Experimental approaches to the study of primordial germ cell lineage and proliferation

All the adult stem cells, where do they all come from? An external source for organ-specific stem cell pools

Stem cells can self-renew and maintain the ability to differentiate into mature lineages. Whereas the "stemness" of embryonic stem cells is not discussed, the primitiveness of a stem cell type within adult organisms is not well determined. Data presently available are either inconclusive or controversial regarding two main topics: maintenance or senescente of the adult stem cell pool; and pluripotentiality of the cells. While programmed senescence or apoptosis following uncorrected mutations represent no problem for mature cells, the maintenance of the stem cell pool itself must be assured. Two different mechanisms can be envisaged for that. In the first mechanism, which is generally accepted, stem cells originate during ontogeny along with the organ which they are responsible for, and remain there during all the lifespan of the organism. Several observations derived from recent reports allow the suggestion of a second mechanism. These observations include: organ-specific stem cells are senescent; adult stem cells circulate in the organism; stem cell niches are essential for the existence and function of stem cells; adult stem cells can present lineage markers; embryo-like, pluripotent stem cells are present in adult organisms, as shown by the development of teratomas, tumors composed of derivatives of the three germ layers; and the fact that the gonads may be a reservoir of embryo-like, pluripotent stem cells in adult organisms. The second mechanism for the maintenance of adult stem cells compartments implies a source external to the organ they belong, consisting of pluripotent, embryo-like cells of unrestricted life span, presenting efficient mechanisms for avoiding or correcting mutations and capable to circulate in the organism. According to this model, primitive stem cells exist in a specific organ in adult organisms. They undergo asymmetrical divisions, which originate one "true" stem cell and another one which enters the pool of adult stem cells, circulating through the entire organism. Upon signals liberated by organ-specific niches, this cell becomes activated to express lineage-specific genes, homes to that particular organ and repopulates its stem cell compartment, differentiating thus in what is seen as the organ-specific stem cell. The gonads are the natural candidates for homing the primitive stem cells in adult organisms. The model proposed in this work for the maintenance of organ-specific stem cell pools from an external source, represented by primitive, embryo-like germinal stem cells present in testes and ovaries, may contribute to the more complete understanding of this complex issue.

Evaluating human embryonic germ cells: concord and conflict as pluripotent stem cells

The realization of cell replacement therapy derived from human pluripotent stem cells requires full knowledge of the starting cell types as well as their differentiated progeny. Alongside embryonic stem cells, embryonic germ cells (EGCs) are an alternative source of pluripotent stem cell. Since 1998, four groups have described the derivation of human EGCs. This review analyzes the progress on derivation, culture, and differentiation, drawing comparison with other pluripotent stem cell populations.

Establishment of oocyte population in the fetal ovary: primordial germ cell proliferation and oocyte programmed cell death

Strict control of cell proliferation and cell loss is essential for the coordinated functions of different cell populations in complex multicellular organisms. Oogenesis is characterized by a first phase occurring during embryo-fetal life and in common with spermatogenesis, during which mitotic proliferation of the germline stem cells, the primordial germ cells (PGC), prevails over germ cell death. The result is the formation of a relatively high number of germ cells depending on the species, ready to enter sex specific differentiation. In the female, PGC enter into meiosis and become oocytes, thereby ending their stem cell potential. After entering into meiosis in the fetal ovary, oocytes pass through leptotene, zygotene and pachytene stages before arresting in the last stage of meiotic prophase I, the diplotene or dictyate stage at about the time of birth. The most part of oocytes die during the fetal period or shortly after birth. It is widely accepted that in mammals a female is born with a fixed number of oocytes within the ovaries, which over the years progressively decreases without possibility for renewal. Once the oocyte reserve has been exhausted, ovarian senescence, driving what is referred to as the menopause in women, rapidly ensues. The fertile lifespan of a female depends by the size of the oocyte pool at birth and the rapidity of the oocyte pool depletion. Which mechanisms control PGC proliferation? Why do most of the oocytes die during fetal life and what are the mechanisms of such massive degeneration? Is it possible to prolong the lifespan of a female by reducing oocyte lost during the fetal life? This review reports some of the most recent results obtained in an attempt to answer these questions.