title: Specification of primordial germ cells in medaka (Oryzias latipes).
abstract: BACKGROUND
  Primordial germ cells (PGCs) give rise to gametes that are responsible for the development of a new organism in the next generation. Two modes of germ line specification have been described: the inheritance of asymmetrically-localized maternally provided cytoplasmic determinants and the induction of the PGC fate by other cell types. PGCs specification in zebrafish appears to depend on inheritance of germ plasm in which several RNA molecules such as vasa and nanos reside. Whether the specification mode of PGCs found in zebrafish is general for other fish species was brought into question upon analysis of olvas expression--the vasa homologue in another teleost, medaka (Oryzias latipes). Here, in contrast to the findings in zebrafish, the PGCs are found in a predictable position relative to a somatic structure, the embryonic shield. This finding, coupled with the fact that vasa mRNA, which is localized to the germ plasm of zebrafish but does not label a similar structure in medaka opened the possibility of fundamentally different mechanisms governing PGC specification in these two fish species.
  
  
  RESULTS
  In this study we addressed the question concerning the mode of PGC specification in medaka using embryological experiments, analysis of RNA stability in the PGCs and electron microscopy observations. Dramatic alterations in the somatic environment, i.e. induction of a secondary axis or mesoderm formation alteration, did not affect the PGC number. Furthermore, the PGCs of medaka are capable of protecting specific RNA molecules from degradation and could therefore exhibit a specific mRNA expression pattern controlled by posttrancriptional mechanisms. Subsequent analysis of 4-cell stage medaka embryos using electron microscopy revealed germ plasm-like structures located at a region corresponding to that of zebrafish germ plasm.
  
  
  CONCLUSIONS
  Taken together, these results are consistent with the idea that in medaka the inheritance of maternally provided asymmetrically-localized cytoplasmic determinants directs cells to assume the germ line fate similar to zebrafish PGCs.
authors.affiliation: Department of Physiological Chemistry I, University of Wuerzburg, Biozentrum, Am Hubland, D-97074 Wuerzburg, Germany. amaury.herpin@biozentrum.uni-wuerzburg.de <amaury.herpin@biozentrum.uni-wuerzburg.de>
authors.forename: Amaury
authors.initials: A
authors.lastname: Herpin
authors.forename: Stefan
authors.initials: S
authors.lastname: Rohr
authors.forename: Dietmar
authors.initials: D
authors.lastname: Riedel
authors.forename: Nils
authors.initials: N
authors.lastname: Kluever
authors.forename: Erez
authors.initials: E
authors.lastname: Raz
authors.forename: Manfred
authors.initials: M
authors.lastname: Schartl
mesh_terms: D000818:Animals; D002454:Cell Differentiation / Q000502:physiology; D029941:Fish Proteins / Q000235:genetics / Q000502:physiology*; D005854:Germ Cells / Q000166:cytology* / Q000000981:diagnostic imaging / Q000502:physiology* / Q000648:ultrastructure; D009990:Oryzias / Q000196:embryology* / Q000235:genetics; D012313:RNA / Q000235:genetics; D014463:Ultrasonography; D015027:Zebrafish
chemical_list: D029941:Fish Proteins; D012313:RNA