A COMPARATIVE LIGHT AND ELECTRON MICROSCOPIC STUDY OF THE PINEAL COMPLEX IN THREE SPECIES OF SNAKES OF PAKISTAN

Abstract

A light and electron microscopic study was carried out on the pineal complex of 3 species nf snakes, namely common cobra (Naja JI(~j((, Family: Elapidae), common krait (HilI/Rums cuemiells, Family: Elapidae) and saw-scaled viper (Echis carinatus, Family: Viperidae) belonging to Class Reptilia, Order Squamata and Suborder Ophidia (Serpentes). The pineal complex in the three species comprises only the pineal organ proper (epiphysis cerebri ). The organ in these species is remarkably similar to the pineal of mammals. In none of these species there is evidence of a ~tral lumen and open communication with the third ventricle of the brain and hence has solid organization. The stalk, in all three species, is co nnected to the posterior commissural-subcommissural area of the brain by a meningeal bridge. The saw-scaled viper is unusual in showing ectopic pineal tissue. The pineal organ in the cobra and the krait is generally similar in its organization. Both the light microscopic and the electron microscopic details reveal three main cell types in the pineal parenchyma which forms cords and shows follicles only rarely. Electron microscopically these cells are distinguishable as electron lucent, electron dense and of intermediate electron density in both species. The electron lucent and the intermediate cell types are designated as pinealocytes. The electron dense cells have been argued to represent supporting elements of the pineal but the alternative possibility of a single cell type in the cobra has also been discussed. These three cell types show species-dependent differences in the cobra and the krait. There is compelling evidence to suggest that the electron lucent and the intermedi ate pinealocytes represent the same cell type in different physiological states. Dense core vesicles ( 100-250 nm), suggesting a secretory potential, are abundant in these cells in the cobra, especially in their processes which terminate on the perivascular space. Although these vesicles are very scanty in the pinealocytes of the krait, the pineal presents a secretory predi sposition in this species too . The presence of synaptic ribbon-like structures in the cobra pinealocytes and rare cells resembling modified photoreceptors in the krait pineal is indicative of their origin fi'om the primitive photoreceptor-cell line. The supp0I1ing cells in the cobra pineal show a range of variation in both nuclear and cytoplasmic features. Their processes contain numerous vacuoles and show an intimate relationship with the pinealocytes. Some of these cells in the cobra but not in the krait appear to undergo pycnotic changes depicting cellular degeneration. The possibility that the pinealocytes themselves undergo degeneration with the dark cells being a transient phase of this degeneration process rather than representing supportive elements has been ciltet1ained but considered to depict merely loss of the supporting cell s. The supporting cells in the cobra do not seem to isolate the pinealocytes from the perivascular space. These cells in the krait show very different morphology, rarely resembling those in the cobra and hardly show any evidence of degeneration. Also, evidence is provided to show that their processes isolate the pinealocytes from the perivascular space. Second order neurons are absent in the pineal of both species. Unmyelinated and myelinated nerve fibers are abundant in the perivascular space in both species, especially the krait. The former also occur in the pineal parenchyma between the pinealocytes. The pineal in both species is very vascu lar, the endothelium of the capillaries being fenestrated . The pineal of the saw-scaled viper is relatively smaller in size. It is very unique in showing several noteworthy features. It has a single category of parenchymal cells, the pinealocytes. Cells resemblig supporting elements are entirely absent. The pinealocytes are unusual in showing a massive microfibrillar body occupying nearly as much space in the cytoplasm as the nucleus. This body is a swirling mass of microfilaments not reported previously in the pineal of other vetiebrates except the water snake, Natr;x nalr;x where microfilaments abound but with a different dispensation. The pinealocytes in the viper also show an outstanding propensity of mitochondria, clusters of which give the semblance of an ellipsoid characteristically seen in the pinealocytes of anamniotes and lizards. The mitochondria contain conspicuous arrays of dense gran ules of 40-160 nm size in their matrix; a feature not reported earlier in the pineal of any other species. The endoplasmic reticulum is mainly of the smooth type. Lysosomes are also very abundant in most pinealocytes. Dense core vesicles (Vesicle = 100-200 nm, Core = 80- 160 nm) abound in both the perikarya and the processes of the pinealocytes and provide strong evidence of their secretory nature. The pinealocytes show some differences in subcellular features . Those which appear to be in an active secretory phase contain much dilated endoplasmic reticulum and partially to completely empty vesicles and fewer lysosomes. Unlike the cobra and the krait, the pinealocyte also release their secretory granules in the residual lumenal spaces and canaliculi which are more common in the viper than in the elapid snakes. It is argued that the canaliculi may communicate with the perivascular space, though no evidence for this could be gleaned presently. There is no evidence of presence of second order neurons. Also, nerve fibers could not be encountered as readily as in the elapid snakes. Evidence for their presence is provided by rare incidence of an axon which could be detected in the parenchyma next to a pinealocyte. The stromal compartment is rich in blood sinusoids. The endothelium of these vessels is fenestrated . The morphological details of the pineal in the three species are compared with the known information for the ophidian group, other Sauropsida as well as the mammals. The need for additional studies on the more primitive snakes than the species studied presently is st ressed Special attention is drawn toward studies based on age and season to develop a better idea regarding cellular details, cell biological endowments and the role of the pineal in regulating physiology and behaviour of snakes.