Introduction Preimplantation development of mammalian embryo

Introduction Preimplantation development of mammalian embryo proceeds with the cleavage division of the zygote into blastomeres and the allocation of these blastomeres and their descendants to the extraembryonic and embryonic cell lineages of the Erlotinib (Rossant and Tam, 2009). In the mouse, the extraembryonic lineages comprise the trophectoderm that contributes to the placental trophoblasts and the primitive endoderm derived from the inner cell mass (ICM). The embryonic lineage is derived from the epiblast of the blastocyst, which forms the primary germ layers during gastrulation. The germ layers are the progenitor of all types of cells and tissues in the body, as well as the hematopoietic tissues in the yolk sac and the vascular tissues of the foeto-maternal interface (Stephenson et al., 2012). During embryogenesis, cells in the mouse embryos transit through different states of developmental potency (Boroviak et al., 2015). In an experimental setting, single blastomeres at the early cleavage stage are able to re-constitute the whole conceptus or contribute unrestrictedly to the full suite of extraembryonic and embryonic tissues in a chimeric conceptus. Cells displaying such attributes are reputed to be totipotent. Following the segregation of the extraembryonic lineages, cells in the embryo become restricted to the epiblast fate. Lineage analysis of individual epiblast cells in chimeras revealed that they can participate in germ layer differentiation and contribute extensively to all the specialized somatic cell types and the germline of the embryo. The multi-lineage potential and the chimera forming capacity of the epiblast cells are hallmarks of pluripotency (Mascetti and Pedersen, 2016). Both totipotency and pluripotency are transitory in the mouse embryo during development, with the dismantling of pluripotency by late gastrulation (Osorno et al., 2012). However, the pluripotent property can be captured in the embryonic stem cells (ESCs) that are isolated from the peri-implantation blastocyst under appropriate in vitro culture conditions (Evans and Kaufman, 1981; Martin, 1981). The ESCs do not only remain pluripotent in lineage differentiation and chimera development but, unlike the parental cells in the embryo, also display limitless self-renewal activity. Self-renewing stem cells that are derived from the rodent embryos under different in vitro conditions display discernibly different molecular properties while remaining pluripotent. Embryos of certain murine strains and rodent species are less amenable for the derivation of ESCs, and non-rodent mammalian species (such as the lagomorphs and primates) require conditions that may be substantially different from those for the mouse for isolating and maintaining the pluripotent stem cells. The diverse cell states and in vitro conditions for the derivation and maintenance of the mammalian embryo-derived stem cells raise the questions of whether there are multiple states of pluripotency of the stem cells of each species, and if there are innate species-specific variations in the pluripotency state that underpin the ability to procure stem cells of comparable state of cell potency in vitro. In this review, we will address these questions by taking a snapshot of our knowledge of the properties of the pluripotent stem cells, focusing on the maintenance of pluripotency and inter-conversion of the different types of pluripotent stem cells from rodents, lagomorphs and primates.
Embryonic stem cell lines from the permissive mouse strain epitomize the naive state of pluripotency Mouse embryonic stem cell (ESC) lines were first derived from the ICM of 129 inbred mouse (Evans and Kaufman, 1981), a permissive strain which has a genetic background known for its propensity to develop testicular teratomas (tumors with cell types of all three germ layers). Two key cell culture supplements for the derivation are leukemia inhibitory factor (LIF) and foetal calf serum (FCS) (Smith et al., 1988) (Fig. 1A); the latter can be replaced by bone morphogenetic protein 4 (BMP4) (Ying et al., 2003). LIF, via gp130, Janus kinase (JAK) 2, and signal transducer and activator of transcription 3 (STAT3) (Niwa et al., 1998), fuels the activity of a complex network of epiblast transcription factors known as the extended pluripotency network. It consists of a core pluripotency factors: Oct4, Sox2, Nanog and other allied factors: Klf2, Klf4, Tfcp2l1, Esrrb, Gbx2, and Sall4, that enable the cells to acquire robust pluripotency (Aksoy et al., 2014; Bourillot et al., 2009; Dunn et al., 2014; Hall et al., 2009; Martello et al., 2012; Martello et al., 2013; Martello and Smith, 2014; Niwa et al., 2009; Qiu et al., 2015; Tai and Ying, 2013; Yang et al., 2010a; Ye et al., 2013; Yeo et al., 2014; Yuri et al., 2009). We shall call this culture condition “Serum/LIF”. The pluripotency network can be further stabilized and the self-renewal activity reinforced by blocking the differentiation-inducing signaling activity mediated by the extracellular regulated kinase (ERK) (Burdon et al., 1999) and by enhancing the metabolic activity and WNT signaling activity by inhibiting glycogen synthase kinase 3 (GSK3) (Martello et al., 2012). For this purpose, two inhibitors are conventionally used, PD0325901 and CHIR99021, which define a new culture condition called “2i/LIF” (Ying et al., 2008). These 2i/LIF ESCs can thrive in the absence of ERK activity, in contrast to those maintained in LIF and serum conditions, and are reputed to have acquired an alternative state of naive pluripotency.