Building Pluripotency Identity in the Early Embryo and Derived Stem Cells

The fusion of two highly differentiated cells, an oocyte with a spermatozoon, gives rise to the zygote, a single totipotent cell, which has the capability to develop into a complete, fully functional organism. Then, as development proceeds, a series of programmed cell divisions occur whereby the arising cells progressively acquire their own cellular and molecular identity, and totipotency narrows until when pluripotency is achieved. The path towards pluripotency involves transcriptome modulation, remodeling of the chromatin epigenetic landscape to which external modulators contribute. Both human and mouse embryos are a source of different types of pluripotent stem cells whose characteristics can be captured and maintained in vitro. The main aim of this review is to address the cellular properties and the molecular signature of the emerging cells during mouse and human early development, highlighting similarities and differences between the two species and between the embryos and their cognate stem cells.

Defining totipotency using criteria of increasing stringency

Totipotency is the ability of a single cell to give rise to all the differentiated cells that build the conceptus, yet how to capture this property in vitro remains incompletely understood. Defining totipotency relies upon a variety of assays of variable stringency. Here we describe criteria to define totipotency. We illustrate how distinct criteria of increasing stringency can be used to judge totipotency by evaluating candidate totipotent cell types in the mouse, including early blastomeres and expanded or extended pluripotent stem cells. Our data challenge the notion that expanded or extended pluripotent states harbor increased totipotent potential relative to conventional embryonic stem cells under in vivo conditions.

Totipotency continuity from zygote to early blastomeres: a model under revision

The mammalian zygote is a totipotent cell that generates all the cells of a new organism through embryonic development. However, if one asks about the totipotency of blastomeres after one or two zygotic divisions, opinions differ. As it is impossible to determine the individual developmental potency of early blastomeres in an intact embryo, experiments of blastomere isolation were conducted in various species, showing that two-cell blastomeres could give rise to a new organism when sister cells were separated. A mainstream interpretation was that each of the sister mammalian blastomeres was equally totipotent. However, reevaluation of those experiments raised some doubts about the real prevalence of cases in which this interpretation could truly be validated. We compiled experiments that tested the individual developmental potency of early mammalian blastomeres in a cell-autonomous way (i.e. excluding nuclear transfer and chimera production). We then confronted the developmental abilities with reported molecular differences between sister blastomeres. The reevaluated observations were at odds with the mainstream view: A viable two-cell embryo can already include one non-totipotent blastomere. We were, thus, led to propose a revised model for totipotency continuity based on the construction of the zygote as a mosaic, which accounts for differential inheritance of totipotency-relevant components between sister blastomeres. This takes place with no preordained mechanisms that would ensure a reproducible partition. This model, which is compatible with the body of data on regulative properties of mammalian early embryos, aims at tempering the rigid interpretation that discounted maternal constraints on totipotency.

Early mammalian development: from basic research to the clinic

Preimplantation embryonic development lays the foundations for the future individual. Fertilization, cleavage, differentiation of the first embryonic cell lineages and implantation of the embryo into the maternal uterus are absolutely critical for proper embryogenesis. Solving unanswered questions as well as creating new ideas and theories constitute the main axis of the basic research, which is driven by the curiosity of scientists and their desire to explore the unknown. We researchers have been exploring the development of mammalian embryos for decades, searching for the answer to the most fundamental question in the whole area of biology: how a complex organism derives from a single totipotent cell, a zygote. Due to obvious ethical concerns, animals, such as mice and, currently more and more often, cattle, pigs and rabbits, have become useful models for studying human embryonic development. Unprecedented advancement in cell and molecular biology techniques witnessed in the last years allows us to deepen our understanding of mammalian embryonic development.