Section 8
Chapter 7,327

Evidence that hypophysis and hypothalamus constitute a single entity from the primary stage of histogenesis

Kawamura, K.; Kikuyama, S.

Development (Cambridge) 115(1): 1-9


ISSN/ISBN: 0950-1991
Accession: 007326516

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To identify the origin of the primordial pituitary cells in early stages of amphibian embryos before the formation of Rathke's pouch or its equivalent, transplantation experiments were carried out on Bufo. Wild-type embryos at the open neurula stage were employed as donors and albino embryos at the corresponding stage as recipients. Since brain and pituitary cells of wild-type animals contain a great number of melanin granules, whereas albino cells are devoid of them, the melanin granules enabled the fate of transplants to be traced. The anterior part of the neural ridge (ANR) was found to be the almost exclusive source of the secretory cells in the epithelial pituitary gland (pars distalis, pars intermedia and pars tuberalis). Transplantation of either the central part of the anterior neural plate (NP) or the ANR revealed that the pituitary primordium and the anlage of the posterior hypothalamus (infundibulum) are closely apposed at the open neurula stage. As morphogenetic movement proceeds, each of the two entities takes a separate migration route. The former part (NP) takes a ventrocaudal route to become the diencephalic floor, and the latter part (ANR) takes a rostroventral route to acquire morphological contact with the anterodorsal wall of the foregut before differentiating into the epithelial hypophysis. At the end of the tail-bud stage, they meet again beneath the diencephalic floor and establish a hypothalamo-hypophyseal connection. Some of the ANR cells are incorporated into the preoptic region of the anterior hypothalamus. None of the NP cells develop into epithelial pituitary cells. We conclude that the epithelial pituitary gland is not stomodeal, but placodal, in origin and that the hypothalamo-hypophyseal complex may be regarded as a single entity from an ontogenetic viewpoint.

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