embryo Development - Essay Example

Factors affect the preimplantation embryo development Development of the preimplantation embryo The preimplantation embryo passes through several metabolic phases, undergoing changes in protein synthesis, energy requirements and amino acid uptake as it develops from a fertilized zygote to the blastocyst stage. Alongside, it also undergoes morphological changes, particularly at compaction when the first differentiative process is observed (Sakkas and Vassalli, n.d.). The preimplantation development of eutherian mammals including human, is notably the same and spans cleavage of the fertilised ovum, compaction and morula formation, and finally, cavitation with formation of blastocyst (Hardy and Spanos, 2002). Embryo cleavage The rate of cleavage has also been linked to genetic influences: Warner et al. (Warner et al, 1987) have described a H-2 linked gene, called the preimplantation embryo development (Ped) gene that influences the rate of cleavage divisions of preimplantation mouse embryos. The Ped gene has two functional alleles, fast and slow, as defined by the rate of development of preimplantation embryos, with the fast allele being dominant. In a more recent study, Brownell and Warner (Brownell and Warner, 1988) demonstrated that the Ped gene phenotype of embryos cultured in vitro is maintained; thus, the control of embryo cleavage is largely dependent on the genes of the embryo itself and is not a function of the uterine environment (Sakkas and Vassalli, n.d.). Embryonic genome activation The variations in the duration of the cell cycle during the early stages of embryo development can perhaps be linked to specific developmental events that occur at this time. The lengthened cycle from the 2- to 4-cell stage in mouse embryos, in particular, may be related to one of the major events of preimplantation development, i.e. embryonic genome activation. The earliest developmental changes are under post-transcriptional maternal control, i.e. they rely on changes in the translation of mRNAs synthesized during oocyte growth, and/or post-translational protein modifications (Sakkas and Vassalli, n.d.). Protein synthesis In concurrence with the activation of the embryonic genome, conventional one dimensional SDS polyacrylamide gel electrophoresis has shown that major changes occur in protein synthesis between days 1 (2-cell stage) and 2 (4- to 8-cell stage) of preimplantation mouse embryo development (13,45). Levinson et al (Levinson et al, 1978) demonstrated that certain stage-specific polypeptides (SSP) are expressed during preimplantation development of mouse embryos; for instance 18 SSP detected in single cell embryos had disappeared by the 4-cell stage (Sakkas and Vassalli, n.d.). Factors affecting embryo metabolism and development One of the most striking changes that the preimplantation embryo experiences is in its energy preferences. Concurrence with compaction, the embryo switches from a dependence on the tricarboxylic acid cycle to a metabolism based on glycolysis. Embryo culture experiments have shown that the mouse oocyte and zygote have an absolute requirement for pyruvate (Biggers et al, 1967); i.e. glucose cannot support early embryo development until the 8-cell stage (Biggers, 1971). This dramatic switch to glucose utilisation as development proceeds may be related to a number of metabolic requirements (Sakkas and Vassalli, n.d.): 1. There is an increase in energy demand around the time of compaction, as protein synthesis increases when the blastocoele is formed. 2. Glucose provides the pentose moieties for nucleic acid synthesis and is required for phospholipid and non-essential amino acid biosynthesis. 