The Pineal of some Chondrichthyes

Abstract

The pineal organs of the two Chondrichthyan species, Scyliorhinus canicula and Raja clavata, were examined by light and electron microscopy. The Chondrichthyan pineal was found to be a blind ended outgrowth originating from the roof of the brain above the third ventricle. The end vesicle occupies a superficial position applied to the underside of the skull roof. The organ was shown to possess the structural characteristics of a photoreceptor containing small numbers of cone-like cells. The latter, however, were always degenerate and did not appear to have any connection with the pineal tract, since no distinct synaptic structures could be discerned. Two types of supporting cell were observed which were classified as "translucent’ or ‘opaque’ depending upon the appearance of the cytoplasm. These cells were sometimes seen to contain dense core granules of unknown composition. The pineal tract was shown to consist mainly of non-myelinated fibres together with a very few myelinated axons. Melatonin could not be isolated from the pineal of Scyliorhinus, although it has been reported in other species of fish (Fenwick, 1970). The effect of continuous light and dark regimes on the pineals of immature specimens was examined. It was found that in the absence of light, proliferation and vacuolation of mitochondria occurred within the various pineal cells. Continuous illumination produced no such effect. The possible functional significance of the Chondrichthyan pineal structure was discussed with reference to other vertebrate species. The structure of the photoreceptor cell was related to the cell-line theory of Collin (1971).