Hypothalamic neurosecretion and metamorphosis inXenopus laevis: IV. The effect of extirpation of the presumed TRF cells and of a subsequent PTU treatment

Temporal pattern of appearance and distribution of cholecystokinin-like peptides during development in Xenopus laevis

Cholecystokinin (CCK) radioimmunoassay and immunocytochemistry were used to localize the expression of CCK-like peptides in the gut, brain, and skin of Xenopus laevis during development from fertilization through metamorphosis to the adult form. CCK-like peptides were first detected in the gut shortly before food was first observed in the lumen of the intestine. This CCK-like immunoreactivity (CCK-li) was localized in endocrine cells in the duodenal mucosa. Gut CCK-li decreased during metamorphosis coincident with gut histolysis. After metamorphosis, gut CCK-li appeared in the newly organized gastric antrum at Nieuwkoop-Faber stage 62-63 and reappeared in the reconstituted duodenum at stage 63-64. In both cases, CCK-li was localized in endocrine cells. In both the gastric antrum and the duodenum, the first appearance of CCK-li preceded the functional activity of its target tissues. CCK-li increased in the brain during brain primary differentiation and again during metamorphic climax coincidentally with the differentiation of the hypothalamo-hypophysial system. Brain CCK-li then declined during the transition from larva to adult. We conclude that the first presence of food in the lumen of the larval gut does not induce the appearance of CCK-li and that CCK may play a role in regulating the development of the gastrointestinal tract and brain in X. laevis.

Forebrain development in control and hypothyroid larvae of Triturus cristatus carnifex

Some cytological and developmental aspects of the forebrain gray matter were studied in the control and the hypothyroid larvae of Triturus cristatus carnifex. The mitotic cell percentages in the periventricular germinal layer, the fusiform shape of the recently divided neuronal cells and the transitory gray matter cleavage, are characteristic of the immature status of the central nervous system (CNS) in this species. The hypothyroidism obtained during two months of thiourea treatment slows down the development in all the forebrain areas of the experimental larvae. Nevertheless, the nucleus olfactorius anterior and the corpus striatum, which have a longer mitotic activity with respect to the other neuronal structures, are more deeply affected by thyroid atrophy. Thyroid activity and forebrain maturation are discussed on the basis of the literature and our data with regard to their influence on behavior and evolution.

Thyroxine-induced activation of hypothalamo-hypophysial axis in neotenic salamander larvae

Thyroxine injected into the hypothalamus of neotenic Ambystoma tigrinum induces metamorphosis by activating hypothalamo-hypophysial stimulatory control of thyroid activity, thereby removing the hypothalamic block to metamorphosis.