The cattle egret is the most abundant and widespread heron species worldwide, with an estimated global population of 4 to 9.85 million birds. Bubulcus ibis ibis belongs to the order Ciconiiformes and has a varied diet based primarily on small invertebrates; it occasionally also captures and consumes frogs, toads, fish, crayfish, small snakes, small mammals, chicks of other bird species, lizards, and earthworms [1][2].

Reproduction occurs at different times of year depending on local climate, location, and food availability. However, the species does not breed everywhere it is present, particularly in colder regions [1][2].

As a native species, its habitat centers on savannas and wetlands; as an exotic species, it occupies agricultural fields and pastures. At night, the cattle egret roost and sleeps colonially in trees [1][2].

Bubulcus ibis ibis is considered a semi-migratory bird distributed worldwide except in Antarctica. It is regarded as beneficial for its ability to remove most ectoparasites from animals, mainly ticks. It has also been observed feeding on larvae of the house fly (Musca domestica) and the blowfly (Calliphora spp.) in fishing villages in India and at organic waste dumps, which is considered useful given that these insects are carriers of pathogens. The species likewise feeds on insects that attack various crops. Nevertheless, it also has negative impacts on the environment [3]-[5].

According to the National Commission of Natural Protected Areas (CONANP), based on data from CONABIO, there are 348 invasive exotic species in Mexico, 16 of which are birds, including the cattle egret Bubulcus ibis ibis. This causes ecosystem imbalance through displacement of native species or extinction of vulnerable species due to predation, competition, and habitat destruction [6].

An additional concern is that these birds act as vectors of diseases transmissible to animal species and humans, posing a public health risk [6]. Many bird species have been shown to be susceptible to infection by influenza A virus. Regarding botulism, caused by toxins produced by Clostridium botulinum, cattle, birds, horses, sheep, dogs, and humans are affected, with susceptibility depending on the toxin type [7]-[9].

With respect to other bacteria, Bacillus spp., Staphylococcus spp., Listeria spp., Streptococcus spp., and Salmonella spp. have been identified, all of which can cause infections in both animals and humans [10]-[12].

Nematodes identified include Heterakis gallinarum, Echinostoma, Capillaria spp., Toxocara spp., and Ascaridia spp.; trematodes include Renicola spp. and Apharyngostrigea spp.; and protozoa include Eimeria spp. and Toxoplasma gondii. The presence of these parasites therefore represents a risk for poultry and humans [13]-[16].

Fungal genera identified include Mucor spp., Rhizopus spp., Penicillium spp., Paecilomyces spp., Scedosporium spp., Alternaria spp., Microsporum spp., and Trichophyton spp., along with yeasts such as Cryptococcus spp., Rhodotorulaspp., and Scedosporium spp. [4][17].

Based on the above, the cattle egret may constitute a public health threat due to its potential for transmitting pathogens to humans, livestock, poultry, and native wildlife, noting that more detailed studies are required to determine its pathogenic capacity [10][12][13].

The objective of this study is to describe the histological characteristics of the cardiovascular and lymphatic systems of Bubulcus ibis ibis, a species with scarce morphological literature available, to provide morphological data that contribute to the morphophysiological knowledge of the species, which is fundamental for the design of assertive population control strategies of B. ibis ibis, in order to help prevent the risk of transmission of pathogens to other species and humans.

Three adult cattle egret carcasses with no apparent physical injuries were obtained incidentally between January and February 2023 in the surroundings of the University Campus of Tulancingo of the Autonomous University of the State of Hidalgo, located in the Tulancingo de Bravo Valley region, in the state of Hidalgo. The specimens presented morphological characteristics compatible with Bubulcus ibis ibis and were fully identified by specialized photographic comparison and phylogenetic corroboration by PCR.

The animals found were refrigerated at 4˚C immediately after discovery, and the respective necropsy was performed the same day for sample collection, provided that the necropsy study did not reveal significant post-mortem changes. In this way, the three specimens for this study were selected.

From each of the three reference individuals, the organs and structures of their cardiovascular and lymphatic systems were carefully dissected and fixed for at least 24 hours in a 3.7% phosphate-buffered aqueous formaldehyde solution for histological study [18].

Cardiovascular system. Longitudinal and transverse sections of the heart were obtained from each bird. Blood vessel samples included three elastic arteries (aorta, pulmonary trunk and carotid artery) and one muscular artery (femoral). In addition, a jejunal sample was obtained to observe arterioles; the veins collected comprised the vena cava and the femoral vein, and the same jejunal sample was used to observe venules; furthermore, brain (to observe continuous capillaries), kidney (to observe fenestrated capillaries), and liver (to observe sinusoid capillaries) samples were also obtained from each individual.

