Comparative scanning electron microscopy of the ventricular surface in four actinopterygian fishes: Acipenser ruthenus, Salmo gairdneri, Tinca tinca and Blennius sanquinolentus (Pisces).

The ventricular surface structure of the brains of actinopterygian fishes representing four distinct evolutionary levels was investigated by scanning electron microscopy (SEM). In the chondrostean Acipenser ruthenus ventricular spaces are wide and the ependymal surface is for the greater part densely covered with cilia; apart from macrophages supraependymal cells (SE) are very scarce. In the teleosts Salmo gairdneri, Tinca tinca and Blennius sanquinolentus the ventricles are slit-like, the densely ciliated areas decrease in size. The following regions carry a variety of supraependymal (SE) cells and fibres: the rostral recessus supraopticus, the hypothalamic walls, especially the infundibulum and the dorsal walls of the rhombencephalic ventricle. There is no tight correlation between areas devoid of cilia and the circumventricular organs in teleosts. The long evolutionary history, independent of other vertebrate lines has caused a series of peculiarities in the brain of actinopterygian fishes, including a peculiar ventricular topography. Observations indicate that the rich spectrum of SE cells found in teleosts reflects a parallel evolution rather than a common heredity of teleostean fishes and higher vertebrates.