Haryana Vet. 47 (December, 2008), pp 15-18Research Article

LIGHT MICROSCOPIC STUDIES ON THE PALATINE TONSIL OF SHEEP

PAWANKUMAR1,R. MAHESH, GURDIALSINGH and S. K.NAGPALDepartment of Veterinary Anatomy and Histology, College of Veterinary Sciences

CCS HaryanaAgricultural University, Hisar-125 004

ABSTRACT

The palatine tonsil of sheep showed stratified squamous keratinized epithelium towards outer surface which wasmodified into reticular epithelium without keratinization in crypt. The crypt appeared to be single but it had branchingpattern in the deeper part where the reticular epithelium associated with lymphoid tissue changed into lymphoepitheliumjust comparable to follicle associated epithelium seen in the nasopharyngeal tonsil. The lymphoid tissue was mainly seenin the form of lymphoid follicles along with a few isolated aggregations. The lymphoid follicles having lightly stainedgerminal centre, darkly stained corona and parafollicular area were constituted by lymphocytes, plasma cells, macrophagesand high endothelial venules. The latter were also observed in isolated aggregations of lymphoid tissue in subepithelialpropria submucosa, a feature not reported earlier in tonsils. The mucous glandular acini present in propria submucosashowed strong PAS positive reaction for glycogen, acidic, and weakly suI fated rnuco-polysaccharides.

Key words: Palatine tonsil, crypt epithelium, lymphoepithelium, sheep

Palatine tonsil, a Iymphoepithelial organ,constitutes a component of Waldeyer's ringwhich is an integrated mucosal immune systemof the pharynx (Ogra, 2000). The anatomical

\ topography and presence of crypts in palatinetonsil favors its exposure to exogenous materialincluding microbial pathogens and their transportto lymphoid tissue in horse (Kumar and Timoney,2005). The crypt epithelium along with mantlezone, interfollicular area and germinal center oflymphoid follicle functions as an immune organby production of immunocytes (Kataura et al.,1992). The tonsils as first line of defence againstforeign antigens have been explained by highincidence of infectious diseases in young cattleduring first months of life when the tonsils arenot well developed (Menesse et al., 1998). Thepresent study on the palatine tonsil of sheepexplores its histoarchitecture for betterunderstanding of early pathogenesis of differentrespiratory diseases.

id MATERIALS AND METHODS

Palatine tonsils were collected from headsof6 young sheep (6-9 months age) procured fromlocal slaughter house immediately after

'Corresponding author

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decapitation: The tissues were fixed in 10 percent neutral buffered formalin and processed forlight microscopy. Paraffin sections (5 11) werestained with routine Harris' hematoxylin andeosin stain, Weigert's method for elastic fibres,Gomori 's method for reticulum, Bielschowsky'smethod for axis cylinder and dendrites, Ayoub-Shklar's method for keratin and pre-keratin(Luna, 1968) and Crossman's trichrome stain forcollagen fibres (Crossman, 1937). The sectionswere also stained by McManus' method forglycogen (PAS), Alcian blue method for muco-substances (pH 2.5), PAS-Alcian blue. methodfor mucosubstances (pH 2.5) and diastasedigestion method (Luna, 1968).

RESULTS AND DISCUSSION

The palatine tonsil of sheep was lined bystratified squamous keratinized epitheliumtowards outer surface which was irregular atplaces (Figs 1, 2). Whereas, the deeper surfacewas uneven due to presence of papillary pegswhich were deeply indented reaching almost tothe level of stratum corneum. The epithelium wascomprised of strata basale, spinosum,granulosum and corneum as reported in horseand goat (Kumar and Timoney, 2005, Kumar et

al., 2006). The stratum basale was comprisedof simple columnar cells having elongated darklystained nuclei and finely granular eosinophiliccytoplasm. Several layers constituted the stratumspinosum and its cells shared the histologicalfeatures similar to those of stratum basale exceptthat the cells of its superficial layers had oval toelongated nuclei which were horizontallyoriented. Stratum granulosum constituted byseveral layers of cells possessing narrow,elongated and basophilic nuclei which werehorizontally placed. The nuclei of the superficiallayers were reduced in dimensions. Stratumcorneum was thick having most of its nucleishowing degenerative changes whereas thecytoplasm was more eosinophilic andhomogeneous. Keratin was observed asdemonstrated by special stain.

The stratified squamous keratinizedepithelium became non-keratinized in the cryptsand had strata basale, spinosum and superficiale.This epithelium decreased in height due toreduced number of cell layers and was called asreticular epithelium because of its associationwith lymphoid tissue and lacked distinct strata(Fig 3). At places, the reticular epithelium wasdrastically reduced with only 1-2 cell layers withpredominance of lymphoid cells and thismodification was called as Iymphoepithelium (Fig3) which was considered as an equivalent tofollicle associated epithelium reported innasopharyngeal tonsil (Kumar and Nagpal, 2007).At places the reticular epithelium not associatedwith lymphoid tissue was also observed (Fig 4).This type of epithelium has not been reportedearlier in tonsils of domestic animals. Theinfiltration of lymphoid cells, mainly theIymphocytes, plasma cells and macro phages wasso extensive in reticular and Iymphoepitheliumthat it obscured the presence of epithelial cellsand reached to the free surface of the crypt.The superficial layers of reticular andlymphoepithelia contained vacuolated cells.

