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The Microanatomy of the Palatine Tonsils of the One-Humped Camel (Camelus dromedarius).

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THE ANATOMICAL RECORD 292:1192–1197 (2009)
The Microanatomy of the Palatine
Tonsils of the One-Humped Camel
(Camelus dromedarius)
Department of Histology and Cytology, Faculty of Veterinary Medicine, Alexandria
University, Egypt
Institute of Functional and Applied Anatomy, Hannover Medical School, Germany
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
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.
Received 7 April 2009; Accepted 28 May 2009
DOI 10.1002/ar.20948
Published online in Wiley InterScience (www.interscience.wiley.
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.
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-
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
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
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).
The palatine tonsils of the one humped camel have a
specific anatomical arrangement which is reflected
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
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.
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.
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