Lymphatic system. Sample of mesentery was collected from each bird to observe lymphatic vessels and lymphoid tissue associated with this structure. Primary lymphoid organs (thymus and cloacal bursa) were also collected, and secondary lymphoid structures from each bird included spleen and jejunum samples to observe diffuse lymphoid tissue and grouped lymphoid nodules associated with their lamina propria, as well as lung samples to observe solitary lymphoid nodules associated with their parenchyma.

After fixation, all samples were trimmed and placed in histocassettes for processing in an automated tissue processor (histokinette), Microm brand, model STP 1201, for 16 hours, to carry out washing, dehydration, clearing, and infiltration [18].

After processing, all samples were embedded in histological-grade paraffin at its melting point (57˚C) and allowed to solidify in blocks at room temperature for at least 24 hours before sectioning [18].

From each block, sections of 6 µm thickness were obtained using a manual rotary microtome, Leica brand, model RM2125RT, and placed in a tissue flotation bath, Premiere brand, model XH-1001, prepared with water and 3 g of gelatin for extension and adhesion to the slides. Preparations were placed in an oven at 60˚C for 5 minutes and then stored at room temperature for staining [18].

Duplicate slides of the referred organs and structures from each bird were stained with the routine Hematoxylin and Eosin (H-E) stain or the Verhoeff method (V) for elastic fibers were used. The former allowed observation of the morphology of the studied organs and structures, while the latter revealed elastic fibers (stained black) associated with the studied tissue. Once stained, a coverslip was placed over the slide, encapsulating the tissue with synthetic resin, which was left to dry for 24 hours [18].

All histological preparations were examined using an Olympus bright-field compound microscope, model BX41, with 4X, 10X, 40X, and 100X objectives. Representative and selected images of each organ and structure were captured in TIFF format using ImagePro v. 4.0 software (MediaCybernetics) with an Evolution VF camera (MediaCybernetics) on a Vaio PC with 4 GB RAM and a Pentium processor.

The histological results shown here are representative and uniform from the observations of the three specimens studied; if slight variations were observed between individuals in the structures observed, they are noted in the respective sections.

3.2.1. Heart

The heart of Bubulcus ibis ibis presents three well-differentiated layers from inner to outer: endocardium, myocardium, and epicardium. The endocardium consists of an endothelium (simple squamous epithelium) beneath which lies a thin layer of dense irregular collagenous connective tissue, the subendothelium (Figure 1(A)). In some areas, this layer thickens to give rise to a subendocardium in which small, grouped muscle cells are located, corresponding to conducting myocytes (Figure 1(A)). Beneath these structures, elongated muscle cells with some branching constitute the main layer of the heart, the myocardium (Figure 1(B)), which forms the bulk of the organ wall. On the outer surface, a thin layer overlying the cardiac myocytes corresponds to the epicardium, consisting of a mesothelium (simple squamous epithelium) that anatomically constitutes the visceral pericardium (Figure 1(C)), beneath which lies a thin layer of dense irregular collagenous tissue corresponding to the subepicardium (Figure 1(C)).

Figure 1.Heart of Bubulcus ibis ibis. A) Endocardium (a) and subendocardium (b) with conducting myocytes. Endothelium (c), dense irregular collagenous connective tissue forming the subendothelium (d), conducting myocytes (e). Bar: 20 μm. 400X. B) Myocardium in longitudinal section. Cardiac myocytes (a) and erythrocytes (b). Bar: 20 μm. 400X. C) Myocardium (a) and epicardium (b). Cardiac myocytes (c), blood vessel (d), mesothelium (simple squamous epithelium) (e), subepicardium (dense irregular collagenous tissue) (f). Bar: 100 μm. 100X. D) Cardiac valve. Valvular endothelium (a), dense irregular collagenous connective tissue (b), cardiac myocytes (c). Bar: 100 μm. 100X. Stain: H - E.

The cardiac valves consist of a number of leaflets depending on their particular structure. Each leaflet is covered by a valvular endothelium, continuous with the mural endothelium of the cardiac chambers, supported by a layer of dense irregular collagenous connective tissue and a layer of muscular tissue also covered by the endothelium (Figure 1(D)).

