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. In hypophysectomized Rana pipiens larvae some 7-15% of the peak numbers of R-B cells are still present after 400 days, more than 4 times the length of the usual larval period. Most or all of these surviving cells are in the tail. The extreme persistence of R-B cells in hypophysectomized larvae is consistent with the view that the R-B cell population can be characterized as being divided into those cells whose death occurs relatively early and those in which cell destruction occurs late, presumably dependent upon different factors. The critical factor for onset of cell death in late larvae may well be the surge in thyroid hormone concentration, which characterizes metamorphic climax.