The eyeless mutant gene (e) in the Mexican axolotl (Ambystoma mexicanum) affects pax-6 expression and forebrain axonogenesis.

Abstract

This study tested the hypothesis that changes in the patterns of pax-6 expression disrupt the anatomy and axonogenesis of the diencephalic areas of the eyeless axolotl. Proper pax-6 expression is necessary for eye and hypothalamus morphogenesis. Since the expression boundaries of pax-6 also provide a permissive environment for axonal outgrowth, an extensive study examining the effects of the eyeless gene (e) in the Mexican axolotl upon pax-6 expression and forebrain axonogenesis was begun. This study used whole embryo in situ hybridization techniques to follow pax-6 expression and whole brain immunocytochemistry to examine axonogenesis and neural differentiation. These studies demonstrated that the mutant gene e in the axolotl alters the response of midanterior neural-plate tissue to signals from the prechordal plate. This response was hypothesized to be a hyper-response to signals (sonic hedgehog?) that suppressed pax-6 expression within the midanterior neural plate and later developmental stages. Alternatively, the affected neuroectoderm of the eyeless embryos may lack competence to express pax-6. Lowered pax-6 expression inhibited eye and forebrain morphogenesis as well as neural axonogenesis and differentiation. Differentiation defects were detected as the suppression of midline dopaminergic neurons within the suprachiasmatic nucleus of eyeless animals. Thus, lowered pax-6 expression by the midanterior neuroectoderm promotes the eyeless condition by inhibiting the role of pax-6 in eye formation. This lowered expression also leads to concurrent alterations in the hypothalamic terrain which disrupt axonogenesis and ultimately promote sterility.