The development of the pars intermedia and its role in the regulation of dermal melanophores in the larvae of the amphibian Xenopus laevis.

The ontogenesis of biosynthesis of pro-opiomelanocortin (POMC)-related peptides in the pars intermedia of Xenopus laevis tadpoles was studied. The results were related to the capacity of the animal to adapt to background color through regulation of pigment dispersion in dermal melanophores. Using immunocytochemical techniques with antisera to alpha-melanophore-stimulating hormone (alpha-MSH), it was revealed that this peptide first appeared at developmental stage 37/38, just prior to the animal's ability to adapt to background. It was shown that pigment dispersion in melanophores between stages 33 and 39 was not dependent on melanotropins of pituitary origin. Using in vitro biosynthetic studies it was possible to follow POMC biosynthetic activity, its processing and the release of peptides from stage 48 onward. Among the newly synthesized peptides observed were a gamma 3-MSH-like peptide, des-N-alpha-acetyl-alpha-MSH, alpha-MSH, and two endorphin-like peptides. By stage 57 a biosynthetic pattern almost identical to that of the adult pars intermedia had evolved. It was concluded that stage 39/40 is a critical stage in the simultaneous development of a number of the components involved in the neuroendocrine control of background adaptation.