Occurrence of Ciliated Vesicle-Containing Reticular Cells in the Mouse Thymus

Transcription

1 Occurrence of Ciliated Vesicle-Containing Reticular Cells in the Mouse Thymus By Takeshi Hoshino Department of Anatomy, Hokkaido University School of Medicine, Sapporo, Japan (Director : Prof. Takashi Ito) With four figures in the text The thymus has generally been regarded as a lymphoepithelial organ, because its reticular cells, unlike those of other lymphoid tissues, are entodermal epithelial in origin. Especially in the medulla of this organ the reticular cells are considerably characteristic in nature. For instance, the unique structures of the thymus, Hassall's corpuscles, are generally regarded as derived from the reticular cells in the medulla. Peculiar cells known as myoid cells in the thymus of amphibia, reptilia and birds are also accepted as specially differentiated reticular cells (H am ma r, 1909 ; B a r g- m a n n, 1943 ; T e i c h m a n n, 1943). Recently the occurrence of inclusion-containing reticular cells was reported in the thymus of the golden hamster (I t o, 1959). These findings may be interpreted as an indication of speciality of the reticular cells of the thymus. In the course of studying the structure of the mouse thymus, reticular cells containing peculiar ciliated vesicles were observed in the medulla. The occurrence of such cells has been reported in the frog (P e n s a, 1905), reptile (D u s t i n, 1909) and dog (H a m- m a r, 1905). However, since there are no reports on mice, this paper presents some histological and histochemical observations of the corresponding reticular cells in this animal. Material and Methods Thymi of 58 inbred Japanese dd-strain mice of both sexes ranging in age from one to 60 days were used in this study. The organs were fixed in various fluids such as Zenker's, Helly's, Bouin's, Susa's, Ciaccio's fluids, a mixture of alcohol, formalin and glacial 209

2 210 Takeshi Hoshino acetic acid (L i l I i e, 1954) or 10% neutral formalin. After fixation, except for a few formalin-fixed materials, the tissues were dehydrated and embedded in paraffin and then serial sections were-prepared. Most of the sections were stained with hematoxylin-eosin, iron hematoxylin (Heidenhain), azan (Heidenhain), mucicarmine (Mayer) and periodic acid-schiff (PAS) plus Mayer's hemalum or Heidenhain's iron hematoxylin. Additional sections were stained with a variety of staining methods for the following special purposes : 0.1% alcoholic toluidin blue stain for metachromasia, Millon reaction (Bensley and Gersh modification), sudan black B stain for Ciaccio type of lipids, aceton-sudan black B method for bound lipids (Berenbaum : Pears e, 1960) and PAS method after incubation with saliva. Frozen sections of formalin-fixed tissues were stained with sudan III plus hemalum or sudan black B. Observations The thymus of the mouse is divided into cortex and medulla The cortex is a compact lymphoid tissue, the reticulum of which is Fig. 1. A ciliated vesicle-containing reticular cell. Susa ; PAS plus Heidenhain's iron hematoxylin. x Fig. 2. The ciliated vesicle is filled with PAS-positive colloidlike substance. Helly ; PAS plus Mayer's hemalum. x obscured by very closely packed lymphocytes. In the medulla the reticular cells are more prominent because of a lower density of lymphocytes. Among the epithelial reticular cells in the medulla there are found, though in small numbers, peculiar forms characterized by the presence of a ciliated vesicle in the cytoplasm (fig. 1).

3 In hematoxylin eosin-preparations it is sometimes not easy to recognize such cells. In sections stained with azan or iron hematoxylin, however, they can be easily found. The intracytoplasmic vesicles are enclosed in a delicate membrane, through which very fine cilia sprout toward the center of the vesicles. The basal bodies at the base of the cilia are found in an orderly row closely surrounding the membrane of the vesicle. Most of the vesicles are spherical in shape, measuring 5 to 9u in diameter. They are usually situated adjacent to the nucleus in the cytoplasm. The vesicles are often filled with faintly eosinophilic colloid-like substance, which is also stainable with anilin blue or mucicarmine, or by the PAS method (fig. 2). The PAS-raction is positive even Fig. 3. The content of the vesicle stains with sudan black B. Helly ; Aceton-sudan black B (Berenbaum). x Fig. 4. A small 8-shaped cystlike space is formed through fusion of two ciliated vesicles. Susa : PAS plus Heidenhain's iron hematoxylin. x after salivary digestion. The substance displays no metachromatic color with toluidine blue. It stains neither with sudan III nor with sudan black B in both frozen and paraffin sections. However, when stained with Berenbaum's aceton-sudan black B method for bound lipids, the colloid-like content is sometimes stained black (fig. 3). In a limited number of vesicles the content material shows a positive, though inconsistent, Millon reaction in paraffin sections after formalin-fixation. These histochemical results indicate that the content of the vesicles is not composed of simple but of complex constituents. Cytological details of the vesicle-containing reticular cells are essentially the same as those of the common reticular cells of the

