THE ANATOMICAL RECORD 202:113-116 (1982) Third Ventricle Suprachoroidal Cells MILENA KEMALI Instituto di Cibernetica del CNR, 80072 Arco Felice, Naples, Italy ABSTRACT Using transmission electron microscopy two types of cells on the choroid plexus of the third ventricle of the frog Rana esculenta have been located. They lie on the microvilli and cilia of the choroid epithelium. Their free surface is in contact with the cerebrospinal fluid. One type, Kolmer-like, has large, dense bodies in its cytoplasm. The cytoplasm of the other type is complete ly filled by large, ovoidal structures which have a limiting membrane and a polymorphic filamentous content. Their functional significance is unknown. Supraependymal cells have often been reported in the ventricular system of the vertebrate brain (Coates, 1973; Paull et al., 1974; Mestres and Breipohl, 1976; Bleier, 1977; Card and Mitchell, 1978; Kemali and Miralto, 1979), and various interpretations have been given to their functional role: Supraependymal cells are believed to be macrophages, glial or neuronal elements stemming from ependymal cells, to have osmosensitive or neurosecretory functions, and to be perceptors of the concentration of various chemical compounds of the cerebrospinal fluid (CSF). Cells lying on the choroid epithelium have not been described frequently (Ling, 1979 Baumgarten et al., 1980).Here we report the Occurrence of suprachoroidal (epiplexus) cells in the third ventricle of the frog. choroid epithelium. One is spherical (on the left), the other is ovoidal in shape (on the right). The latter shows several filopodial processes insinuating into the underlying microvilli. At the electron microscope both types show a dark nucleus with large, dark, chromatin masses; they differ, however, in their c y t e plasmic content. The cytoplasm of the spherical type is completely filled by large inclusions (Fig. 2a) which have a polymorphic filamentous content as shown in detail in Figure 2b. The inclusions, some ovoidal and some spherical in shape, seem to have a limiting membrane. Only small portions of their cell cytoplasm is apparent among the packed inclusions, where some clear vesicles are also visible (arrow in Fig. 2b). The surface of the spherical, large inclusion- MATERIALS AND METHODS Three adult frogs of the species Rana esculenta were anesthetized with M S 222 (1:3,000) and were fixed by perfusion for electron microscopy. The fixative used was a mixture of 1% glutaraldehyde and 1% formaldehyde in phosphate buffer at pH 7.4. The dissected choroid plexus was postfixed in 2% osmium tetroxide and underwent block staining by uranyl acetate. Ultrathin sections were counterstained with lead citrate and observed on a Philips 200 electron microscope. RESULTS We have observed two types of cells lying on the microvilli and cilia of the choroid epithelium of the third ventricle. In Figure 1-which is a light microscope picture of a semithin section lp thick -both types are shown lying over the free surface of the 0003-276X/82/2021-0113$01.50 0 1982 Alan R. Liss, Inc. Fig. 1. Light microscopy of a semithin section, 1, lam thick, showing the two types of suprachoroidalcellslying on the microvilli of the choroid plexus. On the f a r left. the spherical, large inclusion-bearing cell, on the right the ovoidal Kolrner-like cell with several elongations. X 130. Received March 2. 1981; accepted May 14,1981 114 M. KEMALI Fig. 2. a. Frog third ventricle suprachoroidal cell showing a dark nucleus and the cytoplasm completely filled by large inclusions. The free surface of the cell is in contact with the CSF. X 7,500.b. Detail of the intracytoplasmatic i n c h sions showing a polymorphic filamentous content. The arrow points to a small portion of the cytoplasm free of inchsions where some clear vesicles are visible. X 45.400.c. Two types of suprachoroidal cells lying in close contact with microvilli and cilia. One type, on the left, has a dark nucleus and the cytoplasm with dark granules (arrows)and a smooth surface facing CSF. The other type, on the right, shows a portion of its large inclusion-bearing cytoplasm and has an angular surface facing CSF. X 16.400.ven=III ventricle. SUPRACHOROIDAL CELLS bearing cell has an angular aspect which reflects the geometry of the packed inclusions. The other type of suprachoroidal cell is ovoidal in shape and contains large dense bodies in its cytoplasm (Fig. 2c-on the left). It is Kolmer-type (Kolmer,1921)and is similar to the monocytetype of Ling (1979). As shown in Figure 2c, where both types of cells occur in the same section, they lie in close contact with the microvillli and cilia of the choroid epithelium on which they seem to be anchored. The surface facing the ventricle is free and in contact with CSF. DISCUSSION 115 The Kolmer-type is considered in the literature to be a phagocytic cell and the other type, the large inclusion-bearing cell