Germ cell specification and pluripotency in mammals: a perspective from early embryogenesis
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REVIEW ARTICLE
Germ cell specification and pluripotency in mammals: a perspective from early embryogenesis Naoko Irie • Walfred W. C. Tang • M. Azim Surani
Received: 21 March 2014 / Accepted: 19 May 2014 Ó The Author(s) 2014. This article is published with open access at Springerlink.com
Abstract Germ cells are unique cell types that generate a totipotent zygote upon fertilization, giving rise to the next generation in mammals and many other multicellular organisms. How germ cells acquire this ability has been of considerable interest. In mammals, primordial germ cells (PGCs), the precursors of sperm and oocytes, are specified around the time of gastrulation. PGCs are induced by signals from the surrounding extra-embryonic tissues to the equipotent epiblast cells that give rise to all cell types. Currently, the mechanism of PGC specification in mammals is best understood from studies in mice. Following implantation, the epiblast cells develop as an egg cylinder while the extra-embryonic ectoderm cells which are the source of important signals for PGC specification are located over the egg cylinder. However, in most cases, including humans, the epiblast cells develop as a planar disc, which alters the organization and the source of the signaling for cell fates. This, in turn, might have an effect on the precise mechanism of PGC specification in vivo as well as in vitro using pluripotent embryonic stem cells. Here, we discuss how the key early embryonic differences between rodents and other mammals may affect the establishment of the pluripotency network in vivo and in vitro, and consequently the basis for PGC specification, particularly from pluripotent embryonic stem cells in vitro. Keywords Epiblast Human Mouse Pluripotent stem cells Primordial germ cells
N. Irie W. W. C. Tang M. Azim Surani (&) Wellcome Trust/Cancer Research UK, Gurdon Institute, University of Cambridge, Tennis Court Road, Cambridge CB2 1QN, UK e-mail: [email protected]
Introduction In mammals, germ cells are specified at a very early stage of development from the post-implantation epiblast cells following blastocyst implantation. The inner cell mass (ICM) of blastocysts is the source of epiblast cells as well as embryonic stem cells (ESCs). The ICM is segregated into epiblast and hypoblast or the primitive endoderm. Epiblast cells are equipotent and give rise to all the somatic cells and germ cells [1], as well as epiblast stem cells (EpiSCs) in vitro. In mice, precursors of the primordial germ cells (PGCs) are specified in the extreme proximal region of the epiblast adjacent to the extra-embryonic ectoderm (ExE) [2, 3]. Subsequently, nascent PGCs proliferate and migrate through the developing hindgut into the genital ridges [4]. PGCs stain strongly and specifically for alkaline phosphatase (AP) [5–7]. PGCs are also able to become pluripotent stem cells (PSCs) in vitro, called embryonic germ cells (EGCs) under defined culture conditions [8, 9]. Many studies on mammalian development and PGC specification have been conduct
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