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The spatiotemporal sequence of the appearance of cholinergic structures in the brain of Xenopus laevis during development was studied by means of choline acetyltransferase (ChAT) immunohistochemistry. The first ChAT labeling in the central nervous system of Xenopus was obtained at late embryonic stages in the spinal motoneurons, the cranial nerve motor nuclei of the brainstem, and in amacrine cells of the retina. During premetamorphosis, these cholinergic structures maturated significantly and new ChAT-immunoreactive cells were observed in several other nuclei such as the solitary tractnucleus, isthmic nucleus, laterodorsal and pedunculopontine tegmental nuclei, epiphysis, dorsal habenular nucleus, medial amygdala, bed nucleus of the stria terminalis, and dorsal pallidum. Further maturation continued through prometamorphosis and the climax of the metamorphosis together with the appearance of new cell groups in the efferent octaval nucleus, ventral hypothalamic nucleus, anterior preoptic area, suprachiasmatic nucleus, and medial septum. Transient expression of ChAT was only seen in the large Mauthner cells that showed moderate ChAT labeling during pre- and prometamorphosis but became immunonegative at the end of the metamorphosis. The gradual appearance, in general from caudal to rostral brain levels, of ChAT immunoreactivity in Xenopus, was correlated with other developmental events to get insight into the possible roles of acetylcholine during ontogeny. Comparison with the developmental pattern of cholinergic systems in other vertebrates shows that Xenopus possesses abundant features in common with amniotes, suggesting a conservative developmental plan for tetrapods.
Figure 3. Photomicrographs of transverse sections through the brain of Xenopus at stages 40 (a–e), and 45 (f–g), and 44 (h). a: Mesencephalon. The asterisk indicates the retina. b: Isthmic region. The asterisk indicates the retina, which is shown in a higher magnification on the right. Arrows point to trochlear axons. c: Rhombencephalon at the level of the facial motor nucleus. d: Close to the obex at the level of the vagal motor nucleus. e: Spinal cord. f: Mesencephalon. g: Rhombencephalon at the level of the trigeminal motor nucleus. h: Rhombencephalon at the level of the vagal motor nucleus. For abbreviations , see list. Scale bar = 100 μm in a–g; 50 μm in h.
Figure 5. Photomicrographs of transverse sections through the brain of Xenopus at stages 50 (a) and 53 (b–d). a: Dorsal thalamus and epiphysis. b: Oculomotor nucleus. c: Isthmic region. d: Caudal rhombencephalon. Arrows in b point to oculomotor axons, whereas the arrowhead indicates a ventromedial neuron close to the midline. For abbreviations , see list. Scale bar in d = 100 μm in a,c,d, 50 μm in b.
Figure 7. Photomicrographs of transverse sections through the brain of Xenopus at stage 57. a: Rostral aspect of the ventromedial telencephalon. b: Caudal portion of the ventrolateral telencephalon. c: Retina. d: Oculomotor nuclei (arrows point to ventromedially located neurons in the ventral commissure). e: Isthmic region. f: Rhombencephalon at the level of the facial motor nucleus (asterisk indicates the weakly labeled cell group located dorsomedially to the facial motor nucleus). g: Rhombencephalon at the level of the facial motor nucleus showing the Mauthner cell. h: Rhombencephalon at the level of the vagal motor nucleus. i: Ventral aspect of the spinal cord showing lateral and medial motoneurons. j: Thoracic spinal cord. For abbreviations , see list. Scale bar in j = 100 μm in a,b,d,f–j, 50 μm in c, 200 μm in e.
Figure 8. Photomicrographs of transverse (a,c,d) and sagittal (b) sections through the brain of Xenopus at stages 64 (a,c,d) and 66 (b). a: Ventralhypothalamus. b: Hypothalamus and midbrain (arrow points to labeled fibers in the median eminence). c: Isthmic region (arrowheads point to isthmotectal fibers). d: Rhombencephalon at the level of the facial motor nucleus (star indicates the weakly labeled cell group located dorsomedially to the facial motor nucleus). For abbreviations , see list. Scale bar in d = 100 μm in a,d, 200 μm in b,c.
