Review of the Anatomical Localization and Histological Characteristics of the Tonsils of Domestic and Laboratory Animals. Department of Morphology, Faculty of Veterinary Medicine, Ghent University, Salisburylaan 1. Merelbeke, Belgium. Copyright В© 2. 01. Christophe Casteleyn et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. This paper gives an overview of the anatomical localization and histological characteristics of the tonsils that are present in ten conventional domestic animal species, including the sheep, goat, ox, pig, horse, dog, cat, rabbit, rat, and pigeon. Anatomical macrographs and histological images of the tonsils are shown. Six tonsils can be present in domestic animals, that is, the lingual, palatine, paraepiglottic, pharyngeal, and tubal tonsils and the tonsil of the soft palate. Only in the sheep and goat, all six tonsils are present. Proper tonsils are absent in the rat, and pigeon. In human anatomy, the thoracic diaphragm, or simply the diaphragm (Ancient Greek: διάφραγμα diáphragma 'partition'), is a sheet of internal skeletal muscle. Auburn University; Auburn University † Notes, Flashcards & More † † The material on this site is created by StudyBlue users. The Tonsils Revisited: Review of the Anatomical Localization and Histological Characteristics of the Tonsils of Domestic and Laboratory Animals. In the rabbit, only the palatine tonsils can be noticed, whereas the pig does not present palatine tonsils. The paraepiglottic tonsils lack in the ox, horse, and dog. In addition, the dog and cat are devoid of the tubal tonsil and the tonsil of the soft palate. Introduction. An animal’s body is connected with the external environment through the skin and mucosae. As a result, these lining tissues are exposed to many hazards, amongst which foreign antigens play a major immunological role. Compared to the skin, the mucosae represent the weaker barrier. To protect the body, a well- equipped defence mechanism is present in the skin and along many mucosal linings: the SALT (skin- associated lymphoid tissue) and MALT (mucosa- associated lymphoid tissue), respectively [1–3]. According to the anatomical and physiological characteristics of the different mucosae, MALT can further be specified into gut- associated lymphoid tissue (GALT), gastric- MALT, nasal- cavity- associated lymphoid tissue (NALT), larynx- associated lymphoid tissue (LALT), trachea- associated lymphoid tissue (TALT), bronchus- associated lymphoid tissue (BALT), conjunctiva- associated lymphoid tissue (CALT), (salivary) duct- associated lymphoid tissue (DALT), lacrimal duct- associated lymphoid tissue (LDALT), and vascular- associated lymphoid tissue (VALT) [4]. Apart from these MALT sites, aggregations of lymphoid cells are present in the lamina propria mucosae of many other tissues such as the genital tract [5, 6].
Although the various components of MALT are anatomically separated from each other, they are functionally connected through the so- called common mucosal immune system [7]. As such, through lymphocyte recirculation, immune responses initiated in one of the inductive sites of MALT can exert immunity at many effector sites, including the genital tract and the mammary and salivary glands [8–1. T- and B- cell activation at one of the MALT sites can thus result in T- cell responses and the secretion of immunoglobulins at distant effector sites [7, 9, 1. The tonsils are major components of the MALT. They consist of aggregations of lymphoid cells that are present in the mucosa of the nasopharynx (NALT), the oropharynx (GALT), and the laryngopharynx (LALT) [1. As a result, all tonsils together form a ring of lymphoid tissue in the pharyngeal wall, called the “Waldeyer ring” [1. Its location at the crossing of the digestive and respiratory tracts plays a key role in immunity as this is the site where vast amounts of foreign antigens enter the body during feeding and breathing [2. Due to their role in immune responses and in the pathogenesis of several diseases including prion diseases, tonsils have gained much scientific interest during the past decade. Many studies have thoroughly described the immunological characteristics of the tonsils of various mammals, and special attention has been paid to the mechanisms of antigen uptake. However, the larger amount of immunological investigations are limited to those tonsils that are most easily identified, such as the palatine tonsils. The study of other tonsils seems to be hampered by the small amount of basic anatomical and histological data present in the literature. Moreover, available data are scattered in conventional anatomical textbooks, and some inconsistencies between several sources can be noticed. Therefore, this paper offers an overview of the anatomical localization and the histological characteristics of the tonsils that are present in ten conventional domestic animal species. Materials and Methods. Heads of sheep (), goat (), cattle (), pig (), horse (), dog (), cat (), rabbit (), rat (), and pigeon () were obtained after euthanasia of animals that had been used in other studies. The localization of the tonsils was demonstrated by anatomical dissection. To visualize lymphoid tissue that is not macroscopically visible on fresh tissues, the heads were fixed in 2% acetic acid for 4 h after which lymphoid tissue colours white [2. Photographs were taken using a digital camera (Canon EOS 3. D, Diegem, Belgium). Subsequently, samples for histological analysis were collected. These were fixed in 3. Microm STP4. 