CELLULAR SUBSTRUCTURES IN THE OPTIC TECTUM OF ANTARCTIC AND TEMPERATEFISH

The brain of Antarctic fish of the perciform suborder Notothenioidea w as analysed with light- and electronmicroscopical methods. The overall organization and ultrastructure of the optic tectum is very similar t o that of fish from temperate climates. However, unusual structures we re observed in neurons and glial cells, sometimes in high frequencies. The structures are ovoid or elongated, about 200-600 nm in diameter a nd surrounded by two layers of membranes in a uniform distance of abou t 30 nm. The enclosed inter-membrane space is similar to extracellular space, both in size and in cytochemical calcium precipitation, while the interior of the structures resembles cytoplasm. These structures a re sometimes connected to neuronal processes, so that they seem to ori ginate by a sort of budding process, but most of them are isolated as can be concluded from thick sections of up to 800 nm thickness, analys ed with energy-filtering transmission electron microscopy (EFTEM). The se unusual objects are present in high abundance in members of the whi te-blooded Antarctic fish family Channichthyidae. These so-called icef ish lack haemoglobin and exhibit the highest degree of cold adaptation . The red-blooded notothenoid fish had smaller amounts of these struct ures and they were observed even in fish from temperate climates (trou t, carp, cichlid fish). In fish from temperate climates the unusual su bstructures were more abundant during adaptation to cold water tempera tures (winter) than to warm conditions (summer). Therefore, the findin gs may indicate a general phenomenon of cold adaptation with unusual i nteractions of neurons and glial cells, but the precise function is no t yet understood.