Differentiating ability of the pars intermedia in hypothalectomized frog tadpoles

It is generally accepted that hypothalectomy of frog tadpoles at the open n eurula stage results in failure of the pars intermedia to develop. A pale b ody color is assumed to be evidence that the hypothalamus was completely re moved. The present study, however, shows that hypothalectomized Rana japoni ca can develop into either albino, as already reported, or darkly pigmented tadpoles. In order to determine the extent to which the intermediate lobe can develop in these hypothalectomized tadpoles, their adenohypophyses were examined immunohistochemically by using anti-alpha MSH (melanocyte-stimula ting hormone). In all the dark-colored larvae a pars intermedia had formed, though its size was very small. In the pale-colored tadpoles, on the other hand, the pars intermedia frequently failed to differentiate, but it was o bserved in 4 of 13 hypothalectomized larvae. In view of other investigators ' data showing the complete absence of ACTH (adrenocorticotropin) cells in hypothalectomized tadpoles, hypophyses were also stained with anti-ACTH. Im munoreactive ACTH cells were detected in hypothalectomized tadpoles irrespe ctive of the body pigmentation, although their incidence was lower than in normal controls. These data indicate that contact between the infundibulum and adenohypophysis is not absolutely essential for differentiation of MSH and ACTH cells in the frog. (C) 2000 Academic Press.