Cell Populations in the Pineal Gland of the Viscacha (Lagostomus Maximus)
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Histol Histopathol (2003) 18: 827-836 Histology and http://www.hh.um.es Histopathology Cellular and Molecular Biology Cell populations in the pineal gland of the viscacha (Lagostomus maximus). Seasonal variations R. Cernuda-Cernuda1, R.S. Piezzi2, S. Domínguez2 and M. Alvarez-Uría1 1Departamento de Morfología y Biología Celular, Universidad de Oviedo, Spain 2Instituto de Histología y Embriología, Universidad Nacional de Cuyo/CONICET, Mendoza, Argentina and 3Cátedra de Histología, Universidad Nacional de San Luis, Argentina Summary. Pineal samples of the viscacha, which were Introduction taken in winter and in summer, were analysed using both light and electron microscopy. The differences found The pineal gland is mainly involved in the between the two seasons were few in number but integration of information about environmental significant. The parenchyma showed two main cell conditions (light, temperature, etc.), and in the populations. Type I cells occupied the largest volume of measurement of photoperiod length (Pévet, 2000). This the pineal and showed the characteristics of typical gland probably signals the enviromental conditions thus pinealocytes. Many processes, some of which were filled making mammals seasonal breeders (Reiter, 1981). The with vesicles, could be seen in intimate contact with the pineal has been thoroughly investigated; however, the neighbouring cells. The presence in the winter samples number of species in which its ultrastructure have been of “synaptic” ribbons and spherules, which were almost studied is a meager 1.5-2% of all mammalians absent in the summer pineals, suggests a seasonal (Bhatnagar, 1992). Previous studies have focused on rhythm. These synaptic-like structures, as well as the domestic and laboratory animals housed in artificially abundant subsurface cisterns present in type I cells, controlled conditions. Thus, the study of wild species, appeared as basic differential features which allowed which are subjected to natural conditions, will provide these cells to be distinguished from type II cells. These new information about seasonal changes in the gland. latter cells, which can be classified as interstitial cells, The plains viscacha (Lagostomus maximus) is a showed some other distinguishing features, such as rodent of considerable size (up to 80 cm in total length) irregular-shaped nuclei, abundant deposits of glycogen- that inhabits grasslands and lowland desert scrubs, and like particles and structures of unknown function also constructs burrow systems in the barren parts of the consisting of concentric cisterns surrounding a dense pampas at an elevation of 2,680 meters. The body. In the summer, interstitial cells displayed geographical distribution is relatively wide: extreme numerous large round bodies, which contributed to southwestern Paraguay, northern and central Argentina, increase the cellular volume slightly. Regarding other and southern Bolivia (Nowak, 1999). constituents, like glial cell processes, vessels of non- L. maximus is predominantly nocturnal, emerging fenestrated endothelium and sympathetic innervation, no from its burrow before dusk to feed. It is a highly social qualitative differences were observed between the two mammal, living in communities of 15-30 and sometimes seasons studied. up to 50 members (Branch, 1993). Given its high We have presented here some morphological cerebral and sensory development and complex social evidences of the circannual rhythm of the viscacha behavior, this species occupies an intermediate position pineal, as well as ultrastructural criteria for between classically studied rodents and superior distinguishing the main cell populations of this organ, mammals, thus constituting a very interesting matter of which could be useful for studies carried out in other study. mammals. Since a preliminary report on the viscacha pineal was done in the past (Domínguez et al., 1987), our main Key words: Pineal, Viscacha, Pinealocyte, Interstitial objectives were to enlarge this description, giving cell, Synaptic ribbon ultrastructural details that would have been missed then, mainly because of their low frequency of appearance, and to define better the cell populations present in this Offprint requests to: Rafael Cernuda-Cernuda, Departamento de organ. Special attention was paid to the seasonal changes Morfología y Biología Celular, Facultad de Medicina (8º planta), of some morphological features of these cell types in Universidad de Oviedo, 33071 Oviedo, Spain. Fax: 34 98 5103618. e- order to find data representative of the presumed mail: [email protected] circannual rhythmicity of the gland. 