3.2.2. Blood Vessels

Arteries:

1. Elastic: aorta and pulmonary trunk

In Bubulcus ibis ibis, the aorta consists of three layers from inner to outer: tunica intima, tunica media, and tunica externa (adventitia) (Figure 2(A) and Figure 2(B)). The tunica intima consists of an endothelium (simple squamous epithelium) beneath which lies a thin layer of dense irregular collagenous connective tissue forming the subendothelium (Figure 2(A) and Figure 2(B)). The tunica media presents a stratum rich in elastic fibers arranged longitudinally relative to the vessel course, with associated collagen fibers; deeper, the characteristic circular arrangement of smooth muscle cells is observed, interspersed with elastic and collagen fibers also arranged circularly (Figure 2(B)). This arrangement continues to the outer surface of the vessel, where a thin collagen layer supports a mesothelium (simple squamous epithelium) that constitutes the tunica externa, in this case a serosa, given the intracoelomic location of the aorta (Figure 2(A)).

The pulmonary trunk shares the same structure as the aorta, consisting of three layers from inner to outer: tunica intima, tunica media, and tunica externa (Figure 2(C) and Figure 2(D)). The tunica intima is formed by an endothelium beneath which lies a thin layer of dense irregular collagenous connective tissue corresponding to the subendothelium (Figure 2(C) and Figure 2(D)). The tunica media is composed predominantly of elastic fibers arranged longitudinally with associated collagen fibers, as well as smooth muscle cells interspersed with elastic and collagen fibers (Figure 2(D)); this organization extends to the vessel surface, where a thin collagen layer supports a mesothelium that constitutes the tunica externa, in this case a serosa (Figure 2(C) and Figure 2(D)).

2.Muscular: femoral artery

The femoral artery of Bubulcus ibis ibis presents the following layers from inner to outer: tunica intima, tunica media, and tunica externa (adventitia) (Figure 2(E) and Figure 2(F)). The tunica intima consists of an endothelium (simple squamous epithelium) beneath which lies a thin collagen layer. The tunica media is formed by smooth myocytes arranged circularly relative to the vessel course (Figure 2(E) and Figure 2(F)). External to this layer, a zone rich in elastic fibers of varying thickness—some thick and several thin—with their characteristic undulating course, together with thick collagen fibers, forms the outer covering (Figure 2(E) and Figure 2(F)). This elastic fiber-rich region corresponds to what is routinely designated in mammals as the external elastic membrane, while the collagen layer constitutes the tunica externa, in this case the tunica adventitia (Figure 2(E) and Figure 2(F)).

Figure 2. Arteries of Bubulcus ibis ibis. A) Aorta. All three tunicae are identified: tunica intima (a), tunica media (b), tunica externa (c). Endothelium (simple squamous epithelium) (d), subendothelium (dense irregular collagenous connective tissue) (e), smooth myocytes (f), elastic fibers (g), thin collagen layer (h), mesothelium (simple squamous epithelium) (i). Bar: 100 μm. 100X. B) Aorta. Tunica intima (a) and tunica media (b). Endothelium (simple squamous epithelium) (c), subendothelium (dense irregular collagenous connective tissue) (d), smooth myocytes (e), elastic fibers (f). Bar: 100 μm. 100X. C) Pulmonary trunk. All three tunicae are identified: tunica intima (a), tunica media (b), tunica externa (c). Endothelium (simple squamous epithelium) (d), subendothelium (dense irregular collagenous connective tissue) (e), smooth myocytes (f), elastic fibers (g), thin collagen layer (h), mesothelium (simple squamous epithelium) (i). Bar: 100 μm. 100X. D) Pulmonary trunk. Tunica intima (a), tunica media (b), tunica externa (c). Endothelium (simple squamous epithelium) (d), subendothelium (dense irregular collagenous connective tissue) (e), smooth myocytes (f), elastic fibers (g), thin collagen layer (h), mesothelium (simple squamous epithelium) (i). Bar: 100 μm. 100X. E) Muscular artery. Tunica intima (a), tunica media (b), external elastic membrane (c), tunica adventitia (d). Endothelium (simple squamous epithelium) (e), thin collagen layer (f), smooth myocytes (g), elastic fibers (h), collagen (i). Bar: 20 μm. 400X. F) Muscular artery. Tunica intima (a), tunica media (b), external elastic membrane (c), tunica adventitia (d). Endothelium (simple squamous epithelium) (e), thin collagen layer (f), smooth myocytes (g), elastic fibers (h), collagen (i). Bar: 20 μm. 400X. G) Arteriole. Tunica intima (a), tunica media (b), tunica adventitia (c). Endothelium (simple squamous epithelium) (d), thin collagen layer (e), smooth myocytes (f), collagen fibers (g). Bar: 20 μm. 400X. H) Arteriole. Tunica intima (a), tunica media (b), tunica adventitia (c). Endothelium (simple squamous epithelium) (d), thin collagen layer (e), smooth myocytes (f), collagen fibers (g), elastic fibers (h). Bar: 20 μm. 400X. Stain—H - E: A, C, E, and G. Stain—Verhoeff: B, D, F, and H.

