The Microanatomy of the Palatine Tonsils of the One-Humped Camel (Camelus dromedarius).код для вставкиСкачать
THE ANATOMICAL RECORD 292:1192–1197 (2009) The Microanatomy of the Palatine Tonsils of the One-Humped Camel (Camelus dromedarius) MOHAMED ZIDAN1 AND REINHARD PABST2* Department of Histology and Cytology, Faculty of Veterinary Medicine, Alexandria University, Egypt 2 Institute of Functional and Applied Anatomy, Hannover Medical School, Germany 1 ABSTRACT Tonsils form a first line of defense against foreign antigens and are also a route of entry and a replication site for some pathogens. The palatine tonsils are the largest of all the tonsils. Despite their general importance, little is known about the microanatomy of the palatine tonsils of the one-humped camel. Palatine tonsils of 10 clinically healthy male camels were obtained directly after slaughtering for human consumption. The tonsils were examined macroscopically and by light, scanning, and transmission electron microscopy. Palatine tonsils had the unique form of several spherical macroscopic nodules protruding into the pharyngeal lumen. These spherical masses were numerous and close together in the lateral oropharyngeal wall, with a few solitary nodules in the dorsal wall. Each nodule had one or two apical openings to crypts, and was enclosed by an incomplete connective tissue capsule and covered apically with stratified squamous keratinized epithelium. The tonsillar crypt was lined with stratified squamous non keratinized epithelium. Several lymphocytes infiltrated the epithelial layer, forming patches of reticular epithelium. Lymphoid follicles with obvious germinal centers extended under the epithelial surface. Diffusely localized lymphocytes were seen in the interfollicular region. High endothelial venules, dendritic cells, macrophages, and plasma cells were observed among these lymphocytes. The unique arrangement of palatine tonsils in separate units with individual crypts results in a very large surface exposed to antigen and indicates a significant immunological role of palatine tonsils in the camel. Anat Rec, C 2009 Wiley-Liss, Inc. 292:1192–1197, 2009. V Key words: palatine tonsils; histology; ultrastructure; camel Tonsils are lymphoepithelial structures present in the mucosae of the oro-, naso-, and laryngopharynx forming the so-called Waldeyer’s ring (Perry and Whyte, 1998). These tonsils are a part of the integrated pharyngeal mucosal immune system that assists in maintaining mucosal immunity (Pabst and Nowara, 1984; Ogra, 2000). They are typical secondary lymphoid organs (Pabst, 2007), which have no afferent lymphatics but are entered by antigens via their surface structures. The location of the tonsils determines their important role as secondary lymphoid tissue in the immunological response to antigens which enter the body by the oral or nasal route (Brandtzaeg, 1984; Bernstein et al., 2005). They form a first line of defense against foreign antigens in the critical region from the mouth to the esophagus C 2009 WILEY-LISS, INC. V and larynx (Caramelli et al., 2003). Tonsils can also form a route of entry and a replication site for some pathogens of which prions causing bovine spongiform encephalopathy (BSE) are of major importance (Jeffrey et al., 2001; Hunter, 2003; Bellworthy et al., 2005). *Correspondence to: Reinhard Pabst, Anatomie II 4120, Hannover Medical School, Carl-Neuberg-Str. 1, 30625 Hannover, Germany. Fax: þ49-511-532-2948. E-mail: email@example.com Received 7 April 2009; Accepted 28 May 2009 DOI 10.1002/ar.20948 Published online in Wiley InterScience (www.interscience.wiley. com). 1193 PALATINE TONSILS IN THE CAMEL The palatine tonsils form a significant part of Waldeyer’s ring. The structure of palatine tonsils has been described in various species, for example, human (Maeda et al., 1978; Howie, 1980; Nave et al., 2001), equine (Kumar and Timoney, 2005), canine (Belz and Heath, 1995a,b), bovine (Manesse et al., 1998; Velinova et al., 2001), ovine (Cocquyt et al., 2005; Casteleyn et al., 2007, 2008), porcine (Williams and Rowland, 1972), and rabbit (Harrison et al., 1970; Olah and Everett, 1975). LieblerTenorio and Pabst (2006) summarized the structure and function of mucosa-associated lymphoid tissue in farm animals, and described the structure and functional importance of the palatine tonsils in these animals. The camel was not included in this study. It has to be stressed that rodents do not have tonsils and part of the tonsillar function might be taken up by the nose-associated lymphoid tissue (NALT) (Pabst, 2007). The palatine tonsils of humans (Maeda et al,. 