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The amphibian Xenopus laevis can adapt the color of its skin to the light intensity of the background. A key peptide in this adaptation process is alpha-melanophore-stimulating hormone (alpha-MSH), which is derived from proopiomelanocortin (POMC) and released by the endocrine melanotrope cells in the pituitarypars intermedia. In this study, the presence of alpha-MSH in the brain, cranial placode derivatives, and retina of developing Xenopus laevis was investigated using immunocytochemistry, to test the hypothesis that POMC peptide-producing neurons and endocrine cells have a common embryonic origin and a common function, i.e., controlling each other''s activities and/or being involved in the process of physiological adaptation. The presence of alpha-MSH-positive cells in the suprachiasmatic nucleus, ventral hypothalamic nucleus, epiphysis, and endocrine melanotrope and corticotrope cells, which are all involved in regulation of adaptation processes, has been detected from stage 37/38 onward. This is consistent with the presumed common origin of these cells, the anterior neural ridge (ANR) of the neural-plate-stage embryo. The olfactory epithelium and the otic and epibranchial ganglia also contain alpha-MSH, indicating that these placodal derivatives originate from a common placodal domain continuous with the ANR. Furthermore, we demonstrate the presence of alpha-MSH in a subpopulation of retinal ganglion cells (RGCs), which is possibly also derived from the ANR. Immunoreactivity for alpha-MSH in RGCs that are located in the dorsal part of the retina is dependent on the background light intensity, suggesting that these cells are involved in the regulation of background adaptation. Taken together, the results support the hypothesis that POMC peptide-producing cells have a common embryonic origin and are involved in adaptation processes.
Figure 1. Diagrams of coronal sections from rostral to caudal (a–g) through the brain of a Xenopus laevis larva at stage 37/38, showing darkly (solid) and lightly (open) stained cell bodies and fibers. Some neuron groups cannot be designated because of the early developmental stage. For abbreviations , see list. Scale bar = 100 μm.
Figure 2. Coronal sections from rostral to caudal (A–H) through the brain of Xenopus larvae at stages 41 (A), 43 (B,D–F), 45 (H), and 47 (C,G), showing immunoreactive cell bodies (arrows) and fibers. For abbreviations , see list. Scale bars = 50 μm.
Figure 3. Diagrams of coronal sections from rostral to caudal (a–m) through the brain of a Xenopus larva at stage 45, showing darkly (solid) and lightly (open) stained cell bodies and fibers (lines and small dots). For abbreviations , see list. Scale bar = 100 μm.
Figure 4. Sagittal to slightly oblique sections through the brain of a Xenopus larva at stage 45, showing immunoreactive cell bodies and fiber tracts. Note that presence of α-MSH in the optic chiasm is confined to the rostral edge. For other abbreviations , see list. Scale bar = 100 μm.
Figure 5. Coronal sections of Xenopus larvae at stages 43 (A,C) and 45 (B). Immunoreactivity is present in the olfactory epithelium (olf; A), the ganglion of the Vth cranial nerve (gV; B) and the fused ganglion of the VIIth and VIIIth cranial nerves (gVII/VIII; C). For other abbreviations , see list. Scale bar = 50 μm.
Figure 6. Coronal sections through eyes of Xenopus larvae at stages 43 (A) and 45 (B), showing immunoreactivity in perikarya and axons (arrowheads) of retinal ganglion cells. Arrow indicates optic nerve. Scale bars = 50 μm in A, 20 μm in B.
Figure 7. Numbers of α-MSH-immunoreactive profiles of retinal ganglion cell bodies per section per eye in Xenopus larvae at stages 41–47. Values are expressed as means ± SEM. Bars with different superscripts differ significantly (P < 0.05).
Figure 8. Numbers of α-MSH-immunoreactive profiles of retinal ganglion cell bodies per section per eye in the dorsal (A) and ventral (B) halves of the retina of black (solid bars)- and white (open bars)-adapted Xenopus larvae at stages 41–47. Values are expressed as means ± SEM. Bars of one color with different superscripts differ significantly (P < 0.05). Asterisks indicate a significant difference between black- and white-adapted animals (P < 0.05).
Figure 9. Timetable of appearance of α-MSH-positive cell groups (shaded bars) from rostral to caudal in the brain of Xenopus laevis during development (this study) compared with the situation in adults (data from Tuinhof et al., 1998). In adult animals, the epiphysis (E) and the locus coeruleus (LC) are α-MSH-negative but are immunoreactive for POMC (LC) and other POMC-derived peptides (β-endorphin in E, ACTH in LC). For other abbreviations , see list.