4 212 Takeshi Hoshino medulla. The nuclei are usually somewhat eccentrically situated. They are large, oval and vesicular with a distinct nucleolus. The cytoplasm is pale, and homogeneous or very finely granular. In the medulla the vesicle-containing reticular cells occur usually singly among the common cells, but occasionally appear close to Hassall's corpuscles. A limited number of them are found in small groups. When two cells of this type are in close contact ; their vesicles are occasionally fused to form a small 8-shaped cyst-like space (fig. 4). The cells under consideration are constantly encountered in the medulla from birth to 60 days in age, although their occurrence is relatively infrequent. They comprise almost 0.7% of the reticular cells at puberty when the thymus does not yet begin to undergo age involution. Discussion As has already been refered to, occurrence of the intracellular ciliated vesicles in the reticular cells of the thymus has been reported in the frog by P e n s a (1905), in reptile by Dustin (1909) and in dog by Ham ma r (1905). Hammar noted that some of the thymic cysts are formed through fusion of the intracellular vesicles of neighboring cells. The present observations also show, in agreement with Hamm a r's description, that some of the cystic spaces, particularly those of small size, may originate from the ciliated vesicles. As is well known, cysts are of occasional occurrence in the thymus. As to the origin of the thymic cysts particularly in laboratory rodents, detailed work has been made by To gar i and his associates (H i rat a, 1942 ; Kato, 1943 ; Ito, I., 1957 ; Ito, M., 1957 ; T a k a m i, 1958 ; Ka w a s a k i, 1959). According to these studies there are several kinds of thymic cystic spaces which arise from various sources. Here I merely wish to indicate the possibility that ciliated cystic spaces of small size may develop from the vesicles under consideration. However, although a few ciliated vesicles of the reticular cells occasionally come to form cyst-like spaces through fusion, most of them seem to follow the same fate as the common reticular cells in the medulla, because they are intermingled singly among the latter cells, and present no peculiarity except for the occurrence of the vesicles.

5 Occurrence of Ciliated Vesicle-Containing Reticular Cells 213 Summary In the medulla of the mouse thymus, there are found peculiar forms of reticular cells that have a ciliated vesicle in the cytoplasm. Histological and some hitochemical descriptions of these cells are given. The possibility that thymic ciliated cystic spaces of small size may develop from these structure is discussed. References Bar g ma ri n, W. Der Thymus. In : Handbuch der mikroskopischen Anatomie des Menschen, Bd. VI/4. Berlin, Springer (1943). D u s t i n, A. P. Contribution a l'etude du thymus des reptiles. Cellules epitheloides, cellules myoides et corps de Hassall. Arch. de zoolog. exp. et gen. Ser. 5. T. 2. (Cited from Ha m m a r, 1909). Ha m ma r, J. A. Zur Histogenese und Involution der Thymusdrase. Anat. Anz., 27 : (1905). Funfzig Jahre Thymusforschung. Erg. Anat., : (1909). H i r a t a, K. Embryological studies of the thymus of the rat (Japanese). J. Nagoya med. Ass., 55 : (1942). I t o, I. Histogenic studies of the thymus of the rabbit (Japanese). J. Nagoya med. Ass., 73 : (1957). I t o, M. Histology and histogenesis of the thymus of the guinea pig (Japanese). J. Nagoya med. Ass., 73 : (1957). I t 0, T. Uber einschlusshaltige Zellen im Thymus des Goldhamsters. Z. Zellforsch., 49: (1959). K a t o, R. Histological studies of the thymus of the rat (Japanese). J. Nagoya med. Ass., 58 : (1943). Ka w as a k i, I. Histogenic studies of tne thymus of the hamster (Japanese). J. Nagoya med. Ass., 77 : (1959). Lilli e, R. D. Histopathologic Technic and Practical Histochemistry. Blakiston, New York (1954). Pears e, A. G. E. Histochemistry. Theoretical and Applied. J. and A. Churchill, London (1960). Pen s a, A. Osservazioni sulla struttura del Timo. Anat. Anz., 27 : (1905). T a k am i, S. Histogenic studies of the thymus of the mouse (Japanese). J. Nagoya med. Ass., 76: (1958). Teichman n, W. Uber die myoiden Zellen des Thymus. (Untersuchungen am Thymus von Schlangen.) Z. Zellforsch., 32 : (1943).