ABSTRACT
The see-through medaka is a vertebrate model with a transparent body in the adult stage, as well as during the embryonic stages, that was generated from a small laboratory fish, medaka (Oryzias latipes). In this fish model, most of the pigments are genetically removed from the entire body by a combination of recessive alleles at four loci. The main internal organs, namely, heart, spleen, blood vessels, liver, gut, gonads, kidney, brain, spinal cord, lens, air bladder, and gills, in living adult fish are visible to the naked eye or with a simple stereoscopic microscope. This fish is healthy and fertile. A transgenic see-through medaka was produced by using the green fluorescent protein (GFP) gene fused to the regulatory regions of the medaka vasa gene, in which germ cell-specific expression of GFP was visualized. The fluorescent tag also efficiently improved visibility of gonadal tissues. The process of oocyte maturation in the ovary was monitored by repeated observations from the outside of the body during one spawning cycle in the same living females of the transgenic see-through stock. The see-through medaka will provide an opportunity for noninvasive studies of morphological and molecular events that occur in internal organs in the later stages of life.