Jerry J. Kollros

Transitions in the Nervous System during Amphibian Metamorphosis

The tadpole as a whole can be considered to be an aggregate of mosaic territories, each responding in a distinctive way to the increased titers of thyroid hormones (TH) responsible for progressive metamorphic changes. Similarly, the separate elements of the nervous system can be considered, at least as a first approximation, to be composed of a series of mosaic territories. They differ as to the stages when they are first morphologically distinguishable, when they first acquire capacity to respond to TH, in the rates of increase in such sensitivity, i.e., in reduction of threshold concentrations required to elicit detectable change, and in the rates of response to given concentrations. They probably also differ with respect to the hormone concentrations required to elicit maximal rates of tissue response. Some elements of the nervous system grow and differentiate; others shrink, die, and disappear. The extent to which there may be elements of the nervous system that are unresponsive to TH remains largely to be established.

HORMONAL MODIFICATION OF THE DEVELOPMENT OF PLICAL SKIN GLANDS.

Abstract Normal Rana pipiens tadpoles at stages V, VII, and X were immersed in 50 μg per liter of dl-thyroxine solutions, usually for 3, 5, or 7 days. Samples of plical skin were taken 1 day after the end of treatment, and later at the time of forelimb emergence from the opercular cavity (stage XX). In the skin patches taken 1 day after hormone treatment ended, plical gland development was most advanced in the stage X group and least advanced in the stage V group. The differences in skin responsiveness, with respect to gland formation, justify the conclusion that such responsiveness develops gradually, rather than in an all-or-none fashion. Similar gradual onset of responsiveness with respect to regressive changes of labial fringes and cloacal tail piece were suggested. Following the end of hormone treatment, metamorphic changes continued, and in many instances were interrupted by a period of metamorphic stasis. A tendency for an amelioration in the intensity of metamorphic disharmony seen at the end of the period of hormone treatment was noted by the time animals had progressed to stage XX, although numerous examples of continuing disharmony were still seen.

The ultrastructure and development of balancers in Ambystoma embryos with special reference to the basement membrane.

Balancers from embryos and larvae of Ambystoma at various stages of development have been studied by light and electron microscopy with special reference to the basement membrane. The epithelium covering the balancers consists of a superficial and a basal layer of cells. Beneath the latter at stage 34 the basement membrane makes its appearance and is stainable with toluidine blue and the aniline blue component of Mallorys triple. In subsequent stages (34 + to 45 +) a progressive increase in thickness and staining capacity of the basement membrane is observed. Electron micrographs of the basal epithelial cells at stage 34 reveal evaginations originating from their basal surfaces. Most of the cytoplasmic organelles, e.g., mitochondria and endoplasmic reticulum, are found aggregated above the extensions. At a later stage (36 and 36 +) endoplasmic reticulum, pigment and yolk bodies, and a few mitochondria are found in the larger extensions. The surfaces of the evaginations at stages 34 and 34 + are free of formed elements; however, fine filaments are located at the periphery in close contact with the limiting plasma membrane of stages 36 and 36 + larvae. In stages 37 to 45 + no evaginations are observed; the basement membrane has increased in thickness, being constituted mainly of many fine filaments, some of which show preferred orientation close to the basal epidermal cells. A suggestion is made that these filaments may originate from basal evaginations of these deep epidermal cells.

Growth and death of Rohon‐Beard cells in Rana pipiens and Ceratophrys ornata

Rohon‐Beard (R‐B) neurons of the medulla oblongata and spinal cord of anurans originate during gastrulation, become distinguishable just after closure of the neural tube, and are present in maximum numbers at the end of the embryonic period, just before feeding begins. Cell deaths are first seen in the earliest larval stages; in Rana pipiens and Ceratophrys ornata, they may not be complete until the very end of larval development or a day or two later, in the juvenile froglet. This is in sharp contrast with Xenopus laevis, in which the last R‐B cells die well before the onset of metamorphic climax. Cell losses tend to reach completion in the trunk in a craniocaudal progression, that is, first in the medulla oblongata, then sequentially at brachial, postbrachial, and lumbar levels. Nuclei and cells increase in size through embryonic and early larval stages, reaching maxima at stages X–XIV (of 25 larval stages), then shrinking before cell death occurs. While Ceratophrys produces only two‐thirds as many R‐B cells as does R. pipiens, its rate of cell death is slower, gauged by attained stage, and at every stage, X–XXIV, Ceratophrys displays a greater number of surviving cells.

Stage-modified responses to thyroid hormone in anurans.

Abstract Tadpoles from a single clutch of Rana pipiens eggs were cultured at different temperatures in order to achieve at a single time a broad span of developmental stages (I+ through XII) and sizes (20–60 mm). They were placed in dl -thyroxine solutions at concentrations of 50 μg/liter, and maintained at 23°C, until death ensued (approximately 14 days for the largest, and 22 days for the smallest, animals). Initially all animals grew, the larger animals less than the smaller. Tail shortening occurred in all, with latent periods being least in the large and greatest in the small tadpoles. Also, changes in limb characteristics (a measure of metamorphic progress) were much greater in the small than in the large animals, before tail shortening began. After tail shortening began, changes in limb form and rate of tail loss were much alike for tadpoles of all sizes. The differences in latent period correlate inversely with stage and size, and thus compel the conclusion that capacity (i.e., sensitivity) of the tail to respond to thyroid hormone stimulation increases with developmental stage.

Immersion as a method of thyroxine administration in amphibian metamorphosis studies

Abstract Normal and hypophysectomized tadpoles at room temperature (22°–25°C) were supplied dl-thyroxine at concentrations of 0.08–0.8 μg/liter, and hypophysectomized tadpoles in a 15°C bath were supplied the hormone at 0.8–1.6 μg/liter. Each day these tadpoles were supplied with fresh hormone solution, and the older solution was supplied to other hypophysectomized tadpoles initially matched with the first tadpole group as to stage and size. In the 15°C bath the two sets of tadpoles metamorphosed at essentially identical rates. At room temperature the original tadpoles metamorphosed only very slightly more rapidly than did those receiving the “used” hormone solution. This was equally true whether normal or hypophysectomized tadpoles were employed as the original receivers of the hormone. These results demonstrate that tadpoles, under the conditions of the experiment, even though metamorphosing rapidly, utilize extremely little hormone, and that but little hormone is lost by adsorption onto the glass walls of the culture vessels or through chemical alteration in the medium. The method of immersion is therefore satisfactory for providing a relatively unvarying and continuous source of dl-thyroxine.