1978; Howie, 1980) and most animals (Olah and Everett, 1975; Williams and Rowland, 1972) are penetrated by one or many crypts, thus the contact area to antigen is severalfold larger than the covering surface. The palatine tonsils of the dog lack crypts (Banks, 1993). Such information about the palatine tonsils of the camel, however, is completely lacking. Despite the important immunological functions of the palatine tonsils, the only available description of the camel palatine tonsils can be found in the textbook by Smuts and Bezuidenhout (1987), who state that the palatine tonsils of the camel are found in the lateral wall of the oropharynx as a raised area of lymphoid tissue or as diffusely spread lymph nodules. However, there are no detailed anatomical or histological data on the camel palatine tonsils. Therefore, palatine tonsils were studied including the topography and the microscopic structure in the dromedary camel, applying light- and electron microscopic techniques. MATERIALS AND METHODS Specimens The oropharynx and palatine tonsils of 10 clinically healthy male camels (five camels aged 3–5 years and five camels aged 20–25 years) were obtained directly after slaughtering in Koom Hamada slaughterhouse, Behera, Egypt. Palatine tonsils were identified macroscopically, the size measured with a tape measure and dissected for the following studies: Fig. 1. A photograph showing that the palatine tonsils are formed from several spherical macroscopic nodules (arrows) protruding into the oropharyngeal lumen. A few solitary nodules (arrowheads) were observed in the dorsal wall. Each nodule has a macroscopic apical opening or crypt. Bar ¼ 5 mm. mens were examined in a PSEM 500 scanning electron microscope (Philips, Eindhoven, the Netherlands) equipped with a PC-based device for recording digital images at a high resolution (Gebert and Preiss, 1998). Transmission Electron Microscopy Fresh specimens from two young and two old camels, about 1 mm3 in size, were obtained from the tonsils and fixed in 4% phosphate buffered glutaraldehyde (pH 7.4) for 2 hr at 4 C (McDowell and Trump, 1976). After postfixation in 1% solution of phosphate buffered osmium tetroxide at 4 C, specimens were dehydrated in ascending grades of ethanol and embedded in Epon (Serva, Heidelberg, Germany) according to standard protocols. Semithin sections (1 lm) were prepared, stained with toluidine blue, and examined by light microscopy. Suitable areas for electron microscopic examination were selected. From these areas ultrathin sections (50– 70 nm), were cut by a diamond knife and stained with uranyl acetate followed by lead citrate. The sections were observed with a Zeiss EM10 electron microscope (Zeiss, Oberkochen, Germany) at 80 KVs. Light Microscopy Small specimens of the tonsils of all 10 camels were fixed in 10% phosphate buffered formalin and processed for paraffin embedding. Sections (5 lm) were prepared and stained with hematoxilin and eosin, Gomori reticulin and Crossman trichrome stains (see Mills, 2007). Scanning Electron Microscopy Specimens from two young and two old camels were fixed in 10% phosphate buffered formalin, rinsed in phosphate buffered saline (PBS; pH 7.3) and dehydrated in a series of acetone dilutions. Critical-point drying was done using a Balzers CPD 030 (Balzers, Liechtenstein) and sputter-coating using a Polaron E5400 (Polaron, Watford, UK) and a gold-palladium target. The speci- RESULTS Macroscopic examination of the mucosal surface of the oropharynx revealed that the palatine tonsils were formed by several spherical macroscopically visible nodules protruding into the lumen. These spherical masses were numerous and close together in the lateral surface of the oropharyngeal wall, while a few solitary nodules were observed in the dorsal wall. In young camels, the area of the tonsil—without the crypts—on each side was about 24 cm2, with 3–5 nodules per cm2. In the old camels, the area of the tonsils was somewhat larger. Each nodule had a macroscopically visible apical opening or crypt (Fig. 1). Scanning electron microscopic studies showed that some nodules were surrounded by circular mucosal 1194 ZIDAN AND PABST Fig. 3. (a) The surface epithelium (SE) is stratified squamous keratinized epithelium while the crypt epithelium (arrowheads) is stratified squamous