Morphogenesis of the cerebellum of the frog tadpole during spontaneous metamorphosis

Abstract

The cerebellum of the frog tadpole remains in an immature state throughout the premetamorphic (growth) phase. An external granular layer is absent except for a few scattered cells. A well defined Purkinje cell zone is also lacking. Large cells, which presumably are the immature Purkinje cells, are found in the subventricular zone.During the prometamorphic period (accelerated hind leg growth) an external granular layer, several cells thick, becomes established by a migration of cells from the base of the cerebellum. A second migration involving the movement of the external granule cells toward the ventricular region begins at this time. However, the most active period of this latter migration is at the end of the prometamorphic period and during the days following fore leg emergence, viz., metamorphic climax. Concomitantly, a established.Amphibian metamorphosis, in general, entails sweeping morphological and biochemical changes and has, therefore, attracted a great deal of experimental investigation. However, except for a few studies such as those on the neurons of the mesencephalic V nucleus (Kollros and McMurray, '55) and on Mauthner's cells (Moulton, Jurand and Fox, '68), metamorphic changes in the brain have received little attention. This paucity of interest in the central nervous system of the metamorphosing tadpole is not difficult to explain. Once the embryonic development is complete, the tadpole has a brain which is essentially identical to that of the adult. Metamorphosis, therefore, does not bring about any dramatic changes which are readily noticeable in the brain.In the course of exploratory studies on the histological changes in the tadpole brain, our attention was drawn by the abrupt changes in the cerebellum during metamorphosis. Ramdn y Cajal ('11) had observed many years ago that the cerebellum of the frog tadpole had immature Purkinje cells with poorly developed dendritic processes. Larsell ('25) later reported that the tadpole cerebellum did not acquire a well defined Purkinje cell layer J. COMP. NEUR., 146: 133-142. well defined Purkinje cell stratum is until late in larval life. However, no further studies seem to have been pursued on this or other aspects of the morphological changes in the cerebellum during metamorphosis. It is the purpose of this report to describe light microscopic observations on the cerebellar changes during normal metamorphosis. MATERIALS AND METHODSTadpoles and postmetamorphic froglets of the bullfrog, Rana cntesbeiana, and of the leopard frog, R. pipiens, were used.Those of the bullfrog were collected from a pond in Pcekskill, New York and those of R. pipiens were raised from eggs obtained by induced ovulation (Rugh, '34). About 80 animals of each species were sorted into groups of different stages ranging from young tadpoles (Taylor and Krollros ('46) stage VII in the case of R. pipiens) to postmetamorphic froglets of up to ten weeks after the emergence of fore legs.In some cases the upper jaw was cut out and fixed in Bouin's solution for three days. The b...

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