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

Abstract

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 o-melanophore-stimulating hormone (a-MSH), it was revealed that this peptide first appeared at developmental stage 37138, 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 y,-MSH-like peptide, des-N-cl-acetyl+MSH, u-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.

Melanopsin photoreception in the eye regulates light‐induced skin colour changes through the production of α‐MSH in the pituitary gland

“…By stage 42, the light‐sensitive response was firmly established, with significant differences always detected in pigmentation between the light and dark conditions, and for all areas analysed (head, flank and tail) (P < 0.05; 4/4 experiments; Figure G). Importantly, these data are in agreement with the previous qualitative description of skin pigmentation onset in Xenopus tadpoles (Verburg‐van Kemenade et al., ). Of note, the skin pigmentation index of light‐reared embryos decreased between stages 39/40 and 42 (P < 0.05; n = 4), due to melanophore pigment aggregation.…”

“…Although melanopsin mRNAs were detected in melanophores prior to stage 39, light‐induced aggregation of pigment in vivo requires the eye to be functional (Bertolesi et al., ; Verburg‐van Kemenade et al., ). As such, we hypothesized that melanopsin participation in pigment aggregation requires vision and triggers a secondary response, rather than the expected primary response, based on in vitro observations (Isoldi et al., ; Rollag et al., ), mediated by melanopsin within the melanophores themselves.…”

Immunohistochemical studies on the development of the hypothalamo‐hyophysial system in Xenopus laevis

“…P-MSH however showed half equipotent to a-MSH in the background color adaptation through modulation of pigment dispersion in dermal melanophores in anurans (Castrucci et al, 1984). Histochemical studies indicated that 1) intermediate lobe cells in anurans can be detected by both anti-a-MSH and P-MSH antisera (Nyholm and Doerr-Schott, 1977) and 2) the former antiserum tends to show higher cross-reactivity with ACTH cells in the pars distalis (Kar and Naik, 1986;Verburg-van Kemenade et al, 1984). Thus, we used anti-P-MSH antiserum to eliminate this cross-reactivity, although this molecular species is not physiologically relevant, as a consequence of which, cells immunoreactive with this antiserum were found only in the pars intermedia.…”

“…Etkin (1941) concluded that the hypophysial anlage starts to secrete the melanophore-stimulating substance between stage 33/34 and 35/36, judging from pigmentation differences in normal and hypophysectomized Xenopus. Conversely, Verburg-van Kemenade et al (1984) proposed that melanin dispersion in stage 33/34 to 39 is not due to MSH of pituitary origin, based upon similar decapitation and immunohistochemical experiments. In these previous histochemical reports and by other authors (Nyholm and Doerr-Schott, 1977), MSH-immunoreactive cells appeared during stage 37/38 to 39.…”

Forebrain Differentiation and Axonogenesis in Amphibians: I. Differentiation of the Suprachiasmatic Nucleus in Relation to Background Adaptation Behavior

“…observ. ], just prior to the ability to background adapt which occurs during stage 39/40 [Verburg-van Kemende et al, 1984]. Our present study suggests that the DA-ergic cells of the suprachiasmatic nucleus first send out axons just prior to or during stage 37/38.…”