Forebrain differentiation and axonogenesis in amphibians: I. Differentiation of the suprachiasmatic nucleus in relation to background adaptation behavior.
Functional forebrain development is the result of a complex series of early developmental processes which include cell division, cellular rearrangements, tissue-tissue interactions, cellular determinative and differentiation events, and axonogenesis. In these studies, Xenopus laevis embryos were examined for early forebrain neuronal determination, differentiation and axonogenesis with special emphasis on the hypothalamic area known to be involved in regulating pars intermedia function. Whole brain acetylcholine esterase (AChE) histochemistry was used to follow the early pattern of forebrain neuronal differentiation, and whole brain acetylated-tubulin immunocytochemistry was done to follow early forebrain axonogenesis. AChE histochemistry indicated that the source of the tract of the postoptic commissure (stpoc) was the first forebrain area to begin differentiation (stage 22). Whole brain immunocytochemistry for acetylated-tubulin indicated that the tpoc is also the first forebrain tract to develop (at stage 25/26). The main forebrain tracts have developed and become interconnected by stage 35/36. The forebrain undergoes a pronounced extension, with much cellular mixing and rearrangement during stages 37/38 to 43/44. This results in bending and contortions in the already developed tracts. Whole brain immunocytochemistry for tyrosine hydroxylase and extirpation of the stage 14 presumptive suprachiasmatic (SC) area indicated that the dopaminergic cells of the SC are determined by stage 14 and initially undergo differentiation between stages 37/38 and 40. Tadpoles with stage 14 presumptive SC extirpated lacked TH-positive tracts to the pars intermedia, lacked most midline TH-positive forebrain cells, and also failed to background adapt to white background. Thus, the SC tracts to the pars intermedia that inhibit melanotrope secretion probably form during the extension stages of 37/38 and contact the pars intermedia by stage 40 when animals are first capable of background adaptation.
Department of Biology, Loras College, Dubuque, Iowa 52004-0178, USA. eaglegw@lcac1.loras.edu
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D000109:Acetylcholine / Q000502:physiology; D000818:Animals; D001369:Axons / Q000648:ultrastructure*; D001931:Brain Mapping; D002454:Cell Differentiation / Q000502:physiology*; D004298:Dopamine / Q000502:physiology; D008547:Melanophores / Q000166:cytology*; D008675:Metamorphosis, Biological; D009415:Nerve Net / Q000166:cytology; D009474:Neurons / Q000166:cytology; D016548:Prosencephalon / Q000166:cytology*; D012880:Skin Pigmentation / Q000502:physiology*; D012931:Social Environment; D013493:Suprachiasmatic Nucleus / Q000166:cytology*; D014446:Tyrosine 3-Monooxygenase / Q000502:physiology; D014982:Xenopus laevis / Q000196:embryology
D014446:Tyrosine 3-Monooxygenase; D000109:Acetylcholine; D004298:Dopamine