3. The embryo displays a considerable capacity for anaerobic glycolysis by the blastocyst stage (qtd. from Sakkas and Vassalli, n.d.); it is conceivable that this primes the embryo for the anoxic environment it encounters at implantation (Sakkas and Vassalli, n.d.) Morphological changes in the preimplantation embryo Compaction Compaction is the first event of morphogenic and cellular differentiation. The most noteworthy event occurring at compaction is the emergence of 2 distinct cell populations: the blastomeres remaining in contact with the outside are destined to form the trophectodermal lineage while the blastomeres inside the embryo are destined to form the ICM (Sakkas and Vassalli, n.d.). The trigger to the development of a polarized phenotype in the outer cells may be related to the pattern of intercellular contacts: polarization is suppressed when a cell is completely surrounded by other cells, while when contact with other cells is incomplete polarity develops. Once a cell acquires polarity the progeny of the cell will be influenced by the orientation of the subsequent cleavage plane, hence either two polar or one polar and one non-polar cell will arise (Sakkas and Vassalli, n.d.). Blastocyst formation The trophectodermal cells acquire the characteristics of epithelial cells in being flattened and joined together by tight junctional complexes (Ducibella and Anderson, 1975). When the mouse embryo has about 32 cells, trophectodermal cells begin to pump fluid into intracellular spaces and later into extracellular spaces, forming the blastocoelic cavity (qtd. from Sakkas and Vassalli, 2003). The trophectoderm ion transport systems play an important role in establishing ion concentration gradients across the epithelium, and thereby in providing the force that drives water into the blastocoelic fluid. The presence of the tight junctional complex is also necessary and plays a multifunctional role. It provides an impermeable seal allowing fluid accumulation, regulates paracellular transport (Manjewala et al, 1989) and contributes to a polarization of the distribution of the Na, K-ATPase (Watson et al, 1990; Sakkas and Vassalli, n.d.). The blastocyst contains two distinct cell types: the ICM cells which go on to form the embryo proper, and the trophectodermal cells which are involved in the initial contact with and the infiltration of the uterine wall and eventually contribute to the placenta and the extraembryonic membranes (Sakkas and Vassalli, n.d.). Gene Expression and Factors in the Preimplantation Blastocyst It is difficult to identify the genes and factors in vivo that affect the earliest events in mammalian development; maintain the undifferentiated, proliferating state of inner cell mass or epiblast cells; regulate implantation; and direct the differentiation of cells along specific developmental pathways, or cell lineages. The embryo itself is very small and, in vivo, is almost wholly inaccessible to study. Therefore, many of the genetic and molecular influences that are now known to regulate early embryogenesis in vivo were identified by studying mouse embryonic stem cells in vitro (Report on Stem Cells, 2005) Interspecies differences In almost all mammalian species studied to date there is a synchronous cleavage in the first few cell cycles followed by asynchronous divisions, normally after the 8-cell stage. The greatest diversity between species exists at the peri-implantation stage, where many embryos experience long periods at the blastocyst stage, forming either minimal or maximal expanding type blastocysts (Sakkas and Vassalli, n.d.). Chimeric embryos Chimeric embryos (also known as allophenic or tetraparental) are generated by " mixing " cells obtained from genetically different embryos. This can be achieved by placing two morula-stage embryos in direct contact (following removal of their zona pellucida), or by introducing cells from a " donor " embryo either underneath the zona pellucida of a morula-stage recipient, or in the blastocoelic cavity (in contact with the ICM) of a blastocyst-stage recipient. The cells from the two embryos assemble to form a single chimera which, when placed in a foster mother, can develop to term. Depending on the circumstances, the contribution of the two partners can be approximately balanced, with a comparable proportion of cells from each origin in all tissues, or be very markedly skewed in favor of one or the other. Thus, the generation of a chimera does not produce a member of a new species, nor a hybrid, but a unique individual. Furthermore, the relative contribution of the two partners is