non keratinized epithelium. The lymphoid follicles have a well developed germinal center (GC). C ¼ capsule. G ¼ associated glands. H&E. Bar ¼ 200 lm. (b) The photomicrograph shows that the capsule (C) is formed of collagen fibers. Lymphoid follicles with a clear germinal center (GC) surround the crypt (arrow). Crossman’s Trichrome. Bar ¼ 200 lm. (c) This crypt is lined with stratified squamous non keratinized epithelium (E). Groups of lymphocytes (arrows) infiltrate the epithelium. The crypt (CR) is filled with cell debris. H&E. Bar ¼ 25 lm. (d) The parenchyma of palatine tonsils is supported by a reticular fiber network which is diffuse in the germinal centers (GC) and condensed in the corona (CO). I, interfollicular area. Gomori reticulin. Bar ¼ 100 lm. Fig. 2. Scanning electron micrograph of a nodule of a palatine tonsil. (a) The nodule is surrounded with a circular ridge (arrowhead). The mucosal surface of the nodules containing one opening (arrow) leading to a corresponding crypt. Bar ¼ 500 lm. (b) The nodule is surrounded with a circular ridge (arrowhead). The mucosal surface of the nodules is irregular and contains two openings of crypts. Bar ¼ 500 lm. ridges. The luminal surface of the nodules was irregular and contained one or two apical openings leading to a corresponding crypt (Fig. 2a,b). The mucosal surface of each nodule was covered by a keratinized stratified squamous epithelium continuous with that of the buccal mucosa. The abluminal side of the nodule was enclosed by a dense irregular connective tissue capsule. The basic structure of the lamina propria mucosa consisted of irregular connective tissue fibers which continued up to the incomplete capsule and extended under the epithelial surface (Fig. 3a–d). The tonsillar crypt was a nonbranched invagination into the nodule. Each nodule possessed one main crypt. A second smaller crypt was found in some nodules. Each crypt was lined with stratified squamous epithelium, nonkeratinized, and continuous with the keratinized epithelium covering the mucosal surface of the nodule, as seen on the sections of some crypts (Fig. 3b). Patches of the crypt epithelium were frequently infiltrated by lymphocytes forming reticular epithelium. This epithelium was less in layers of cells (Fig. 3c). The debris of cell remnants formed a hyalinized mass in the crypts. Clusters of mucous glandular acini were localized among the nodules but there was no evidence of openings in the crypts (Fig. 3a). The lymphoid parenchyma constituted the majority of tonsillar nodules. It was formed by lymphoid follicles and interfollicular areas (Fig. 3a). The lymphoid follicles encircled the tonsillar crypt and were arranged as one layer of lymphoid follicles extending between the crypt and the surface epithelium or the capsule. Well-developed germinal centers were observed in all lymphoid follicles (Fig. 3a,d). The germinal centers were formed from lymphocytes, lymphoblasts, plasma cells, follicular dendritic cells, and a few macrophages, as identified by TEM (Fig. 4a). Interfollicular regions were rich in lymphocytes, plasma cells, macrophages, and interdigitating cells (Fig. 4b). High endothelial venules supported a meshwork of fine reticular fibers. The high endothelial venules were prominent in the interfollicular area (Fig. 5a–c). Several migrating lymphocytes were observed in the wall of the high endothelial venules. The lining endothelium of the high endothelial venules had a vesiculo-vacuolar organelle in the form of grape-like clusters in the peripheral cytoplasm. Several migrating lymphocytes were observed traversing the vesiculo-vacuolar organelle (Fig. 5d). DISCUSSION The palatine tonsils of the one humped camel have a specific anatomical arrangement which is reflected PALATINE TONSILS IN THE CAMEL 1195 Fig. 4. (a) Electron micrograph of the germinal center showing lymphoblasts (LB), follicular dendritic cells (D), and plasma cells (P). Bar ¼ 1 lm. (b) Electron micrograph of the interfollicular area showing macrophages (M) located among lymphocytes (L), and plasma cells (P). Bar ¼ 1 lm. partially in their histological structure. Each palatine tonsil is uniquely formed from aggregations of separate spherical nodules, which is different from other species (Liebler-Tenorio and Pabst, 2006) and increases the surface area of the tonsils. The palatine tonsils of the one humped camel can be classified as tonsils with crypts, similar to humans, ruminants, horses, and swine but