3.Arteriole: intestine (jejunum)

For the description of an arteriole, blood vessels located in the periphery of the intestine (jejunum) were used as reference. Despite their small size, these vessels also present the three layers described above. The tunica intima is formed by an endothelium beneath which lies a thin collagen layer corresponding to the subendothelium (Figure 2(G) and Figure 2(H)), connecting with the tunica media, which consists of two to three layers of smooth myocytes arranged obliquely relative to the vessel axis (Figure 2(G) and Figure 2(H)); externally, a thin layer of collagen fibers constitutes the tunica adventitia (Figure 2(G) and Figure 2(H)).

3.3.1. Large: Vena Cava

In the vena cava of Bubulcus ibis ibis, three well-differentiated layers are identified: tunica intima, tunica media, and tunica externa, in this case a serosa (Figure 3(A) and Figure 3(B)). From inner to outer, the tunica intima consists of an endothelium beneath which lies a thin collagen layer corresponding to the subendothelium (Figure 3(A) and Figure 3(B)). The tunica media is formed by smooth myocytes arranged circularly relative to the vessel course (Figure 3(A) and Figure 3(B)). External to the tunica media, and greater in thickness, the tunica serosa is formed by thick collagen fibers (Figure 3(A)) and elastic fibers (Figure 3(B)).

3.3.2. Medium: Femoral Vein

The femoral vein of Bubulcus ibis ibis presents the same three tunicae. The tunica intima consists of an endothelium beneath which lies a thin collagen layer forming the subendothelium (Figure 3(C) and Figure 3(D)). The tunica media is formed by two to three layers of smooth myocytes arranged circularly relative to the vessel course (Figure 3(C) and Figure 3(D)). External to this, a layer approximately twice the thickness of the tunica media, formed by thick collagen fibers (Figure 3(C)) and elastic fibers (Figure 3(D)), constitutes the tunica adventitia.

3.3.3. Small: Small Intestine (Jejunum)

Small veins located in the periphery of the small intestine (jejunum) were used for their histological description. The vessel wall comprises the tunica intima, media, and adventitia. The tunica intima consists of an endothelium supported by a thin collagenous subendothelial layer (Figure 3(E) and Figure 3(F)). The tunica media contains two to three layers of smooth myocytes arranged obliquely relative to the vessel axis (Figure 3(E) and Figure 3(F)). The tunica adventitia consists of dispersed collagen fibers (Figure 3(E)) and elastic fibers (Figure 3(F)) and is approximately twice as thick as the tunica media.

Figure 3.Veins of Bubulcus ibis ibis. A) Large vein (vena cava). All three tunicae are identified: tunica intima (a), tunica media (b), tunica externa (serosa) (c). Endothelium (d), subendothelium (thin collagen layer) (e), smooth myocytes (f), thick collagen fibers (g). Bar: 100 μm. 100X. B) Large vein (vena cava). Tunica intima (a), tunica media (b), tunica externa (serosa) (c). Endothelium (d), subendothelium (thin collagen layer) (e), smooth myocytes (f), elastic fibers (g), thick collagen fibers (h). Bar: 100 μm. 100X. C) Medium vein (femoral). All three tunicae with their corresponding structures: tunica intima (a), tunica media (b), tunica adventitia (c). Endothelium (d), subendothelium (thin collagen layer) (e), smooth myocytes (f), thick collagen fibers (g). Bar: 20 μm. 400X. D) Medium vein (femoral). Tunica intima (a), tunica media (b), tunica adventitia (c). Endothelium (d), subendothelium (thin collagen layer) (e), smooth myocytes (f), thick collagen fibers (g), elastic fibers (h). Bar: 20 μm. 400X. E) Small vein (jejunum). All three tunicae with their corresponding structures: tunica intima (a), tunica media (b), tunica adventitia (c). Endothelium (d), subendothelium (thin collagen layer) (e), smooth myocytes (f), collagen fibers (g). Bar: 20 μm. 400X. F) Small vein (jejunum). Tunica intima (a), tunica media (b), tunica adventitia (c). Endothelium (d), subendothelium (thin collagen layer) (e), smooth myocytes (f), collagen fibers (g), elastic fibers (h). Bar: 20 μm. 400X. Stain—H-E: A, C, and E. Stain—Verhoeff: B, D, and F.

