Human primordial germ cells and embryonic germ cells, and their use in cell therapy
Introduction
Embryonic germ (EG) cells result from the adaptation of primordial germ cells (PGCs) to survive and self-renew in culture [1, 2, 3]. They usually have a normal karyotype (initially at least), and display a pluripotency that enables differentiation into a wide range of tissue types of all three primary cell lineages either in vitro or on forming teratomas when transplanted to suitable sites of the body (i.e. to the testis or kidney capsule). Although human embryonic germ (hEG) cells were first derived [3] around the same time as their pluripotent counterparts, human embryonic stem (hES) cells [4], there has been far less attention focused on the potential use of hEG cells for applications in regenerative medicine than on the use of hES cells. The cause of this disparity is mainly twofold. First, although both stem cells are of controversial origin and their derivation necessitates comprehensive ethical and, in some countries, legislative approval, the recovery of PGCs from early human foetal tissue presents additional practical issues of timing (as tissue is recovered after termination of pregnancy) that can limit access to suitable samples to a greater extent than for obtaining pre-implantation embryos for hES cell derivation. Second, although the initial generation of hEG cells is relatively simple [5•], the maintenance of well-defined cell lines through extended passage in culture has proved to be quite difficult to date [6•, 7•]. This has restricted the wide distribution of well-characterised hEG cell lines to investigators to explore potential cell therapies. Despite these problems, EG cells represent an important alternative to ES cells because they potentially have a different epigenetic status to hES cells that might ultimately prove to be significant for cell therapy applications (Table 1). Moreover, EG cells are important tools for studying factors involved in PGC survival, proliferation and regulation that have clinical relevance. For example, testicular cancer in men arises from carcinoma in situ cells, which probably originate from PGCs that escape normal differentiation processes as is also the case for EG cells [8]. In this review we examine some of the main factors involved in the specification and maintenance of PGCs in vivo and in vitro and the culture conditions that influence the derivation of EG cells. The potential of PGCs and EGs to undergo differentiation to functional cell types for clinical applications is also considered.
Section snippets
Primordial germ cells
For a long time the activity of tissue nonspecific alkaline phosphatase (TNAP) has been used to mark the migration of PGCs from the base of the allantois through the hindgut to the dorsal body wall, and their entry into the genital ridges. This migration occurs about 10.5 days after conception in the mouse, and between weeks 5 and 8 of human gestation. Little is known of the specification of human PGCs because of the difficulties in obtaining tissue at this early stage of foetal development,
Primordial germ cells to embryonic germ cells
PGCs will neither form embryoid bodies (EBs) nor contribute to chimeric formation, and they have a short lifespan in culture such that they only proliferate for a week or so before dying or differentiating. The mechanism of how PGCs are converted to immortal EG cells that can form EBs and in mice can generate chimeric embryos remains largely unknown.
Both murine and human EG cells are derived by dissociating the genital ridges containing PGCs to a cell suspension (Figure 1). This suspension is
Embryonic germ cells to embryoid bodies and differentiation
hEG cell lines differentiate to a variety of cell types in vitro [5•] including neural and muscle cells [22, 23] and, notwithstanding the difficulties of maintaining hEG cell lines, their potential for use in cell therapy has been investigated in several studies. Perhaps the most impressive report to date is the use of EB-derived cells from hEG cell lines to restore neurological function in rats with diffuse motor neuron injury [24•]. Transplanted EB cells displayed engraftment throughout the
Embryonic stem cells to primordial germ cells and embryonic germ cells
In the past, ES cells have generally been termed pluripotent because they were believed not to form trophoblast or germline cells. It is apparent, however, that hES cells can spontaneously generate trophoblast, and this is also the case for mouse ES cells if Oct4 expression is conditionally downregulated. Remarkably, it is now clear that murine ES cells can also develop into PGCs in vitro and subsequently form follicle structures with oogonia [26••]. The latter develop to blastocysts,
Conclusions
Human EG cells are equivalent in pluripotent potential to hES cells and show considerable potential for applications in regenerative medicine. The establishment and distribution of well-characterised EG cell lines to the wider research community, however, remains very limited in comparison with recent rapid progress with hES cells. This is partly because of the lack of robust techniques available for maintaining and proliferating hEG cell lines. A clearer understanding of the factors involved
References and recommended reading
Papers of particular interest, published within the annual period of review, have been highlighted as:
• of special interest
•• of outstanding interest
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Cited by (37)
Angiotensin-converting enzyme (CD143) specifies emerging lympho-hematopoietic progenitors in the human embryo
2012, BloodCitation Excerpt :In contrast, the majority of ACE+ cells express a marker of stem cell pluripotency, Oct-4A,31 which is a variant of the transcription factor Oct-4 and surface Ag SSEA-1 (Figure 4). Oct-4 is expressed in embryonic stem cells and by stem cells from amniotic fluid, neonatal human cord blood, and adult nonhematopoietic organs32-34; however, the association of Oct4 and SSEA-1 in humans is assumed to be restricted to primordial germ cells (PGCs).35 PGCs migrate from the posterior part of the embryo to reach the genital ridges that run adjacent to kidney anlagen and therefore are included within the AGM region.
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