Primordial germ cells (PGCs) are unipotent progenitors of sperm and egg which retain an innate ability to generate pluripotent stem cells in vivo, called embryonal carcinomas (ECCs), and in vitro, known as embryonic germ cells (EGCs). It is unknown whether mechanisms similar to those involved in the generation of these cells are also involved in maintaining the pluripotent status of other stem cells such as embryonic stem cells (ESCs) and induced pluripotent stem cells (IPSCs). In this study, the first genome wide assessments of human PGCs and EGCs were performed and compared with other pluripotent ESCs, IPSCs and ECCs.
PGCs are unipotent in that they are lineage-restricted to become germ cells. They do not exhibit self-renewal and do not survive past one week under standard tissue culture conditions 
. In the mouse, it is known that PGCs are derived from a region of the epiblast that mainly gives rise to the extra-embryonic mesoderm. In humans, PGCs first appear between the third and fourth week post-fertilization in the endoderm of the dorsal wall of the yolk sac, near the allantois, then proceed to migrate through the hindgut during the fourth week and dorsal mesentery in the fifth week to reach the genital ridge 
Under appropriate conditions, human PGCs can generate pluripotent stem cells. EGCs are pluripotent stem cells derived from PGCs in cell culture using specific growth factors and media formulations. The term EGCs, was given by Donovan et al.
who derived them from mouse PGCs, in order to distinguish them from ESCs. Since then, a handful of other laboratories have also reported the generation of human EGC lines 
. ECC lines are another source of pluripotent cells that are cultured from adult teratocarcinomas from which there is genetic, immunological, and morphological evidence suggesting a PGC origin 
. Lastly, IPSCs can be generated from PGCs by lentiviral transduction of pluripotent regulators similar to those used to generate IPSCs from somatic cells 
EGCs share the general properties of pluripotent stem cells including unlimited self-renewal and the ability to give rise to cells that represent all three embryonic germ layers. In contrast, EGCs are unlike any other pluripotent stem cell as they are derived from differentiated cells without IPSC-like targeted genetic manipulations and unlike ECCs, maintain a stable karyotype. Furthermore, while human EGCs can generate a variety of cell-types representative of the three germ layers in vitro, they do not form teratomas in vivo like their mouse EGC counterparts. Therefore, EGCs may exist in a uniquely, partial, or intermediate pluripotent state. As such, comparisons between EGCs and PGCs with other pluripotent stem cells provide a powerful model to identify factors that are associated with different states of pluripotency.
Distinct states of pluripotency have been revealed by several laboratories which have shown that pluripotent stem cells exhibit differences in their clonal or self-renewing and differentiating capacities 
. For instance, mouse ESCs and IPSCs in the “naïve state” demonstrate single cell clonal ability, rounded colony morphology, and are not dependent on FGF2 and TGFβ/Activin signaling. In contrast, conventional human ESCs and IPSCs and mouse epiblast-derived stem cells exist in a “primed state” of pluripotency exhibiting flattened colony morphology, insufficient clonal expansion, and a dependence on FGF2 and TGFβ/Activin signaling. These differences in pluripotent states have been attributed to species differences as well as the developmental state of the stem cell origin and yet they are inter-convertible depending on the cell culture environment. For instance, the primed state of human ESCs and IPSCs was shown to be convertible to the naïve mouse ESC-like state given the appropriate factors in cell culture 
. It has also been shown that mouse EGCs will behave similar to the naïve state of mouse ESCs under similar culture conditions 
. However, it remains unknown whether human EGCs could also be converted to a naive state. Indeed, there is considerable interest in deciphering the range of multiple pluripotent states in human cells as they could be utilized to partition out mechanisms that regulate distinct attributes of the pluripotent phenotype.
Currently, the pluripotent state of conventional human EGCs is unknown. For instance, like human ESCs, conventional human EGCs express SSEA3, SSEA4 and TRA antigens, TRA-1-60 and TRA-1–80, which are inefficient at clonal expansion and require FGF2 in cell culture 
. However, similar to mouse ESCs, human EGCs share rounded morphology, express SSEA1 and require LIF for their survival. Given that human EGCs share features in common with both mouse ESCs and human ESCs, it is likely that conventional EGCs fall in their own unique state of pluripotency. Therefore, the following study provides new insight into this question and reveals the genomic signature of EGCs which will identify new candidate genes for regulating pluripotency.
Comparisons between EGCs and PGCs will also help establish a unique signature of human PGCs which have not been demonstrated before while also providing further insight into whether ESCs originate from a PGC origin. Indeed, several lines of evidence suggest that PGCs and ESCs may originate from an early germ cell progenitor 
. For instance, several reports have demonstrated that mouse ESCs express genes associated with immature male and female germ cells such as Stella
, deleted in azoospermia-like (Dazl)
. Similarly, one study that examined the differentiation of human ESCs into germ cells 
, detected the expression of eight genes characteristic of early germ cells in ESCs and none from six genes expressed by later germ cells. Most significantly, this study demonstrated gene expression of DAZL
by ESCs but not by human inner cell mass (ICM
). In a recent study, Scholer et al
demonstrated significant similarities in gene profiles between mouse PGCs and ESCs. Despite these relevant studies, it is largely unknown how human PGCs resemble ESCs at the molecular level compared to other PGC-derived stem cells.
Global expression analyses have been extensively performed to examine the molecular signature of human pluripotent stem cells. These studies have characterized gene expression profiles of human ESC, IPSC, and ECC lines using microarray and other high-throughput analyses in attempts to identify candidates involved in self-renewal or pluripotency 
. In addition to comparisons with mouse stem cells, a meta-analysis performed by Assou et al., 
provides further statistical significance to genes that have been found enriched in ESCs by these studies. Importantly, the meta analysis revealed the ability of the microarray studies to consistently reveal patterns of similarities and differences in gene-expression patterns among human ESC lines including high levels of OCT4
expression in human ESCs, three factors which are critical for maintaining pluripotency in mouse 
and human stem cells 
. Other genes associated with pluripotency are also found including UTF1, REX1, FOXD3
and members of the FGF2, MAPK-ERK, TGFB/activin/nodal, Wnt/b-catenin and Akt/PkB signaling pathways. Together this data demonstrates the efficacy of genomic profiling for determining the “stem-cell” orchestra and provide a foundation for further comparisons of these cells with PGCs and EGCs. Two other studies have also reported on the genomic profiles of PGCs for the purpose of identifying mouse unipotent PGC specific genes that would distinguish them from mouse ESCs 
. Scholer and colleagues 
revealed 4 new unipotent PGC specific genes that are highly conserved in mouse and human while Mise et al
demonstrated that mouse EGCs were more similar to ESCs compared to mouse PGCs and adult germ line derived stem cells.
In this study, we conduct the first genome-wide comparison study of human PGCs and EGCs with a number of diverse pluripotent stem cell types including ESCs, ECCs, and IPSCs. Genomic analyses classified these cell types into three distinct groups, 1) PGCs, 2.) EGCs, and 3.) IPSCs, ESCs ECCs, that reflected their developmental potential. These comparisons included qualitative and quantitative differences in known pluripotent associated gene expression as well as revealing a novel signature for human EGCs. Thus, these findings provide important information for identifying potential mechanisms required for PGC reprogramming into the pluripotent state. Comparisons between human PGCs and ESCs will also help facilitate the identification of factors associated with germ line development and may help establish factors related to a common germ cell progenitor potentially shared between these cell types.