Morphogenesis of the hypothalamus and hypophysis: their association, dissociation and reassociation before and after "Rathke".

Recent progress in the experimental morphology of the development of amphibian pituitary gland is reviewed. A series of transplantation experiments were carried out using wild-type embryos of the toad as a donor and albino embryos as a recipient. Melanin granules in the wild-type cells allowed tracing of the developmental fate of the grafts as a visible cell marker. These studies have demonstrated that the pituitary gland is not a stomodeal derivative, as it has long been believed to be under the name of "Rathke's pouch". The adenohypophysis is of neural origin. The anterior part of the neural ridge (ANR) in the neuroectoderm of the open neurula gives rise to the whole adenohypophysis, i. e., pars distatis, pars intermedia and pars tuberalis. The presumptive hypothalamus is apposed caudally to the pituitary primordium. A part of the ANR contributes neurons to the preoptic hypothalamus even after closure of the neural tube. The anlagen of the olfactory system, which include the nasal epithelia and the olfactory bulbs, are situated on both sides of the pituitary primordium in the neural ridge. In both hypothalamic-hypophyseal and olfactory systems, the peripheral and central parts derive from closely affiliated cell populations, suggesting their clonal relationships. Development of the hypophysis and hypothalamus is interdependent. On one hand, a connection with the embryonic hypothalamus is essential for the pituitary proopiomelanocortin cells to develop. On the other hand, neither the hypothalamic median eminence nor its axonal supply develops without the presence of the pituitary primordium. Novel aspects of the ontogeny and phylogeny of these organs are discussed with special reference to the role of the neural ridge in the generation of a spectrum of chemoreceptive organs.