Investigations on the development of the myelencephalon in the clawed frog, Xenopus muelleri

The brain stem harbours centres of origin and termination of all the cranial nerves with the exception of the olfactory nerve. Comparative studies on its development in different species is important for understanding both organization and functioning of cell masses and fibre systems. The development of myelencephalon of Xenopus muelleri, a pipid frog that retains a lateral line system throughout life, was studied in Nissl-and Kluver-Barrera stained serial sections. Also normal stages of development of X. muelleri tadpoles from hatching to metamorphosis using external morphological features were determined. The results show that normal stages of development of X. muelleri conform to that of South-African clawed frog, X laevis. An increase in total body length correlates positively with an increase in neuron size in the myelencephalon during larval development. Cell masses were formed in the later half of larval stages (stages 48 to 58). Large neurons migrate first before small neurons and motor nuclei migrate before sensory ones. Myelinogenesis started at the time when the tadpoles entered the phase of metamorphosis with the sensory pathways myelinating before the motor ones. In the adult brain, three distinct sulci, the sulcus medianus inferior, sulcus intermedius ventralis and sulcus limitans of His, could be distinguished. With the aid of cytoarchitectonic criteria about 18 cell groups were delineated; four of these are motor nuclei, seven are sensory centres, five nuclei are considered to be components of the reticular formation, and the remaining two nuclei can be indicated as “relay nuclei”. In order to provide a basis for experimental work, the topographical position of the nuclei is illustrated in photomicrographs of representative levels and in graphical reconstructions. In this study, recommendations for tracing the origin, migration and final destination of the cell masses and myelination of fibres are put forward.