Germ line development in fishes

Classical work on germ cells in fishes has dealt with three main issues; th eir embryonic origin, the proliferation, and migration pathway during embry onic and larval development. Until recently, primordial germ cells (PGCs) h ave been studied in a number of fishes using morphological criteria only. T he identification of the Drosophila vasa homolog gene of zebrafish now allo ws comparison of these morphological data with vasa RNA expression patterns in zebrafish. Teleost PGCs can be distinguished from somatic cells by thei r distinct morphology, at the earliest during gastrulation, and in most fis hes their number varies between 10 and 30 during pregonial development. Mit osis is generally not observed in PGCs at extragonadal locations, whereas t hey are mitotically active once at the gonadal ridges. During gastrulation, PGCs appear to translocate from the epiblast to the hypoblast and during s omitogenesis they are found associated with the most peripheral yolk syncit ial layer (YSL). From the peripheral YSL they migrate through the median me soderm into the dorsal mesoderm and then to the dorsal mesentery, where the y establish the gonad primordia with mesenchymal cells. Vasa RNA positive c ells, the PGCs of the zebrafish conform to these general observations. Inte restingly, classical descriptive and experimental data can now be reevaluat ed using vasa as a molecular marker of the fish germ line. The power of zeb rafish genetics together with possibilities of experimental embryology shou ld accelerate research on aspects of vertebrate germ line development such as PGC migration, division and apoptosis, as well as (in) fertility. The pr esent review summarizes some of the classical data on germ line development in fishes in relation to recent data on vasa expression in zebrafish and c ompares these findings, where appropriate, with those in other model organi sms. Special emphasis is placed on vasa gene expression as a potential univ ersal germ line marker and suggestions are made for novel, zebrafish specif ic approaches to investigate the vertebrate germ line.