unpredictable, and each chimeric individual is therefore different (Sakkas and Vassalli, n.d.). Growth factor expression and function in preimplantation embryo There is increasing evidence that even before implantation, human development is regulated by embryonically and maternally derived factors. Studies in some mammalian species have shown by the preimplantation embryo and the reproductive tract. Furthermore, a number of growth factors have been shown to affect the rate of embryo development, the proportion of embryos developing to the blastocyst stage, blastocyst cell number, metabolism and apoptosis. Growth factor ligands and receptors are also expresses in human embryos and the maternal reproductive tract, and supplementation of culture medium with exogenous growth factors affects cell fate, development and metabolism of human embryos in vitro. Autocrine, paracrine and endocrine pathways that may operate within the embryo and between the embryo and the reproductive tract before implantation are proposed. Furthermore, evidence suggest that growth factors play an important role in blastocyst development, regulation of cellular events and in maternal embryonic dialoque (Hardy and Spanos, 2002). DNA Methylation and Genomic Imprinting Affect Embryonic Development Also essential to preimplantation embryonic development is the genomic imprinting process, which makes some genes to turned on or off, depending on whether they are inherited from the mother or the father. Several mechanisms of genomic imprinting exist in mammals. A common method of imprinting is DNA methylation (Report on Stem Cells, 2005). DNA methylation is a genome-wide phenomenon; it occurs in many genes depending on the stage of development and the differentiation status of a cell. When the methyl groups are bound at their designated sites in DNA, transcription factors cannot bind to the DNA and gene transcription is turned off. DNA methylation is also responsible for the rearrangement of the structure of chromatin, the combination of DNA and protein that forms the chromosomes. DNA methylation patterns change during development, and their rearrangement in different tissues at different times is an essential method for controlling gene expression (Gilbert, 2000; Report on Stem Cells, 2005). The role of genes regulating programmed cell death in human preimplantation embryo development It is a general characteristic of undifferentiated cells—including embryonic cells in vivo or in vitro—that when they stop dividing, they differentiate, become quiescent or senescent (stop their progress through the cell cycle and enter a period of temporary or permanent "rest"), or die. In vivo or in vitro, the process of cell death can occur by necrosis or apoptosis. Its DNA disintegrates in a characteristic manner, blebs (small pouches) form in the cell membrane, and the cell dies. The genetic controls for apoptosis differ, depending on the cell type, but all engage activating proteases called caspases, enzymes that destroy the protein components of cells (appendix a). Therefore, one central difficulty in successful IVF lies in the production of high quality embryos, as most embryos will give in to embryo arrest, fragmentation, or both. Even those embryos that are competent to reach the blastocyst stage in vitro are usually subject to apoptosis in both the inner cell mass and trophectodermal lineages (Jurisicova andActon, 2004). One cause of developmental arrest may be chromosomal abnormalities (Bongso et al. 1991; Munne et al. 1995), which could contribute to a disruption in mRNA transcripts and ensuing protein synthesis, leading to interference with normal embryo development (Devreker & Englert 2000). There is no plain explanation for developmental arrest, as there is a broad spectrum of cellular defects that occur during cleavage arrest, such as redistribution of mitochondria (Muggleton-Harris & Brown 1988; Acton et al. 2004) and abnormal subcellular localisation of some proteins (Ohashi et al. 2001), including Bcl-2 family members (Jurisicova et al. 2003; Jurisicova andActon, 2004). Moreover, these factors are also regulated by maternal age, as