Control of melanotrope cell activity in Xenopus laevis.

The melanotrope cells of the amphibian pars intermedia secrete the peptide melanophore stimulating hormone (aMSH), 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 melanotrope cells function as a homogeneous or heterogeneous population in meeting the physiological demand for aMSH. The demand for aMSH 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 melanotrope cells per lobe did not change as a result of background adaptation. Therefore, we conclude that enlargement of the pars intermedia (as found after adaptation to darker backgrounds) involves hypertrophy rather than hyperplasia of the melanotropes. Ultrastructural examination indicated that the majority of melanotrope cells in black-adapted animals are biosynthetically active, whereas the cells of white-adapted animals are relatively inactive and show secretory granule storage. The pars intermedia of grey-adapted toads contained both active and inactive cells, indicating that melanotrope cells respond as a heterogeneous cell population in meeting the endocrine demand imposed by background.