3.4.1. Continuous: Brain

Brain tissue was used to identify and describe this capillary type, as it is a primary site of localization. These capillaries present two layers: an endothelial cell layer and a basement membrane (Figure 4(A)). The inner layer consists of endothelial cells with a continuous arrangement, and the basement membrane is identified as the outer layer (Figure 4(A)).

3.4.2. Fenestrated: Kidney

Kidney sections were used for observation of fenestrated capillaries, as the kidney is their most common site of localization. The renal glomerulus, consisting of a tuft of these capillaries, is observed (Figure 4(B)). These capillaries structurally present an endothelial cell layer, a basement membrane, and a layer formed by podocytes (Figure 4(B)).

3.4.3. Sinusoidal: Liver

Liver sections were used for observation of sinusoidal capillaries, as the liver is their primary site of localization. These capillaries are identified by their sinusoidal course, irregular diameter, and the presence of erythrocytes within their lumen (Figure 4(C)). Both layers are observed: the inner endothelial cell layer and an outer thin basement membrane (Figure 4(C)).

Figure 4.Capillaries of Bubulcus ibis ibis. A) Continuous capillary (brain). Endothelial cells (a), basement membrane (b), erythrocyte (c). Bar: 20 μm. 400X. B) Fenestrated capillary (kidney). Renal corpuscle (a), capillary tuft—glomerulus (b), endothelial cells (c), basement membrane (d), erythrocyte (e). Bar: 20 μm. 400X. C) Sinusoidal capillary (liver). Endothelial cells (a), basement membrane (b), erythrocyte (c). Bar: 20 μm. 400X. Stain: H - E.

Primary Lymphoid Organs and Structures

1.Thymus

The thymus of Bubulcus ibis ibis showed consistent involution with minimal variations in the three individuals studied. In general, its structure is composed mainly of adipose tissue, with some areas where it is still possible to observe scattered lymphocytic cells and loose collagenous connective tissue (Figure 5(A) and Figure 5(B)).

2.CloacalBursa

The cloacal bursa of Bubulcus ibis ibis presented similar histological characteristics in the three specimens studied. Externally presents a serous tunic composed of a mesothelium with associated connective tissue (Figure 5(C)). Beneath this lies a capsule composed of smooth muscle and dense irregular collagenous connective tissue, which projects thin trabeculae into the interior of the organ (Figure 5(C)). Between these, well-organized lymphocyte aggregates constitute bursal lymphoid nodules of varying size, each presenting a corona and a germinal center separated by a characteristic capillary network (Figure 5(D)). The cloacal bursa presents a lumen lined by an epithelium varying from pseudostratified columnar to simple cuboidal (Figure 5(D)). Between the epithelium and the capsule, the bursal lymphoid nodules described above collectively form cone-shaped projections directed toward the organ lumen (Figure 5(D)).

Figure 5.Primary lymphoid organs and structures of Bubulcus ibis ibis. A) Thymus. Involuted area showing adipose tissue (a). Bar: 100 μm. 100X. B) Thymus. Residual area with lymphocytic cells (a) and loose collagenous connective tissue (b). Bar: 100 μm. 100X. C) Cloacal bursa. Serous tunic and capsule. Mesothelium (a), smooth muscle (b), dense irregular collagenous connective tissue (c), trabeculae (d). Bar: 100 μm. 100X. D) Cloacal bursa. Cone-shaped projections directed toward organ lumen. Serous tunic (a), capsule (b), trabeculae (c), bursal lymphoid nodules (d), corona (e), capillary network (f), germinal center (g), pseudostratified columnar epithelium (h), simple cuboidal epithelium (i), lumen (j). Bar: 200 μm. 40X. Stain: H-E.

3.6.1. Diffuse Lymphoid Tissue

Intestine (Jejunum):

This type of lymphoid tissue is present in the intestine. Lymphocytic infiltration was present in the intestinal mucosa, specifically beneath the lamina propria (Figure 6(A)). This finding was consistent in the three birds studied, with virtually no variation in terms of the degree of lymphocytic infiltration of the jejunal lamina propria.