828 Cell types of the viscacha pineal Materials and methods as type AB according to Vollrath (1981), has got a considerable size, with 12 mm in length, 2.5 mm in A total of 7 adult male viscachas were used in this width, and an average weight of 4.23 mg. The study, 3 of them were captured during the summer and 4 histological features of this gland and the differences during the winter. The animals were killed in their observed between the two seasons analysed can be found habitat during the night by shooting with a rifle with below. telescopic sight. This method avoided stress and prolonged pain to the animal. Special care was taken to Light microscopy aim at the thoracic region, in order to avoid damage of the head, as well as to cause a rapid death and to prevent The analysis of the semithin sections revealed that the animals from getting refuge in their burrows. Within pineals of summer and winter showed a fairly similar a period of 4-5 minutes, the brain was exposed, the appearance (Figs. 1a,b). At least, two types of cells pineal gland dissected “in situ” and fixed in a phosphate could be easily recognized in the pineal parenchyma. buffered (0.1 M, pH 7.3) 3% paraformaldehyde/2.5% Most of the cells possessed a round/oval nucleus with glutaraldehyde solution for 2 hours at room temperature. prominent nucleolus (type I cells), but some others This solution was prepared 1-2 hours before use. The presented an irregular-shaped nucleus (type II cells). The samples were then postfixed in 1% OsO4 at 4 ºC for 2 type II cells had many dense inclusions in their hours, washed is distilled water, dehydrated in a series of cytoplasm in the summer samples (Fig. 1a), whereas acetone of increasing concentration and embedded in such inclusions were very scarce or absent in winter Epon, following a conventional protocol. 2 blocks of (Fig. 1b). Spaces occupied by vessels and connective tissue were sampled per gland. Semithin (1 µm thick) tissue were intensely stained, probably due to the and ultrathin (about 70 nm thick) sections were obtained abundant collagen deposits, and few details could be in a Reichert Jung ULTRACUT ultramicrotome. 4 areas observed. separated by 200 µm were sampled per block. Semithin No statistical differences were found between the sections were stained with toluidine blue solution and parameters measured in both groups, with the exception photographed with a Polaroid DMC digital camera. of the volume density of type II cells, which was slightly Ultrathin sections were mounted on copper grids and increased (p<0.05) in summer (Table 1). stained with lead citrate and uranyl acetate. Stained ultrathin sections were examined and photographed in a Electron microscopy Zeiss EM 109 transmission electron microscope. Type I and type II cells constituted the main Morphometric analysis was carried out at the light components of the parenchyma of this organ (Fig. 2). In microscope level to assess: (1) the volume density of the addition, some glial (Type III) cells could also be main cell types and the space occupied by connective observed. Nerves and blood vessels penetrated from a tissue, nerves and vessels, and (2) the numerical density connective capsule and were distributed throughout the of the main cell types per unit area of section gland. A detailed description of all of these elements can The point-counting method of Weibel (1979) was be found below. employed to calculate the relative volume of the gland components. Five semithin sections (1 µm width) Ultrastructural features of Type I cells separated by at least 100 µm were randomly chosen from each gland. Five fields were randomly selected and Type I cells, pinealocytes, the most abundant cell photographed at a final magnification of x1000, 0.01 type in the pineal parenchyma, were irregular-contoured mm2 being the area of gland tissue examined in each photograph. Measurements were made by placing a transparent screen with a square point lattice spacing 1 cm over each photograph and counting the number of Table 1. Morphometric data of the main pineal components. intersections on each component. The volume fraction of Comparisons between summer and winter samples. a particular component was calculated by dividing the SUMMER WINTER total number of points over that component by the total number of points over the glandular tissue. Volume density (%) The number of cells per unit area of section was Type I cells (pinealocytes) 70±1.7 71.9±1.5 estimated by counting their nuclei in 5 random fields of Type II cells 11.6±1.4 7.2±1.1* 2500 µm2 in each of the 5 sections mentioned above. Vessels and perivascular spaces The data obtained were statistically compared using (nerves, connective tissue, etc) 18.4±1.6 20.9±1.4 the Student t test. Numerical density (No. of nuclei/2500 µm2) Type I cells (pinealocytes) 10.1±0.7 11.5±0.5 Results Type II cells 4.2±0.4 4.0±0.3 The pineal of the viscacha, which can be classified Each value is the mean±S.E.M. *P<0.05. Fig. 1. Semithin sections of pineal taken in summer (a) and in winter (b). The general appearance is fairly similar in both of them. Small black arrows point to cells with irregular-shaped nuclei (type II cells). Note that in the summer pineal (a), these cells possess numerous dense inclusions (white arrows point to two large-sized bodies).