increased frequencies of embryo fragmentation were observed during early cleavage stages in embryos from older IVF patients (Ziebe et al. 1997), and by an increased cell death index of blastocysts from older female mice (Jurisicova et al. 1998; Jurisicova andActon, 2004). Significance of optimal culture conditions Optimal culture conditions are also significant factors that affect the preimplantation embryo development for all species. Water is the basis of all culture media, and therefore ultrapure water should be employed. The main energy sources of a medium are lactate, pyruvate, and glucose. The concentrations of the first two vary in different media, whereas the latter is needed solely for the later stages (morula to blastocyst) of development (Biological factors in culture media). However, despite the conditions of culture during the various steps of in vitro embryo production (IVP) can certainly affect developmental rates, the comparatively low level of effectiveness achieved, manifested by the frequent failure of up to 60% of immature oocytes to reach the blastocyst stage, is almost surely related to the essential quality of the oocyte at the onset of maturation (Lonergan et al..,2001). Conclusion The advances that have taken place in the fields of experimental embryology and molecular genetics over the last 15 years lead us into a new era in the study of mammalian biology and in medicine. Animal breeding already enabled man to considerably speed up the development and selection of desired traits. But the unparalleled power given by these techniques that allow direct manipulation on the genetic heritage places us in a distinct kind of relationship with the living world. Man is not anymore the innocent observer of evolution that he used to be. Therefore, the responsibility that this gives us should not be underestimated (Sakkas and Vassalli, n.d.). Reference List Acton BM, Jurisicova A, Jurisica I & Casper RF 2004 Alterations in mitochondrial membrane potential during preimplantation stages of mouse and human embryo development. Molecular Human Reproduction 10 23–32 Biggers, J.D. (1971): In: The Biology of the Blastocyst, edited by R.J. Blandau, pp. 319-382. University of Chicago Press, Chicago. Biggers, J.D., Whittingham, D.G., and Donahue, R.P. (1967): Proc. Natl. Acad. Sci. USA, 58:560-567. Bongso A, Ng SC, Lim J, Fong CY & Ratnam S 1991 Preimplantation genetics: chromosomes of fragmented human embryos. Fertility and Sterility 56 66–70 Borland, R.M., Biggers, J.D., and LeChene, C.P. (1977): Dev. Biol., 68:440-452. Brownell, M.S., and Warner, C.M. (1988): Biol. Reprod., 39:806-811. Devreker F & Englert Y 2000 In vitro development and metabolism of the human embryo up to the blastocyst stage. European Journal of Obstetrics, Gynecology, and Reproductive Biology 92 51–56 Ducibella, T., and Anderson, E. (1975): Dev. Biol., 47:45-58. Epstein, W., and Smith, S.A. (1973): Dev. Biol., 33:171-184. Gardner, R.L. (2001). The initial phase of embryonic patterning in mammals. Int. Rev. Cytol. 203, 233–290. Gilbert, S.F. (2000). Developmental biology. (Sunderland, MA: Sinauer Associates). Gossler, A. (1992). Early embryonic development of animals, Hennig, W., Nover, L., and Scheer, U. eds. (Berlin, New York: Springer-Verlag). Gilbert, S.F. (2000). Developmental biology. Sunderland, MA: Sinauer Associates. Guillemot, F., Nagy, A., Auerbach, A., Rossant, J., and Joyner, A.L. (1994). Essential role of Mash-2 in extraembryonic development. Nature. 371, 333–336. Hara, T., Tamura, K., de Miguel, M.P., Mukouyama, Y., Kim, H., Kogo, H., Donovan, P.J., and Miyajima, A. (1998). Distinct roles of oncostatin M and leukemia inhibitory factor in the development of primordial germ cells and sertoli cells in mice. Dev. Biol. 201, 144–153. Hardy K. and Spanos S. (2002). Growth factor expression and function in the human and mouse primplantation embryo. Department of Reproductive Scienceand Medicine, Institute of Reproductive and Developmental Biology, Faculty of Medicine, ImperialCollege, Hammersmith Hospital, Du Cane Road, London W120NN, UK Hogan, B., Beddington, R., Constantini, F., and Lacy, E. (1994). Manipulating the