Human pre-implantation embryo development

Abstract

Understanding human pre-implantation development has important implications for assisted reproductive technology (ART) and for human embryonic stem cell (hESC)-based therapies. Owing to limited resources, the cellular and molecular mechanisms governing this early stage of human development are poorly understood. Nonetheless, recent advances in non-invasive imaging techniques and molecular and genomic technologies have helped to increase our understanding of this fascinating stage of human development. Here, we summarize what is currently known about human pre-implantation embryo development and highlight how further studies of human pre-implantation embryos can be used to improve ART and to fully harness the potential of hESCs for therapeutic goals.

Fig. 1.

Stages of human pre-implantation embryo development. Phase-contrast images of human embryo development from day (d) 0 to day 7. Following fertilization, embryos undergo a series of mitotic cell divisions. Arrowheads in d0 and d1 indicate pronuclei. On or around day 4, the embryo compacts, resulting in the formation of a morula that consists of cells (or blastomeres) in a compact cluster contained within the zona pellucida (the glycoprotein layer that surrounds the embryo). The blastocyst, which forms on day 5, is a fluid-filled structure composed of an inner cell mass (white arrowhead) and trophectoderm (gray arrowhead). On day 6, the blastocyst ‘hatches’ from the zona pellucida and it is ready to implant into the uterine wall on day 7.

Fig. 2.

Imaging human embryo development and predicting developmental potential. (A) Timeline of human embryo development from the zygote to the blastocyst stage, highlighting critical times between stages that predict successful development. (B) Molecular events and time-lapse images of human embryo development. ESSP1 describes maternally inherited oocyte mRNAs destined for degradation, ESSP4 genes are stably expressed, and ESSP2 refers to a set of embryonic activated genes. (C) Automated tracking demonstrates modeling via computer algorithms to predict success or failure. (D) Critical features that influence developmental success include the duration of first cytokinesis, the time between the first and second mitotic division, and the synchronicity of appearance of the third and fourth blastomeres. (E) Potential application of prediction of embryo developmental potential in ART. Figure reproduced with permission from Wong et al. (Wong et al., 2010).

Fig. 3.

Cell fate decisions and their timing in mouse versus human early embryo development. Prior to implantation, both human and mouse embryos similarly undergo cell divisions culminating in the development of a blastocyst comprising a discernible ICM and TE. Mouse zygotic/embryo genome activation (ZGA/EGA) begins at the 2-cell stage (Flach et al., 1982), whereas human EGA begins at ∼4- to 8-cell stage on day 3, although minor human EGA may occur as early as the 2-cell stage (Taylor et al., 1997; Vassena et al., 2011). The timing of compaction and blastocyst formation also differs significantly, with human embryos showing delayed development compared with mouse embryos; the mouse blastocyst forms between days 3 and 4, whereas human blastocysts form between days 5 and 6. Both human and mouse pre-implantation blastocysts comprise an outer layer of trophectoderm (TE) cells, which form the trophoblast lineage of the placenta, and an inner cell mass (ICM) that segregates into epiblast (Epi) and primitive endoderm (PE) layers. Epiblast cells eventually give rise to all the tissues of the future fetus, whereas the PE gives rise to extra-embryonic endoderm (ExEn) cells that will form the yolk sac. In the mouse, the TE gives rise to a proliferative stem cell pool of extra-embryonic ectoderm (ExEc) cells that bud off differentiated polyploid trophoblast giant (TG) cells. By contrast, human TE gives rise to villous cytotrophoblast (VCT) cells, a multinucleated syncytium (Syn) and extravillous trophoblast cells (not shown). The dashed arrow indicates the possibility of earlier minor gene activation.

Fig. 4.

Genetic networks of human pre-implantation development. Maternal transcripts inherited from the oocyte are degraded through subsequent rounds of cell division. Human genome activation principally occurs between the 4- and 8-cell stages, and perhaps as early as the 2-cell stage. It is unclear when genes associated with the restriction of the TE or ICM cell lineage are expressed in human embryos, but data suggest that these lineage-associated genes are expressed in human embryos later than in mice at around the early blastocyst stage. Human embryos can be cultured in vitro for 7-8 days post-fertilization and, in vivo, human embryos implant around day 7. The derivation of epiblast stem-like cells from human pre-implantation embryos suggests that human embryos might be capable of reaching a more mature stage in vitro than mouse pre-implantation embryos, which can be cultured in vitro up to 4 days post-fertilization. The dashed arrows indicate the possibility of earlier minor gene activation and lineage-associated gene expression. Epi, epiblast; TE, trophectoderm; PE, primitive endoderm.

Fig. 5.

Derivation of human embryonic stem cells (hESCs). (Left) hESCs cultured on feeders (e.g. fibroblasts cells) and (right) in ‘feeder-free’ conditions (supportive fibroblast cells have been substituted with extracellular matrix components). Scale bars: left, 50 μm; right, 100 μm.