Amos G. Gona

Failure of synthetic TRF to elicit metamorphosis in frog tadpoles or red-spotted newts☆

Abstract Synthetic TRF was administered intraperitoneally to bullfrog tadpoles (40 μg per day per animal of 16- to 18 g body weight) and red efts (20 μg per day per animal of 1.0- to 1.2g body weight). No indication of metamorphosis-stimulating effect was elicited in either of these amphibian species.

Effects of prolactin and thyroxine in hypophysectomized and thyroidectomized red efts of the newt Notophthalmus (Diemictylus) viridescens

Abstract In an earlier investigation of the role of hormones on the second metamorphosis of intact newts, we unexpectedly found that small doses of thyroxine (T 4 ) act synergistically with prolactin. The present report presents evidence which seems to establish the existence of such a synergism. Hypophysectomized-thyroidectomized (HX-TX) red efts were injected on alternate days with 0.01 ng T 4 and 0.25 unit of ovine prolactin. This treatment brought about second metamorphorsis, including spontaneous migration of all the animals into water (100% water drive), by day 10. In contrast, only 10% of HX-TX efts treated with prolactin alone showed water drive, and 90% retained their rough skin; all died by day 10. As for the intact animals treated with only prolactin, 30% showed water drive during the same period.

Effects of thyroxine, thyrotropin, prolactin, and growth hormone on the maturation of the frog cerebellum

Abstract The effects of thyroxine (T 4 ), thyrotropin (TSH), prolactin, and growth hormone (GH) on the maturation of cerebellum were studied in premetamorphic tadpoles of the bullfrog, Rana catesbeiana . Intraperitoneal injections of 1 μg T 4 or 0.1 unit TSH, administered on alternate days, induced the morphogenetic changes reported previously in the cerebellum of tadpoles during normal development. In the TSH-injected animals, an external granular layer several cells thick was established, and a massive migration of the external granule cells into the internal granular layer also occurred. Prolactin at a dose of either 10 units or 0.1 unit per injection had no noticeable effect, supporting our view that this hormone does not promote metamorphic changes. Growth hormone at a dose of 0.1 unit seemed to induce cerebellar changes. However, the equivalent of the TSH impurity (0.001 unit) contained in 0.1 unit of GH also gave similar results, suggesting that GH itself may not have any effect on cerebellar maturation in the tadpole.

Ultrastructural studies on cerebellar histogenesis in the frog: The external granular layer and the molecular layer

Maturational changes of the cerebellum of frog tadpoles were studied with the electron miscroscope. In the premetamorphic tadpole, parallel fiber-like processes (PFP) were present in the incipient molecular layer, long before the appearance of the external granular layer (EGL). These PFP showed synaptic contacts with the precociously developed Purkinje cell dendrites. It appears that these PFP may be responsible for inducing the precocious elaboration of the Purkinje cell dendritic arborization. In the metamorphosing tadpoles, the EGL cells migrating into the internal granular layer were frequently seen in close association with the ependymoglial cell processes, which extend from the pia down toward the ependymal surface. This observation lends support to the hypothesis that glial processes guide the migrating EGL cells.

Calbindin Immunoreactivity in Purkinje Cells of the Bullfrog Cerebellum during Thyroxine-Induced Metamorphosis

Calbindin-immunoreactive Purkinje cells were identified in the cerebella of frog tadpoles that had been treated with thyroxine to accelerate metamorphosis. The dorsal part of the cerebellar plate contained the full complement of Purkinje cells which were all CaBP-immunoreactive, while in the ventral part of the cerebellum Purkinje cells acquired CaBP-immunoreactivity only after several days of thyroxine treatment. The ventral group of Purkinje cells was separated from the dorsal group by a distinct gap, which is the site of a shallow sulcus in adult frogs. Additionally, following thyroxine treatment, the numbers of CaBP-immunoreactive Purkinje cells in the ventral group were only half the numbers seen in frogs that metamorphosed spontaneously. We suggest that the variation in the CaBP-immunoreactivity of the dorsal and ventral groups of Purkinje cells, along with the gap in the Purkinje cell layer between the two groups, may be indicative of two distinct populations of Purkinje cells, with distinct patterns of generation, maturation, and perhaps, origin and connectivity, in the cerebellum of frogs.

