Cell lineage allocation within the inner cell mass of the mouse blastocyst

Abstract

At the time of implantation, the early mouse embryo consists of three distinct cell lineages: the epiblast (EPI), primitive endoderm (PrE), and trophectoderm (TE). Here we will focus on the EPI and PrE cell lineages, which arise within the inner cell mass (ICM) of the blastocyst. Though still poorly understood, our current understanding of the mechanisms underlying this lineage allocation will be discussed. It was originally thought that lineage choice was strictly controlled by the position of a cell within the ICM. However, it is now believed that the EPI and PrE lineages are defined both by their position and by the expression of lineage-specific transcription factors. Interestingly, these lineage-specific transcription factors are initially co-expressed in early ICM cells, suggesting an initial multi-lineage priming state. Thereafter, lineage-specific transcription factors display a mutually exclusive salt-and-pepper distribution that reflects cell specification of the EPI or PrE fates. Later on, lineage segregation and likely commitment are completed with the sequestration of PrE cells to the surface of the ICM, which lies at the blastocyst cavity roof. We discuss recent advances that have focused on elucidating how the salt-and-pepper pattern is established and then resolved within the ICM, leading to the correct apposition of cell lineages in preparation for implantation.

Fig. 10.1. Mouse preimplantation development leading to blastocyst formation

During preimplantation development, the mouse embryo undergoes cleavages that culminate in the proper segregation of three lineages at the blastocyst stage. This process involves two cell fate decisions: the first decision occurs when the inner cell mass (ICM) is segregated from the extraembryonic trophectoderm (TE); the second decision occurs within the ICM and involves the segregation of the pluripotent epiblast (EPI) from the extraembryonic primitive endoderm (PrE)

Fig. 10.2. Models for lineage allocation within the ICM

(a) In the positional-based model, all ICM cells are morphologically and molecularly equal, being able to contribute to either EPI or PrE. Lineage allocation is strictly controlled by cell position: cells positioned in the interior of the ICM give rise to EPI, whereas cells that underlie the blastocyst cavity form PrE, possibly as a result of positional induction. (b) In the three-phase-based model, lineage allocation is controlled by the sequential activation of key transcription factors as well as dynamic cell movements within the ICM. Initially, EPI- and PrE-specific transcription factors are co-expressed in all cells of the ICM. That is followed by a mutually exclusively expression pattern, where cells expressing EPI-or PrE-specific transcription factors are distributed in a salt-and-pepper fashion irrespectively of their position within the ICM. Finally, randomly positioned PrE precursor cells will sort to the surface of the ICM ensuring the spatial segregation of the EPI and PrE lineages before implantation

Fig. 10.3. Lineage allocation in the ICM depends on FGF/MAPK signaling

At around the 32-cell stage, when EPI- and PrE-specific transcription factors are expressed homogeneously in all ICM cells, a differential expression of the ligand Fgf4 and the receptor Fgfr2 starts to become evident. EPI precursor cells express and secrete Fgf4, whereas PrE precursor cells express Fgfr2. Binding of Fgf4 to Fgfr2 activates the FGF/MAPK signaling pathway, which is required for the induction of PrE-specific transcription factors, thus promoting PrE formation. When the signaling is not active, such as in Fgf4 mutants or by inhibiting the receptor Fgfr2, PrE is not formed and all ICM cells contribute to EPI. On the contrary, in the presence of excess Fgf4, all ICM cells contribute to PrE

Fig. 10.4. Two models for the establishment of the salt-and-pepper distribution of cells in the ICM

The two “waves” of asymmetric cell divisions, which take place at the 8- to 16-cell and 16- to 32-cell stages, generate the inner cells of the ICM. (a) In the “time-outside/time-inside” model, cells that are internalized with the first wave are biased to form EPI, whereas cells that are internalized with the second wave are biased to form PrE. (b) In the “random/no lineage-biased” model, cells could contribute to EPI or PrE irrespectively of whether they have internalized during the first or the second wave

Fig. 10.5. Cell sorting ensures the spatial segregation of PrE and EPI.

After the salt-and-pepper distribution of cells within the ICM, dynamic cell rearrangements take place, involving the movement of EPI and PrE precursor cells to inner locations of the ICM and surface of the blastocyst cavity, respectively. Some PrE precursor cells that are located deep into the ICM may undergo apoptosis or convert to an EPI fate. On the contrary, some EPI precursor cells that lie next to the blastocyst cavity might convert to a PrE identity. Moreover, positional signals from the blastocyst cavity (such as Wnt9α) might induce the migration of PrE precursor cells toward the cavity and the maintenance of PrE identity in these cells