Making Germ Cells from Human Embryonic Stem Cells - Male Germline Stem Cells Developmental and Regenerative Potential - page 78

to study germ cell development has great benefits. Using this system we can more
easily probe the early genetic and epigenetic events of gamete development and
uncover the genetic requirements, signaling pathways, and molecular programs that
are unique to the human germ cells. Now that numerous investigations in the mouse
and human have generated germ cells and even more mature gametes from ES cells,
the road has been paved for achieving the formation of bona fide eggs and sperm
that are fully functional and capable of generating healthy viable offspring.
The creation of gametes in vitro would provide several potential scientific and
clinical applications. Establishment of this system could provide a novel and faster
approach for the generation of transgenic and knockout mice. Additionally, as the
oocyte is the only cell type that is capable of complete genomic reprogramming to
generate a totipotent embryo, human ES cells might also serve as an in vitro method
of generating an oocyte-like cell capable of nuclear reprogramming. Oocytes gener-
ated in culture could provide an excellent alternative reprogramming cell for SCNT.
As eggs produced in vivo are limited in number and difficult to obtain, oocytes
generated in vitro might be advantageous for SCNT and might have great regenera-
tive and therapeutic applications.
As infertility is remarkably common in humans relative to other species and
often results from problems with gamete quantity or quality, the ES cell-germ cell
model system might also provide a greater understanding of various reproductive
disorders and pathologies resulting from aberrant germ cell development. Moreover,
as the exact cause of infertility is often unknown, developments and insight gained
from the study of gametes in vitro could promote the creation of genetic screens and
assays for the diagnosis of infertility. Other important potential applications of
germline stem cell biology include using iPS cells to generate gametes in vitro as
an alternative to hESC-derived germ cells. This possibility is exciting as it would
allow the reprogramming of an adult cell type, such as a skin cell or fibroblast, into
a pluripotent cell that could subsequently be differentiated into a mature male or
female gamete. An iPS cell-derived gamete would be specific to the individual from
whom it was obtained. For individuals who are unable to conceive naturally and
either make little or no gametes or have failed with assisted reproductive technolo-
gies, this might one day present a way to generate functional gametes that are
genetically unique to the individual. Importantly, a mechanism to create mature
gametes “in the dish” by the use of embryonic or pluripotent stem cells holds great
promise for the conservation of endangered species and may one day have clinical
applications for the treatment of human infertility.
Acknowledgments We thank past and present members of the Reijo Pera laboratory for their
scientific contributions, diligence, and creativity in their investigations of stem cells and germ
cell biology. We give special thanks to Vanessa Angeles, Raul Calvijo, Marty Flores, Kelly
Haston, Kehkooi Kee, and Cory Nicholas for allowing us to include published and unpublished
images and data in this chapter, and Drs. Shawn Chavez and Kehkooi Kee for their critical
reviews of this chapter. We also thank our families and friends for their continued support of our
scientific endeavors. We would like to acknowledge generous funding and fellowships provided
over the past several years by: California Institute for Regenerative Medicine (CIRM); National
Institutes of Health (NIH); National Institute of Child Health and Human Development (NICHD,
specifically grant R01HD044876 to SMSH and RARP); National Institute on Aging (NIA);