mouse embryo a laboratory manual, (Cold Spring Harbor, New York: Cold Spring Harbor Laboratory Press). Jurisicova A, Antenos M, Varmuza S, Tilly JL & Casper RF 2003 Expression of apoptosis-related genes during human preimplantation embryo development: potential roles for the Harakiri gene product and Caspase-3 in blastomere fragmentation. Molecular Human Reproduction 9 133–141 Jurisicova A, Rogers I, Fasciani A, Casper RF & Varmuza S 1998 Effect of maternal age and conditions of fertilization on programmed cell death during murine preimplantation embryo development. Molecular Human Reproduction 4 139–145 Jurisicova, Andrea and Beth M Acton. (2004). Deadly decisions: the role of genes regulating programmed cell death in human preimplantation embryo development. Society for Reproduction and Fertility.Available from: [31 Jan. 2006]. Lonergan, Patrick., Dimitrios RIZOS, Fabian WARD,,and Maurice P. BOLAND. (2001). Factors influencing oocyte and embryo quality in cattle. Available from: [31 Jan. 2006]. Loutradis D, Drakakis P, Kallianidis K, Sofikitis N, Kallipolitis G, Milingos S, Makris N, Michalas S. (2000). Biological factors in culture media affecting in vitro fertilization, preimplantation embryo development, and implantation. Available from: [31 Jan. 2006]. Levinson, J., Goodfellow, P., Vadeboncouer, M., and McDevitt, H. (1978): Proc. Natl. Acad. Sci. USA, 75:3332-3336. Manjewala, F.M., Cragoe, E.J., and Schultz, R.M. (1989): Dev. Biol., 133:210-220. Munne S, Alikani M, Tomkin G, Grifo J & Cohen J 1995 Embryo morphology, developmental rates, and maternal age are correlated with chromosome abnormalities. Fertility and Sterility 64 382–391 Muggleton-Harris AL & Brown JJ 1988 Cytoplasmic factors influence mitochondrial reorganization and resumption of cleavage during culture of early mouse embryos. Human Reproduction 3 1020–1028. Ohashi A, Minami N & Imai H 2001 Nuclear accumulation of cyclin B1 in mouse two-cell embryos is controlled by the activation of Cdc2. Biology of Reproduction 65 1195–1200 Paria, B. C.,and S K Dey. (n.d.). Preimplantation embryo development in vitro: cooperative interactions among embryos and role of growth factors. Available from: [31 Jan. 2006]. Report on Stem Cells. (2005). The official National Institutes of Health resource for stem cell research. Appendix A: Early Development. Available from: [31 Jan. 2006]. Sakkas, D. and J.D. Vassalli. (2003). THE PREIMPLANTATION EMBRYO: DEVELOPMENT AND EXPERIMENTAL MANIPULATION. Geneva Foundation for Medical Education and Research. Available from: [31 Jan. 2006]. Van Blerkom, J., and Brockway, G.O. (1975): Dev. Biol., 44:148-157. Warner, C.M., Gollnick, S.O., and Godbard, S.B. (1987): Biol. Reprod., 36:606-610. Watson, A.J., Damsky, C.H., and Kidder, G.M. (1990): Dev. Biol.,141:104-114. Ziebe S, Petersen K, Lindenberg S, Andersen AG, Gabrielsen A & Andersen AN 1997 Embryo morphology or cleavage stage: how to select the best embryos for transfer after in-vitro fertilization. Human Reproduction 12 1545–1549 Bibliography Acton BM, Jurisicova A, Jurisica I & Casper RF 2004 Alterations in mitochondrial membrane potential during preimplantation stages of mouse and human embryo development. Molecular Human Reproduction 10 23–32 Biggers, J.D. (1971): In: The Biology of the Blastocyst, edited by R.J. Blandau, pp. 319-382. University of Chicago Press, Chicago. Biggers, J.D., Whittingham, D.G., and Donahue, R.P. (1967): Proc. Natl. Acad. Sci. USA, 58:560-567. Bongso A, Ng SC, Lim J, Fong CY & Ratnam S 1991 Preimplantation genetics: chromosomes of fragmented human embryos. Fertility and Sterility 56 66–70 Borland, R.M., Biggers, J.D., and LeChene, C.P. (1977): Dev. Biol., 68:440-452. Brownell, M.S., and Warner, C.M. (1988): Biol. Reprod., 39:806-811. Devreker F & Englert Y 2000 In vitro development and metabolism of the human embryo up to the blastocyst stage. European Journal of Obstetrics, Gynecology, and Reproductive Biology 92 51–56 Ducibella, T., and Anderson, E. (1975): Dev. Biol., 47:45-58. Epstein, W., and Smith, S.A. (1973): Dev. Biol., 33:171-184. Gardner, R.L. (2001). The initial phase of embryonic patterning in mammals. Int. Rev. Cytol. 