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 study was undertaken to see the self-differentiating ability of these cells in an ectopic environment. Bufo japonicus embryos at the tailbud stage received implants of either the ANR from open-neurula-stage embryos, or the pituitary primordium from tailbud-stage embryos (the ANR derivative beneath the forebrain floor) into the tail. Development of the pars intermedia and the pars distalis was monitored immunohistochemically using antisera against both synthetic alpha melanophore-stimulating hormone (alphaMSH) and bullfrog prolactin (PRL). Neither the immunoreactive alphaMSH cells nor the immunoreactive PRL cells differentiated 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 pituitary primordium transplanted from the tailbud-stage embryos, immunoreactive 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 the pars intermedia and pars distalis is discussed.