3.6.2. Solitary Lymphoid Nodules

Lung:

Solitary lymphoid nodules were found in different areas of the lung parenchyma in the three individuals studied; two representative nodules from one of the samples are shown here: the first with subpleural localization (Figure 6(B)) and the second in the pulmonary parenchyma adjacent to a pulmonary bronchus (Figure 6(C)). Lymphocytic cells are present in both, although the boundary between the germinal center and the corona cannot be precisely determined (Figure 6(B) and Figure 6(C)).

3.6.3. Aggregated Lymphoid Nodules

1.Small Intestine (Jejunum): Intestinal Plaques

Figure 6. Secondary lymphoid organs and structures of Bubulcus ibis ibis. A) Diffuse lymphoid tissue (jejunum). Lymphocytic infiltration (a), lamina propria (b). Bar: 20 μm. 400X. B) Solitary lymphoid nodule (lung, subpleural location). Lymphocytic cells (a), capillaries (b), pleura (c). Bar: 100 μm. 100X. C) Solitary lymphoid nodule (lung, peribronchial location). Lymphocytic cells (a), capillaries (b), bronchus (c). Bar: 100 μm. 100X. D) Aggregated lymphoid nodules (jejunum). Intestinal plaque formed by aggregated lymphoid nodules (a). Lymphoid nodule (b), lymphocytic cells (c). Bar: 100 μm. 100X. E) Spleen. Capsule (a) and parenchyma (b). Trabeculae (c), dense irregular collagenous connective tissue (d), smooth muscle (e). Bar: 100 μm. 100X. F) Spleen. Red and white pulp. Splenic lymphoid nodules (a), germinal center (b), corona (c), lymphocytic cells (d), nodular arteriole (e), splenic cords (f). Bar: 100 μm. 100X. Stain: H - E.

The intestine is one of the sites of localization of this lymph nodule type. Aggregated lymphoid nodules forming intestinal plaques were present in the submucosa of the intestine (Figure 6(D)). Within each nodule, lymphocytic cells are present, although the boundary between the corona and the germinal center cannot be observed with clarity (Figure 6(D)).

2.Spleen

The spleen of Bubulcus ibis ibis presents a thin capsule formed by dense irregular collagenous connective tissue and smooth muscle, with trabeculae of the same composition distributed throughout the parenchyma; both structures form part of the organ stroma (Figure 6(E)). The parenchyma contains red pulp and white pulp. In the white pulp, splenic lymphoid nodules are present; the germinal center formed by lymphocytic cells and their corona, where these cells are also present, along with nodular arterioles (Figure 6(F)). In the red pulp, splenic cords formed by large numbers of erythrocytes and lymphocytes are visible (Figure 6(F)).

3.6.4. Lymphatic Vessels and Associated Lymphoid Tissue

1.Mesentery

Figure 7. Associated lymphoid tissue and lymphatic vessels of Bubulcus ibis ibis. A) Mesentery. Lymphatic vessel. Tunica intima (a), tunica media (b), tunica adventitia (c), associated lymphoid tissue (d). Bar: 100 μm. 100X. B) Mesentery. Lymphoid tissue aggregates are associated with blood vessels. Associated lymphoid tissue (a), blood vessel (b). Bar: 20 μm. 400X. C) Common carotid artery. Structure of undetermined nature (a) adjacent to the blood vessel (b). Serous tunic (c), parenchyma (d). Bar: 200 μm. 40X. D) Common carotid artery. Parenchyma of the associated structure to it. Serous tunic (a), lymphocytic cells (b), capillaries (c), acidophilic material (d). Bar: 100 μm. 100X. Stain: H - E.

In the absence of lymph nodes, lymphocyte aggregates are visible in certain body regions. A section of structures associated with the mesentery reveals a lymphatic vessel presenting a thin tunica intima formed by an endothelium and dense irregular connective tissue, beneath which lie smooth muscle cells forming the tunica media; externally, a thin tunica adventitia of dense irregular collagenous connective tissue is present (Figure 7(A)). Associated lymphoid tissue is in the periphery of the vessel (Figure 7(A)). A lymphocytic tissue aggregate associated with blood vessels is also present (Figure 7(B)).

2.CarotidArtery

Associated with serosa of this blood vessel, found a very interesting structure (Figure 7(C)). Its parenchyma presents a homogeneous appearance of lymphocytic cells and capillaries with erythrocytes; in some areas, these cells form small cavities with acidophilic material (Figure 7(D)).