Local action of thyroxine implants on cerebellar maturation in frog tadpoles

Abstract Implants of a mixture of thyroxine and cholesterol (1 : 10) or of cholesterol alone were placed in the fourth ventricle of bullfrog tadpoles. Fourteen days after surgery, the brains of all tadpoles were processed for light microscopic examination. The cerebella of the control tadpoles, with implants containing only cholesterol, showed no indication of any maturational changes. The cerebella of the tadpoles with thyroxine-containing implants, on the other hand, showed advanced maturational changes, although no external metamorphic changes were exhibited by these animals. It is concluded that thyroxine induces cerebellar maturation by direct action on this part of the brain.

Action of human placental lactogen on second metamorphosis in the newt Notophthalmus viridescens

The effects of human placental lactogen (HPL) on metamorphosis were studied in red efts (land stage) of the newt Notophthalmus viridescens. Three groups received the following injections: (i) low (10 μg) doses of HPL; (ii) high (100 μg) doses of HPL; (iii) low doses of HPL plus 0.01 μg thyroxine (T4). The low dosage level was most effective in inducing migration into water (water drive) and increasing the tail height. The higher dose was least effective in inducing water drive and had no effect on tail height. No synergistic effect was noted with HPL plus T4 treatment. Thus, HPL has a clear-cut prolactin-like effect in that both substances can induce second metamorphosis. However, HPL differs strikingly from prolactin in the failure of high doses to induce water drive and also in the absence of synergism with T4.

Effects of thyroidectomy and season on the external granular layer of the cerebellum in metamorphosing bullfrog tadpoles (Rana catesbeiana)

Transient formation of the cerebellar external granular layer (EGL) occurs during metamorphosis in frog tadpoles and is dependent on thyroid hormone. Late prometamorphic bullfrog tadpoles at similar stages of development were thyroidectomized and then killed after 1 month and 2 months during the fall and winter. The cerebellum was studied by qualitative and quantitative light microscopy. Thyroidectomy resulted in an inhibition of external metamorphic changes such as hind limb growth. Examination of the cerebellum after thyroidectomy showed the EGL greatly decreased in thickness after 1 month and in some cases was altogether absent after 2 months. Because the disappearance of EGL cells was a result of their inward migration into the IGL, we conclude that thyroid hormone was not necessary for granule cell migration. In addition, metamorphic change normally occurs in the late spring and summer in bullfrog tadpoles, and is inhibited during the fall and winter (metamorphic stasis). During the winter, the volume of the EGL was significantly less compared with tadpoles undergoing active metamorphosis. Although lowered temperature appears to contribute to the large decline in EGL thickness, significant seasonal differences in the EGL volume ratio were still observed in tadpoles maintained under conditions of constant temperature and light. Thyroxine administered to thyroidectomized tadpoles during the winter increased the thickness of the EGL. Therefore, the seasonal decline in the EGL thickness reflects a decrease in thyroid hormone activity, which in turn appears controlled by both internal (hypothalamic) and environmental (e.g., temperature) factors.

Ultrastructural studies on the ventricular surface of the frog cerebellum.

SummaryUltrastructural studies of the ventricular surface of the frog cerebellum showed regional differences. In the midline region of the adult cerebellum was found a band of profusely ciliated squamous ependymal cells. In the rest of the cerebellum the ependymal cells were columnar and each had a single cilium. In the cerebellum of the premetamorphic tadpole, the squamous ependymal cells of the midline region also were monociliated. During metamorphosis they gradually became multiciliated. Additionally, supraependymal cells and synaptic elements were present on the ventricular surface of the cerebellum of adult frogs as well as in late metamorphic tadpoles. In contrast, supraependymal cells were rarely observed in premetamorphic tadpoles, and it was concluded that the supraependymal system develops during metamorphosis. It is postulated that the band of cilia may be associated with the circulation of cerebrospinal fluid, and supraependymal synaptic elements function in neuroendocrine regulation.

Cerebellar maturation in the frog tadpole after excision of the margin of the cerebellar plate.

Abstract Autoradiographic studies previously revealed that the ependymal cells along the margin of the cerebellar plate in the frog tadpole gave rise to the external granule cells. In the present study, the cerebellar margin was excised surgically in premetamorphic bullfrog tadpoles, and the latter were induced to metamorphose by treatment with thyroxine. Two weeks after surgery, the brains were processed and examined by light microscopy. Two main effects were observed. First, there was a substantial decrease in the thickness of the external granular layer. Second, the Purkinje cells were distributed in a wider band than seen in sham-operated control animals. There appeared to be an inverse relationship between the thickness of the external granular layer and that of the Purkinje cell layer.