The crypt usually appeared as a singleopening towards the free surface but in thedeeper part it was divided and contained varyingnumber of lymphoid cells and cell debris. Thecrypts increased the epithelial surface area incontact with the tonsillar parenchyma and thus,

ingested material might be carried by penetrationof crypt system into proximity with thesubstantially larger volume of lymphoid tissue.These factors indicated a specific functionalrelationship between environment of oral cavityand tonsillar Iympho-epithelial tissue (Koburg1967). The desquamation and loss ofmesenchymal cells into lumen might haveeliminated most of the material taken up by thetonsil and permitted only a sample to reach thelymphoid tissue (Trautmann and Fiebiger, 1952,Williams and Rowland, 1972). The crypt regionof human palatine tonsil was involved in theuptake of antigen and their presentation tolyrnphocytes (Howie, 1980). The lamellatedstructures resembling Pacinian's corpusclesreported in stratified squamous epithelium ofpalatine tonsil of horse (Kumar and Timoney,2005) and goat (Kumar et aI., 2006) were notobserved in present study.

Propria submucosa having loose irregularconnective tissue and large number of small bloodcapillaries was mainly occupied by the lymphoidtissue towards the crypt (Figs 1,2,4) and glandulartissue adjacent to the crypt. The reticular fibresin addition to formation of the basementmembrane were few in number. Theconcentration of collagen and elastic fibres wasmore towards the superficial part of propriasubmucosa which was lacking the lymphoidtissue (Fig 4) and in the portion separating thelymphoid and glandular tissues.

The lymphoid tissue was mainly organizedinto lymphoid foillicles along with isolated smallaggregations without follicle in the subepithelialpropria submucosa (Figs 1, 3). The follicularlymphoid tissue encapsulated by bundles ofcollagen and elastic fibres was separated fromadjacent one by trabeculae like structure whichwas mainly formed by fine collagen and elasticfibres along with few reticular fibres. The folliclesof different size and shape stacked one abovethe other were separated from each other byinterfollicular areas (Fig 1). Most of follicles haddarkly stained corona facing the crypt epithelium,parafollicular area and lightly stained germinalcentre which were constituted by small and largelymphocytes, plasma cells, macrophages, highendothelial venules (REVs) and blood capillaries

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Fig 1. Stratified squamous keratinized epithelium towardsouter surface (0), reticular epithelium (C) along withcrypt and dense arrangement of lymphoid tissue (L)in the propria submucosa of palatine tonsil of sheep.

(H. & E. x 40)

Fig 2. Outer surface epithelium, lymphoid tissue arrangedin follicles (F) and isolated aggregate (I) of palatinetonsil of sheep. (H. & E. x 100)

Fig 3. Palatine tonsil showing modification of reticularepithelium into Iymphoepithelium (E). Note higherconcentration of lymphocytes linearly arranged insubepithelial portion (L). (H. & E. x 100)

Fig 4. Palatine tonsil showing distribution of collagen fibresjust below reticular epithelium in superficial part ofpropria submucosa (blue colour). Note absence offibres in the area of lymphoid tissue (L).

(Crossman trichrome x 100)

Fig 5. Photomicrograph of palatine tonsil showing moredistribution of acidic than neutralmucopolysaccharides in glandular acini (0). Noteabsence of reaction in inter and intraglandularducts (D). (PAS AB x 100)

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along with a meshwork of fine reticular fibers asreported in the horse and goat (Kumar andTimoney 2005, Kumar et aI., 2006). The humanpalatine tonsil was unique among the peripherallymphoid organs due to presence oflarge numberof secondary lymphoid follicles with activegerminal centres (Curran and Jones 1978). Atfew places in the subepithelial propria submucosa,lymphocytes were densely populated andarranged in a linear pattern. The HEV s localizedto parafollicular area contained varying numberoflymphocytes. The HEVs were also observedin the subepithelial propria submucosa havingisolated aggregations of lymphoid tissue, a sitewhich had not been reported earlier. These HEV salso had large number of lymphocytes whichmight be in the process of trafficking throughinter or intra-endothelial migration. The lymphoidtissue extended up to the deeper part of propriasubmucosa and was separated from the adjacentglandular tissue and the tunica muscularis byelastic fibres and few collagen fibres.

The glandular tissue was localized in thedeeper propria submucosa adjacent to lymphoidtissue. However, it was most superficial justbelow the epithelium in the portion wherelymphoid tissue was absent (Fig 5). The acinishowed strong PAS positive reaction forglycogen, acidic mucopolysaccharides andweakly sulphated mucosubstances. Theconcentration of neutral mucopoly-saccharideswas very less (Fig 5). Alcianophilic reaction wasweak in the horse (Kumar and Timoney, 2005)and moderate in goat (Kumar et aI., 2006). ThePAS reaction was moderate after diastasedigestion indicating presence ofmucopolysaccharides other than glycogen. Thecells of intra and inter glandular ducts were devoidof the PAS positive reaction though the PASpositive material was noticed in the lumen ofthese ducts (Fig 5). The mucous glandular aciniwere surrounded by fine reticular, collagen andelastic fibres. The concentration of elastic fibresincreased drastically in the area separating theglandular tissue and fasciculi of striated muscles.The muscle fasciculi of striated muscles wereoriented in different directions and wereseparated by loose connective tissue having large

elastic fibres oriented in different directions. Thistype of arrangement has not been reported inthe tonsils of domestic animals. The immuno-histological features of the palatine tonsil suggestit as part of mucosa associated lymphoid tissueand a targeted organ for the study of earlypathogenesis of different diseases and deliveryof oral vaccines.

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