different from carnivores which lack crypts (Banks, 1993). The crypts play an important role in immune responses by specialized cells similar to M cells in the epithelium of Peyer’s patches, as shown for rabbit tonsils (Gebert et al., 1995) and these will be relevant for trapping antigens (Nave et al., 2001). Each nodule contains only one to two crypts which increase the epithelial surface area potentially exposed to antigen (Casteleyn et al., 2008). Also the presence of one or two crypts in each nodule indicates that the camel palatine tonsils possess a large number of crypts bringing all the lymphoid follicles into close contact with reticulated epithelium of the crypts to magnify the immune response of the lymphoid follicles to antigen. Thus, the camel palatine tonsil plays a greater immunological role than tonsils without crypts or tonsils with a limited numbers of crypts as in other species (Slı́pka and Slı́pka, 1996; Casteleyn et al., 2007). An interesting aspect is that there is no evidence of a branching of the individual crypt as is known with primary, secondary, and tertiary human crypts. The tonsillar surface area which is important for the exposure of the immune system to foreign material is directly related to the amount of crypts (Casteleyn et al., 2007). In the pig, it has been shown that during a bacterial infection the lymphocyte subpopulation in the crypt epithelium changes with time after infection, for example, CD4, CD8, MHC-II cells (Salles et al., 2002). The crypt epithelium was formed by stratified squamous epithelium with patches of reticular epithelium. This arrangement may be attributed to antigen stimulation where the less strongly stimulated area may have retained stratified squamous nonkeratinized epithelial tissue (Perry, 1994). The reticular epithelium is a modified epithelium in which lymphoid and other mononuclear cells transform the squamous epithelium into a reticular network of intercellular passageways. This lymphoepithelial barrier samples and translocates antigens to the underlying lymphoid tissue (Perry and Whyte, 1998). Several studies in sheep documented that the reticular epithelium facilitates the uptake of prions responsible Fig. 5. (a) The interfollicular area (I) is rich in high endothelial venules (arrows). LF, lymphoid follicles. H&E. Bar ¼ 50 lm. (b) Higher magnification of (a) showing a longitudinal section in a high endothelial venule (arrow) with several lymphocytes infiltrating its wall. H&E. Bar ¼ 25 lm. (c) The interfollicular area is supported by a reticular fiber network (R). The high endothelial venule (arrow) is encircled by reticular fiber and several migrating lymphocytes distributed in the wall. Gomori reticulin. Bar ¼ 25 lm. (d) Electron micrograph showing the vesiculo-vacuolar organelle (VV) of two adjacent endothelial cells (E) in the wall of a high endothelial venule. Migrating lymphocytes (L) traversing the organelle. Bar ¼ 1 lm. for BSE (Bernstein et al., 2005; Bellworthy et al., 2005). This may be of clinical importance as prions can be detected in tonsils 1 year before the expected onset of clinical prion diseases (Schreuder et al.; 1998; Roels et al., 1999; Thuring et al., 2005). The crypt contains a hyalinized mass of cells. These cells in the crypt lumen appear to be shed from the reticular epithelium (Kumar and Timoney, 2005). These masses of cell debris can block the crypt lumen and enhance the formation of small crypt absesses. The lymphoid tissue constitutes the majority of tonsillar nodules and is organized into lymphoid follicles with clear germinal centers and interfollicular lymphoid tissue obviously separated. This is similar to the palatine tonsils of other species (Perry and Whyte, 1998; Kumar and Timoney, 2005). Follicular dendritic cells extend among the lymphoblasts of the germinal centers. These follicular dendritic cells bind antigen–antibody complexes to their surface for long periods and are essential for the generation of effective humoral antibody responses (Heinen et al., 1995). The clear germinal centers indicate the active role of camel palatine tonsils in the immune response. It has been documented for the pig tonsil by the vincristine arrest technique that the mitotic rate in the follicles is about seven times higher than in the interfollicular area (Pabst and Fritz, 1986). In agreement with Kumar and Timoney (2005), the interfollicular regions were rich in high endothelial venules. Several migrating