Principles of Self-Organization of the Mammalian Embryo

Abstract

Early embryogenesis is a conserved and self-organized process. In the mammalian embryo, the potential for self-organization is manifested in its extraordinary developmental plasticity, allowing a correctly patterned embryo to arise despite experimental perturbation. The underlying mechanisms enabling such regulative development have long been a topic of study. In this Review, we summarize our current understanding of the self-organizing principles behind the regulative nature of the early mammalian embryo. We argue that geometrical constraints, feedback between mechanical and biochemical factors, and cellular heterogeneity are all required to ensure the developmental plasticity of mammalian embryo development.

Figure 1.. Overview of Preimplantation Development and Fate Decision Processes

(A) Preimplantation development starts with fertilization, after which the embryos undergo several rounds of cleavage divisions. At the 8-cell stage the embryo becomes compacted and establishes apico-basal polarity. The loss of totipotency commences at the 4-cell stage as different sister cells differentiate the contribution to blastocyst lineages. The first cell fate completes at the 32-cell stage; the second cell fate decision completes at the late blastocyst stage. (B) In the morula stage embryo (between the 16–32 cell stages), the Hippo signaling component protein AMOT becomes tethered to the apical domain, resulting in the inactivation of Hippo signaling and translocation of Yap and Taz proteins to the nucleus, where they activate Cdx2 and Gata3 expression to specify trophectoderm (TE) fate. In the apolar cells, the lack of apical domain activates Hippo signaling, and the resulting lack of Cdx2 and Gata3 expression allows the cells to express pluripotency factors (such as Nanog and Sox2) to specify inner cell mass (ICM) fate. (C) During the second cell fate decision, ICM cells express Nanog and Gata6 in a salt-and-pepper manner. The expression of Gata6 prompts the expression of FGF receptor Fgfr1/2, whereas Nanog prompts the expression of FGF. The communication between cells expressing different levels of Nanog and Gata6 by FGF signaling allows the separation of Nanog and Gata6 in two cell populations: the former will specify as epiblast (EPI), whereas the latter will specify as primitive endoderm (PE).

Figure 2.. Crosstalk between Morphogenesis and Lineage Specification in the Preimplantation Embryo

(A) At the 16-cell stage, asymmetric cell divisions generate polar and apolar cells. Cortical tension is higher in apolar cells and lower in polar cells. As a result, when an apolar cell is surrounded by polar cells, the tension difference drives the apolar cell to internalize to become ICM. (B) At the blastocyst stage, the cavity expansion stretches the TE cells, which induces the recruitment of tight junction proteins. The tight junction proteins in turn prompt cavity expansion to form a positive feedback loop. The maximum size of the blastocyst is set by the maximum cavity expansion pressure within which TE layer can suffer.

Figure 3.. Gene Regulatory Networks Control Preimplantation Development

(A) During the first cell fate decision, the robust separation between TE and ICM lineages are contributed to by the feedback mechanisms between cell polarity and its downstream events, as well as the mutual inhibition between TE and ICM transcription factors (B) During the second cell fate decision, FGF signaling and Nanog and Gata6 expression form a tri-stable network to produce EPI and PE lineages.

Figure 4.. The Regulation of Timing of Cell Polarization

(A) Formation of the apical domain is regulated by two key conditions: (1) zygotic genome activation (ZGA), which activates the expression of two transcription factors—Tfap2c and Tead4; and (2) the Rho GTPases activity. The activities of Tfap2c, Tead4, and Rho GTPases are high at the 8-cell stage when they induce the formation of the apical domain. (B) Tfap2c and Tead4 regulate the cooperative recruitment of apical proteins, allowing apical proteins to concentrate as a cluster, whereas Rho GTPases signaling allows the apical proteins to spread laterally in order to form the cap shape of the domain. (C) Overexpression of Tfap2c, Tead4, and RhoA advance the timing of cell polarization to the 4-cell stage. Scale bars, 15um.

Figure 5.. Heterogeneities in Fate Regulators Control Differential Fate Outcomes

Heterogeneous levels of fate regulators at the 2- and 4-cell stages contribute to the heterogeneous expression of lineage transcription factors at the 8-cell stage to influence the fate outcomes of sister blastomeres. The non-coding RNA lincGET is expressed unequally between the sister blastomeres at the 2-cell stage and contributes to the heterogeneous activity of Carm1 at the 4-cell stage. The Carm1 activity in turn positively regulates the expression or DNA binding dynamics of pluripotency factors (Nanog and Sox2), whereas it inhibits Cdx2 expression, and as a result, the cells that have high Carm1 activity preferentially contribute to ICM, whereas the cells that have low Carm1 activity contribute to TE.