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Comparative Morphology of the Vertebrate Pineal Complex

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Publisher Summary This chapter explores whether there is evidence for the assumption that the mammalian pineal organ is a complex rather than a single organ with a uniform function. It is pointed out that the pineal organ is structurally very complex, especially in rodents. The uneven development of the pineal organ in rodents and the topographical relationships of the pineal organs in other mammalian orders have prompted a classification which is based on the shape, the size, and localization of the pineal organ. A tentative classification of the pineal organs of thoroughly studied species shows that closely related species have similar types of pineals. Based on the different localizations of the pineal organs in relation to the third ventricle, the problem of a release of pineal hormones into the CSF is discussed. It is concluded that comparative morphological studies do not support the notion that secretion directly into the CSF is an important principle. The separation of pineal tissue into superficial and deep pineals in some rodents poses the question as to whether the two parts are structurally and functionally identical. The available data suggest that they are dissimilar. The chapter also discusses the possible subdivision of pineal tissue into cortical and medullary regions. Recent results obtained in rats illustrate the importance of carefully analyzing the pineal parenchyma in order not to miss a perhaps meaningful mosaic-like architecture of the organ. It is concluded that in some species the use of the term pineal complex is justified from a purely morphological point of view.