Vascularization of the pineal complex in the lizard Tiliqua rugosa

Abstract

The vascularization of the pineal complex in the lizard Tiliqua rugosa was investigated by vascular corrosion and latex casting techniques. The fine structure of the pineal capillaries was also studied by transmission electron microscopy. The pineal complex in T. rugosa consists of an elongated pineal gland proper and a separate, distinct parietal eye. The pineal complex derives an abundant blood supply from branches of the middle and posterior cerebral arteries. Scanning electron microscopy of vascular corrosion casts revealed a dense and extensive pineal capillary bed which drains ultimately into a wide longitudinal sinus suggesting an efficient pathway for the rapid removal of substances secreted by the gland. The parietal eye, which receives a unilateral left-sided blood supply from the unpaired anterior pineal artery, is shown to be a highly vascularized structure. The close morphological relationship between the pineal gland and dorsal sac, where the two structures apparently share the same blood vessels, suggests a functional relationship between them. The pineal capillaries are fenestrated with tight junctions between adjoining endothelial cells. Podia-like abluminal extensions of the endothelial cells were observed in close relation to unmyelinated nerve bundles. The basal margin of the pineal parenchyma is highly invaginated with thin finger-like cytoplasmic protrusions into the pericapillary space. Distinct bands of microfibrils form "struts" anchoring the pineal parenchyma to the endothelial wall. These features may have a role in the transfer of materials between the pineal gland and the blood stream.