203, 233–290. Gilbert, S.F. (2000). Developmental biology. (Sunderland, MA: Sinauer Associates). Gossler, A. (1992). Early embryonic development of animals, Hennig, W., Nover, L., and Scheer, U. eds. (Berlin, New York: Springer-Verlag). Gilbert, S.F. (2000). Developmental biology. Sunderland, MA: Sinauer Associates. Guillemot, F., Nagy, A., Auerbach, A., Rossant, J., and Joyner, A.L. (1994). Essential role of Mash-2 in extraembryonic development. Nature. 371, 333–336. Hara, T., Tamura, K., de Miguel, M.P., Mukouyama, Y., Kim, H., Kogo, H., Donovan, P.J., and Miyajima, A. (1998). Distinct roles of oncostatin M and leukemia inhibitory factor in the development of primordial germ cells and sertoli cells in mice. Dev. Biol. 201, 144–153. Hardy K. and Spanos S. (2002). Growth factor expression and function in the human and mouse primplantation embryo. Department of Reproductive Scienceand Medicine, Institute of Reproductive and Developmental Biology, Faculty of Medicine, ImperialCollege, Hammersmith Hospital, Du Cane Road, London W120NN, UK Hogan, B., Beddington, R., Constantini, F., and Lacy, E. (1994). Manipulating the mouse embryo a laboratory manual, (Cold Spring Harbor, New York: Cold Spring Harbor Laboratory Press). Jurisicova A, Antenos M, Varmuza S, Tilly JL & Casper RF 2003 Expression of apoptosis-related genes during human preimplantation embryo development: potential roles for the Harakiri gene product and Caspase-3 in blastomere fragmentation. Molecular Human Reproduction 9 133–141 Jurisicova A, Rogers I, Fasciani A, Casper RF & Varmuza S 1998 Effect of maternal age and conditions of fertilization on programmed cell death during murine preimplantation embryo development. Molecular Human Reproduction 4 139–145 Jurisicova, Andrea and Beth M Acton. (2004). Deadly decisions: the role of genes regulating programmed cell death in human preimplantation embryo development. Society for Reproduction and Fertility. Available from: [31 Jan. 2006]. Lonergan, Patrick., Dimitrios RIZOS, Fabian WARD,,and Maurice P. BOLAND. (2001). Factors influencing oocyte and embryo quality in cattle. Available from: [31 Jan. 2006]. Loutradis D, Drakakis P, Kallianidis K, Sofikitis N, Kallipolitis G, Milingos S, Makris N, Michalas S. (2000). Biological factors in culture media affecting in vitro fertilization, preimplantation embryo development, and implantation. Available from: [31 Jan. 2006]. Levinson, J., Goodfellow, P., Vadeboncouer, M., and McDevitt, H. (1978): Proc. Natl. Acad. Sci. USA, 75:3332-3336. Manjewala, F.M., Cragoe, E.J., and Schultz, R.M. (1989): Dev. Biol., 133:210-220. Munne S, Alikani M, Tomkin G, Grifo J & Cohen J 1995 Embryo morphology, developmental rates, and maternal age are correlated with chromosome abnormalities. Fertility and Sterility 64 382–391 Muggleton-Harris AL & Brown JJ 1988 Cytoplasmic factors influence mitochondrial reorganization and resumption of cleavage during culture of early mouse embryos. Human Reproduction 3 1020–1028. Ohashi A, Minami N & Imai H 2001 Nuclear accumulation of cyclin B1 in mouse two-cell embryos is controlled by the activation of Cdc2. Biology of Reproduction 65 1195–1200 Paria, B. C.,and S K Dey. (n.d.). Preimplantation embryo development in vitro: cooperative interactions among embryos and role of growth factors. Available from: [31 Jan. 2006]. Report on Stem Cells. (2005). The official National Institutes of Health resource for stem cell research. Appendix A: Early Development. Available from: [31 Jan. 2006]. Sakkas, D. and J.D. Vassalli. (2003). THE PREIMPLANTATION EMBRYO: DEVELOPMENT AND EXPERIMENTAL MANIPULATION. Geneva Foundation for Medical Education and Research. Available from: [31 Jan. 2006]. Van Blerkom, J., and Brockway, G.O. (1975): Dev. Biol., 44:148-157. Warner, C.M., Gollnick, S.O., and Godbard, S.B. (1987): Biol. Reprod., 36:606-610. Watson, A.J., Damsky, C.H., and Kidder, G.M. (1990): Dev. Biol.,141:104-114. Ziebe S, Petersen K, Lindenberg S, Andersen AG, Gabrielsen A & Andersen AN 1997 Embryo morphology or cleavage stage: how to select the best embryos for transfer after in-vitro fertilization. Human Reproduction 12 1545–1549 Read More