Scanning electron microscopical investigation of the larval development and the morphological differentiation of the paraventricular organ (PVO) of the South African clawed toad Xenopus laevis Daudin.

The development and the differentiation of the surface of the paraventricular organ (PVO) in tadpoles of the South African Clawed Toad Xenopus laevis Daudin from stages 46 to 66 and one adult animal was studied by means of light microscopy, scanning and transmission electron microscopy. At the light microscopical level, the PVO is visible from stage 46 on. From stage 48 on the organ is divided into two parts, a narrow rostral one and a more flattened and broad caudal one. From stage 58 on the rostral part of the PVO lies in the sulcus organi paraventricularis similar to the caudal part which lies in the sulcus lateralis infundibuli. The position of the PVO on both sides of the third ventricle is unchanged over all stages studied. The growth of the organ is finished at stage 65. The structure of the surface of the rostral and the caudal part of the organ differs up to the stage of 57. From stage 58 on the surfaces of the two parts of the PVO assimilate by developing a network of fibers which is orientated transversally. The different cell types of the PVO which can be seen at the level of transmission electron microscopy are fully developed at stage 57 but their position does not correspond to that of the adult animal. There is no difference between the right and left half of the organ in all groups studied.