DEVELOPMENT OF THE ECTOPICALLY TRANSPLANTED PRIMORDIUM OF EPITHELIAL HYPOPHYSIS (ANTERIOR NEURAL RIDGE) IN BUFO-JAPONICUS EMBRYOS

It has recently been demonstrated that the epithelial pituitary of the toad is not stomodeal, but placodal, in origin. The placodal cells in the anterior part of the neural ridge (ANR) of the open neurulae are the exclusive source of the epithelial pituitary gland. The present st udy was undertaken to see the self-differentiating ability of these ce lls in an ectopic environment. Bufo japonicus embryos at the tailbud s tage received implants of either the ANR from open-neurula-stage embry os, or the pituitary primordium from tailbud-stage embryos (the ANR de rivative beneath the forebrain floor) into the tail. Development of th e pars intermedia and the pars distalis was monitored immunohistochemi cally using antisera against both synthetic alpha melanophore-stimulat ing hormone (alphaMSH) and bullfrog prolactin (PRL). Neither the immun oreactive alphaMSH cells nor the immunoreactive PRL cells differentiat ed from the neural ridge when it was dislocated from the original site at the open neurula stage. On the other hand, in grafts of the pituit ary primordium transplanted from the tailbud-stage embryos, immunoreac tive PRL cells developed invariably and immunoreactive alphaMSH cells were detected at an incidence of 72%. The significance of the role of brain tissue surrounding the pituitary anlage in differentiation of th e pars intermedia and pars distalis is discussed.