might also belong to the same functional type. In fact, Bleier and Albrecht (1980) described supraependymal macrophages of the third ventricle which contain many membrane bound inclusions of various sizes within their cytoplasm and that may constitute a resident macrophage system of the ventricles of the brain. Although the large inclusion-bearing cells have also a somewhat ultrastructural similarity in their cytoplasmic content to basophilic leucocytes and mast cells (see the Cell Fine Structure atlas of Lentz, 1971),we are unable to say if they also share possible functional similarity. The suprachoroidal cells described here are features of the adult frog since they have not been observed in the third ventricle choroid plexus of the tadpole (Kemali and Gioffre, 1981). The functional role of these suprachoroidal cells is unknown and we do not know if it is the same for both types. However, we might suppose that they participate in the functional events which occur at the level of the third ventricle choroid plexus. The choroid plexus plays a role in the production of CSF and in the regulation of its composition. The choroid epithelium secretes the ACKNOWLEDGMENTS constituents of the CSF from blood to ventricle We thank the Stazione Zoologica of Naples and, at the same time, it absorbs a variety of solutes from CSF into the blood with an active for the use of their Philips 200 electron transport mechanism (Wright, 1979). I t has microscope. been shown that the third ventricle choroid plexus plays an important role in maintaining LITERATURE CITED the circadian rhythm of melatonin levels in CSF and blood (Smulders and Wright, 1980). Baumgarten, F. v., H.G. Baumgarten, and H.G. Schlossberger (1980)The disposition of intraventricularly These authors suggest that melatonin reaches injected "C-5. 6-DHT-Melanin in, and possible routes of the CSF directly from the pineal gland, either elimination from the rat CNS. Cell Tissue Res., 212: by diffusion from systemic circulation or by 279-294. local vascular compartment. This hormone, Bleier, R. (1977)Ultrastructure of supraependymal cells and ependyma of hypothalamic third ventricle of mouse. J. through an active transport process located at Comp. Neurol., 174359-376. the brush border surface of the choroid R., and R. Albrecht (1980) Supraependymal epithelium, then returns to the blood where it Bleier, macrophages of third ventricle of hamster: Moris metabolized by the liver and excreted in the phological, functional and histochemical characterization in situ and in culture. 3. Comp. Neruol, 192489-504. urine. The level of melatonin in the CSF might, in Card, J.P., and J.A. Mitchell (1978) Electron microscopic demonstration of a supraependymal cluster of neuronal addition, be regulated by the suprachoroidal cells and processes in the hamster third ventricle. J. cells. In fact, Baumgarten et al. (1980) have Comp. Neurol., 18e43-58. demonstrated that epiplexus cells are able to Coates. P.W. (1973)Supraependymal cells in the recesses of the monkey third ventricle. Am. J. Anat., 136533-539. accumulate synthetic melanin by phagocyKemali. M., and A. Miralto (1979) Light- and electrontosis. microscopic structure of e l l s protruding into the We have observed the large inclusionmesencephalic ventricle of Scyllium stellare (Elasmobranchii, Selachii). Cell Tissue Res., 200.153-157. bearing cell type also in the layer separating the pineal parenchima from the habenular com- Kemali, M., and D. Gioffrb (1981)Development of the third ventricle choroid epithelium of the frog: An electron missure in the frog epiphysis (Kemali, unmicroscopic study. Zool. Jb. Anat., 105r353-361. published observation). However, on a purely Kolmer, W. (1921) Uher eine eigenartige Beziebung von Wanderzellen zu den Choroidealplexus des Gehirns der morphological basis, we cannot say if the Wirbeltiere. Anat. Am. 54:15-19. suprachoroidal cells are involved in melatonin T.L. (1971) Cell Fine Structure. W.B. Saunders Co., regulation. We are also unable to say if both Lentz, Philadelphia. types of epiplexus cells described here play the' Ling. E.A. (1979) Ultrastructure and origin of epiplexus cells in the telencephalic choroid plexus of postnatal rats same role. 116 M. KEMALI studied by intravenous injection of carbon particles. J. Anat., 129:479-492. Mestres, P., and W. Breipohl (1976) Morphology and distribution of supraependymal cells in the third ventricle of the albino rat. Cell Tissue Res.. 168:303-314. Paull, W.K., D.E. Scott, and W.G. Boldosser (1974) A cluster of supraependymal neurons located within the in- fundibular recess of the rat third ventricle. Am. J. Anat., 14@129-133. SmuMers, A.P., and E.M. Wright (1980) Role of choroid plexus in transport of melatonin between blood and brain. Brain Res., 191:555-558. Wright. E.M. (1979) Relations between thechoroidplexuses and the cerebrospinal fluid. Trends Neurosci., 2;13-15.