lymphocytes were observed in their lining epithelium. The high endothelial venules are specialized vessels that support active lymphocyte transmigration from peripheral blood to secondary lymphoid organs depending on molecules on the lymphocytes and corresponding receptors on the endothelial cells (Zidan 1196 ZIDAN AND PABST et al., 2000). It has been documented for humans that T and B lymphocytes leave the palatine tonsils via efferent lymphatics (Zidan et al., 2000). In the current study, it was found that the lining endothelium of the high endothelial venules had a vesiculo-vacuolar organelle in the form of grape-like clusters in the peripheral cytoplasm. Several migrating lymphocytes were observed traversing the vesiculo-vacuolar organelle. This organelle provides a major mode of extravasation of macromolecules at sites of augmented vascular permeability induced by vascular permeability factor/vascular endothelial growth factor and cytokines (Dvorak and Feng, 2001). It remains to be shown whether specific homing receptors direct the entry of specific lymphocyte subsets into the tonsil, as has been argued for high endothelial venules in tonsils of other species (Zidan et al., 2000). Clusters of mucous glandular acini were observed among the nodules of camel palatine tonsils. Kumar and Timoney (2005) described similar glands in equine palatine tonsils. An opening of the glands into the crypts was never seen. In conclusion, the palatine tonsils in the camel may play a major role in immune responses and in the propagation of pharyngeal infections. Abundant epithelial lymphocyte infiltration suggests that these tonsils are perfectly adapted to sample foreign antigens. Immunological processes, both humoral and cellular, are initiated in the different specialized compartments of the palatine tonsils, such as the crypt epithelium, lymphoid follicles, and extrafollicular region (Nave et al., 2001). Further studies, in particular immunohistochemical characterization of lymphocyte subsets and the ultrastructure of reticular crypt epithelium, may be helpful to understand the basic immunological role and clinical importance of the peculiar nodular palatine tonsils of the one humped camel. ACKNOWLEDGEMENTS The excellent technical assistance of K. Westermann, S. Fassbender, and G. Preiss is acknowledged. The authors would also thank M. Peter for preparing the illustrations and S. Fryk for polishing the English text. This article was partly presented as a poster at the Joint Meeting of the Anatomische Gesellschaft and the Nederlandse Anatomen Vereniging, in Antwerpen, Belgium, March 2009. LITERATURE CITED Banks WJ. 1993. Lymphatic system and immunity. In: Applied veterinary histology, 3rd ed. Mosby Yearbook. St. Louis, MO. p 277–297. Bellworthy SJ, Hawkins SAC, Green RB, Blamire I, Dexter G, Dexter I, Lockey R, Jeffrey M, Ryder S, Berthelin-Baker C, Simmons MM. 2005. Tissue distribution of bovine spongiform encephalopathy infectivity in Romney sheep up to the onset of clinical disease after oral challenge. Vet Rec 156:197–202. Belz GT, Heath TJ. 1995a. The epithelium of canine palatine tonsils. Anat Embryol (Berl.) 192:189–194. Belz GT, Heath TJ. 1995b. Intercellular and lymphatic pathways of the canine palatine tonsils. J Anat 187:93–105. Bernstein JM, Baekkevold ES, Brandtzaeg P. 2005. Immunobiology of the tonsils and adenoids. In: Mestecky J, Lamm ME, Strober W et al., editors. Mucosal immunology Vol. 2, 3rd ed. San Diego: Elsevier, Academic Press. p 1547–1572. Brandtzaeg P. 1984. Immune functions of human nasal mucosa and tonsils in health and disease. In: Bienenstock J, editor. Immunology of the lung and upper respiratory tract. New York: McGrawhill. p 28–95. Caramelli M, Acutis P, Bozzetta E, Casalone C, Gagna C, Ru G. 2003. Bovine spongiform encephalopathy in Italian herds. Vet Rec 153:711–712. Casteleyn C, van den Broeck W, Simoens P. 2007. Histological characteristics and stereological volume assessment of the ovine tonsils. Vet Immunol Immunopathol 120:124–135. Casteleyn C, Cornillie P, Simoens P, van den Broeck W. 2008. Stereological assessment of the epithelial surface area of the ovine palatine and pharyngeal tonsils. Anat Histol Embryol 37: 366–368. Cocquyt G, Baten T, Simoens P, van den Broeck W. 2005. Anatomical localisation and histology of the ovine tonsils. Vet Immunol Immunopathol 107:79–86. Dvorak AM, Feng D. 2001. The vesiculo-vacuolar organelle (VVO). A new endothelial cell permeability organelle. J