Germ line development in fishes

Classical work on germ cells in fishes has dealt with three main issues; their embryonic origin, the proliferation, and migration pathway during embryonic and larval development. Until recently, primordial germ cells (PGCs) have been studied in a number of fishes using morphological criteria only. The identification of the Drosophila vasa homolog gene of zebrafish now allows comparison of these morphological data with vasa RNA expression patterns in zebrafish. Teleost PGCs can be distinguished from somatic cells by their distinct morphology, at the earliest during gastrulation, and in most fishes their number varies between 10 and 30 during pregonial development. Mitosis is generally not observed in PGCs at extragonadal locations, whereas they are mitotically active once at the gonadal ridges. During gastrulation, PGCs appear to translocate from the epiblast to the hypoblast and during somitogenesis they are found associated with the most peripheral yolk syncitial layer (YSL). From the peripheral YSL they migrate through the median mesoderm into the dorsal mesoderm and then to the dorsal mesentery, where they establish the gonad primordia with mesenchymal cells. Vasa RNA positive cells, the PGCs of the zebrafish conform to these general observations. Interestingly, classical descriptive and experimental data can now be reevaluated using vasa as a molecular marker of the fish germ line. The power of zebrafish genetics together with possibilities of experimental embryology should accelerate research on aspects of vertebrate germ line development such as PGC migration, division and apoptosis, as well as (in) fertility. The present review summarizes some of the classical data on germ line development in fishes in relation to recent data on vasa expression in zebrafish and compares these findings, where appropriate, with those in other model organisms. Special emphasis is placed on vasa gene expression as a potential universal germ line marker and suggestions are made for novel, zebrafish specific approaches to investigate the vertebrate germ line.