Morphology of the pars intermedia and the melanophore-stimulating cells in Xenopus laevis in relation to background adaptation.

The melanophore-stimulating hormone (MSH) cells of the amphibian pars intermedia secrete the peptide alpha-melanophore-stimulating hormone (alpha-MSH), which induces pigment dispersion in dermal melanophores. The purpose of the present study was to determine (1) whether prolonged activation of the secretory activity of the pars intermedia is associated with hypertrophy, hyperplasia, or both and (2) whether the MSH cells function as a homogeneous or heterogeneous population in meeting the physiological demand for MSH. The demand for MSH was manipulated by adapting animals for at least 3 weeks to white, two shades of grey, or black backgrounds. Morphometric analysis showed that the intermediate lobe volume was positively correlated with the degree of pigment dispersion in the melanophores. The number of MSH cells per lobe was not affected by the degree of pigment dispersion. Therefore, we conclude that enlargement of the tissue associated with MSH cell activation involves hypertrophy rather than hyperplasia. Ultrastructural examination indicated that the majority of MSH cells in black-adapted animals are biosynthetically active, whereas the cells of white-adapted animals are relatively inactive and show granule storage. The pars intermedia of grey-adapted toads contained both active and inactive cells, indicating that MSH cells respond as a heterogeneous cell population in meeting the endocrine demand imposed by background.