Histochem Cytochem 49:419–432. Gebert A, Willführ B, Pabst R. 1995. The rabbit M cell marker vimentin is present in epithelial cells of the tonsil crypt. Acta Otolaryngol (Stockh) 115:697–700. Gebert A, Preiss G. 1998. A simple method for the acquisition of high-quality digital images from analog scanning electron microscopes. J Microsc 191:297–302. Harrison BH, Patt CI, Albright JT. 1970. The epithelial origin of lymphocytes in the palatine tonsil of rabbits. Tissue Cell 2: 33–45. Heinen E, Bosseloir A, Bouzahzah F. 1995. Follicular dendritic cells: origin and function. Curr Top Microbiol Immunol 201:15–47. Howie AJ. 1980. Scanning and transmission electron microscopy on the epithelium of human palatine tonsils. J Pathol 130:91–98. Hunter N. 2003. Scrapie and experimental SE in sheep. Br Med Bull 66:171–183. Jeffrey M, Martin S, Thomson JR, Dingwall WS, Begara-McGorum I, González L. 2001. Onset and distribution of tissue PrP accumulation in scrapie-affected Suffolk sheep as demonstrated by sequential necropsies and tonsillar biopsies. J Comp Pathol 125: 48–57. Kumar P, Timoney JF. 2005. Histology, immunohistochemistry and ultrastructure of the equine palatine tonsil. Anat Histol Embryol 34:192–198. Liebler-Tenorio EM, Pabst R. 2006. MALT structure and function in farm animals. Vet Res 37:257–280. Maeda S, Mogi G, Oh M. 1978. Microcrypt extensions of tonsillar crypts. Ann Otol Rhinol Laryngol 94:1–8. Manesse M, Delverdier M, Abella-Bourges N, Sautet J, Cabanié P, Schelcher F. 1998. An immunohistochemical study of bovine palatine and pharyngeal tonsils at 21, 60 and 300 days of age. Anat Histol Embryol 27:179–185. McDowell EM, Trump BF. 1976. Histologic fixatives suitable for diagnostic light and electron microscopy. Arch Pathol Lab Med 100:405–415. Mills SE. 2007. Histology for pathologists, 3rd ed. Philadelphia, PA: Lippincott. Nave H, Gebert A, Pabst R. 2001. Morphology and immunology of the human palatine tonsil. Anat Embryol 204:367–373. Ogra PL. 2000. Mucosal immune response in the ear, nose and throat. Pediatr Infect Dis J 19:S4–S8. Oláh I, Everett NB. 1975. Surface epithelium of the rabbit palatine tonsil: scanning and transmission electron microscopic study. J Reticuloendothel Soc 18:53–62. Pabst R. 2007. Plasticity and heterogeneity of lymphoid organs. What are the criteria to call a lymphoid organ primary, secondary or tertiary? Imunol Lett 112:1–8. Pabst R, Fritz FJ. 1986. Comparison of lymphocyte production in lymphoid organs and their compartments using the metaphasearrest technique. Cell Tissue Res 245:423–430. Pabst R, Nowara E. 1984. The emigration of lymphocytes from palatine tonsils after local labelling. Arch Otorhinolaryngol 240:7–13. PALATINE TONSILS IN THE CAMEL Perry ME. 1994. The specialized structure of crypt epithelium in the human palatine tonsil and its functional significance. J Anat 185:111–127. Perry M, Whyte A. 1998. Immunology of the tonsils. Immunol Today 19:414–421. Roels S, Vanopdenbosch E, Langeveld JPM, Schreuder BEC. 1999. Immunohistochemical evaluation of tonsillar tissue for preclinical screening of scrapie based on surveillance in Belgium. Vet Rec 145:524–525. Salles MWS, Perez-Casal J, Willson P, Middleton DM. 2002. Changes in the leucocyte subpopulations of the palatine tonsillar crypt epithelium of pigs in response to Streptococcus suis type 2 infection. Vet Immunol Immunopathol 87:51–63. Schreuder BEC, van Keulen LJM, Vromans MEW, Langeveld JPM, Smits MA. 1998. Tonsillar biopsy and PrPSc detection in the preclinical diagnosis of scrapie. Vet Rec 142:564–568. 1197 Slı́pka J, Slı́pka JJ. 1996. The palatine tonsil as an evolutionary novelty. Acta Otolaryngol (Suppl.)523:8–11. Smuts MMS, Bezuidenhout AJ. 1987. Anatomy of the dromedary. Oxford: Clarendon press. Thuring CMA, van Keulen LJM, Langeveld JPM, Vromans MEW, van Zijderveld FG Sweeney T. 2005. Immunohistochemical distinction between preclinical bovine spongiform encephalopathy and scrapie infection in sheep. J Comp Pathol 132:59–69. Velinova M, Thielen C, Mélot F, Donga J, Eicher S, Heinen E, Antoine N. 2001. New histochemical and ultrastructural observations on normal bovine tonsils. Vet Rec 149:613–617. Williams DM, Rowland AC. 1972. The palatine tonsils of the pig— an afferent route to the lymphoid tissue. J Anat 113:131–137. Zidan M, Jecker P, Pabst R. 2000. Differences in lymphocyte subsets in the wall of high endothelial venules and the lymphatics of human palatine tonsils. Scand J Immunol 51:372–376.