20. D, Prosan, Merelbeke, Belgium), and embedded in paraffin using an embedding station (Microm EC3. Prosan). Tissue sections (8 μm thick) were made (Microm HM3. Prosan) from all blocks, mounted on slides, stained with hematoxylin (Haematoxylin (C. I. 7. 52. 90), Merck KGa. A) and eosin (Eosine yellow (C. I. 4. 53. 80), VWR international bvba/sprl), and examined with a motorized microscope (Olympus BX6. Olympus Belgium) linked to a digital camera (Olympus DP5. Olympus Belgium). Results. 3. 1. Sheep (Figure 1)Figure 1: Anatomical localization and histological characteristics of the ovine tonsils. Median section through an ovine head showing the location of the lingual tonsil (1), the palatine tonsil (2) of which a larger magnification is shown in the lower left insert, the paraepiglottic tonsil (3) that is located at the lateral basis of the epiglottis (lower right insert), the tonsil of the soft palate (4) consisting of scattered lymphoid nodules at the nasopharyngeal side of the soft palate (higher magnification after 2% acetic acid fixation for 4 h shown in the upper right insert), the pharyngeal tonsil (5) of which a larger magnification is presented in the upper left insert, and the tubal tonsil (6) consisting of scattered lymphoid nodules located caudoventral to the opening of the auditory tube (higher magnification after 2% acetic acid fixation for 4 h also shown in the upper right insert). Histological section through the paraepiglottic tonsil. Histological images of the lingual tonsil. The upper view shows aggregations of lymphoid cells (arrows) in between the lingual muscles and salivary glands. The lower image demonstrates the presence of an aggregation of lymphoid cells in the connective tissue core of a vallate papilla. Histological sections through the palatine (d) and tubal (e) tonsils, the tonsil of the soft palate (f), and the pharyngeal tonsil (g), respectively. A tonsillar follicle is encircled in (d); the lymphoid tissue in (e) and (f) is indicated by the arrows. Six tonsils can be observed in the sheep, that is, the lingual tonsil (tonsilla lingualis), the palatine tonsil (tonsilla palatina), the paraepiglottic tonsil (tonsilla paraepiglottica), the pharyngeal tonsil (tonsilla pharyngea), the tubal tonsil (tonsilla tubaria), and the tonsil of the soft palate (tonsilla veli palatini). The oropharyngeal lingual tonsil is not macroscopically visible since it consists of small aggregations of lymphoid cells that are mainly present within the connective tissue cores of the vallate gustatory papillae. These are located at the lateral sides of the root of the tongue. Small lymphoid cell aggregations or scattered lymphoid cells are also located in between the lingual muscles and the salivary glands in the region of the vallate papillae. The lingual tonsil is entirely covered by a keratinized stratified squamous epithelium. The palatine tonsil is bilaterally present in the oropharynx and is located between the palatoglossal and palatopharyngeal arches. It is an ovoid structure, the size of a hazelnut, that contains one to three narrow elongated entrances (fossulae tonsillares) to the underlying crypts (cryptae tonsillares) which have a few diverticula. Each crypt lies in the centre of a tonsillar follicle (folliculus tonsillaris) that contains lymphoid tissue composed of numerous, mainly secondary, lymphoid follicles (lymphonodulus) divided by interfollicular regions. Each tonsillar follicle is surrounded by a capsule of connective tissue. Within the crypts, the overlying nonkeratinized stratified squamous epithelium is irregularly modified into a reticular epithelium due to heavy infiltration by lymphoid cells. The bilaterally present paraepiglottic tonsil is located in the laryngopharynx lateral to the base of the epiglottis. It is macroscopically visible as a few nodular mucosal elevations that are separated by deep invaginations. Dense aggregations of lymphoid cells and primary and secondary lymphoid follicles that are separated by interfollicular regions are present within the mucosal elevations. The lining epithelium is stratified squamous and slightly keratinized. Lymphoid cell infiltration can be present in the epithelium directly overlying secondary lymphoid follicles. The tonsil of the soft palate is located at the dorsal surface (nasopharyngeal side) of the soft palate, but is only macroscopically visible after 4 h fixation in 2% acetic acid, after which white, scattered nodules appear. The rostral part of the tonsil consists of a continuous layer of lymphoid cells separated from the respiratory epithelium by a thin layer of connective tissue. Primary and secondary lymphoid follicles, which are covered by a thin stratified squamous epithelium, are seen in the middle part.
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