Pathways of lymph flow through superficial inguinal lymph nodes in the pig

The Internal Conduit System of the Swine Inverted Lymph Node

Lymph nodes (LN) are the crossroad where naïve lymphocytes, peripheral antigens and antigen presenting cells contact together in order to mount an adaptive immune response. For this purpose, LN are highly organized convergent hubs of blood and lymphatic vessels that, in the case of B lymphocytes, lead to the B cell follicles. Herein take place the selection and maturation of B cell clones producing high affinity antibodies directed against various antigens. Whereas the knowledge on the murine and human LN distribution systems have reached an exquisite precision those last years, the organization of the antigens and cells circulation into the inverted porcine LN remains poorly described. Using up to date microscopy tools, we described the complex interconnections between afferent lymphatics and blood vessels, perifollicular macrophages, follicular B cells and efferent blood vessels. We observed that afferent lymphatic sinuses presented an asymmetric Lyve-1 expression similar to the one observed in murine LN, whereas specialized perifollicular sinuses connect the main afferent lymphatic sinus to the B cell follicles. Finally, whereas it was long though that mature B cells egress from the inverted LN in the T cell zone through HEV, our observations are in agreement with mature B cells accessing the efferent blood circulation in the efferent, subcapsular area. This understanding of the inverted porcine LN circuitry will allow a more accurate exploration of swine pathogens interactions with the immune cells inside the LN structures. Moreover, the mix between similarities and differences of porcine inverted LN circuitry with mouse and human normal LN shall enable to better apprehend the functions and malfunctions of normal LN from a new perspective.

Research Relevant Background Lesions and Conditions: Ferrets, Dogs, Swine, Sheep, and Goats

Animal models provide a valuable tool and resource for biomedical researchers as they investigate biological processes, disease pathogenesis, novel therapies, and toxicologic studies. Interpretation of animal model data requires knowledge not only of the processes/diseases being studied but also awareness of spontaneous conditions and background lesions in the model that can influence or even confound the study results. Species, breed/stock, sex, age, anatomy, physiology, diseases (noninfectious and infectious), and neoplastic processes are model features that can impact the results as well as study interpretation. Here, we review these features in several common laboratory animal species, including ferret, dog (beagle), pig, sheep, and goats.

An animal model to train Lichtenstein inguinal hernia repair

PURPOSE

Inguinal hernia repair is a common surgical procedure, and the majority of operations worldwide are performed ad modum Lichtenstein (open tension-free mesh repair). Until now, no suitable surgical training model has been available for this procedure. We propose an experimental surgical training model for Lichtenstein's procedure on the male and female pig.

METHODS

In the pig, an incision is made 1 cm cranially to the inguinal sulcus where a string of subcutaneous lymph nodes is located and extends toward the pubic tubercle. The spermatic cord is located in a narrow sulcus in the pig, thus complicating the procedure if operation should be done in the inguinal canal. The chain of lymph nodes resembles the human spermatic cord and can be used to perform Lichtenstein's hernia repair.

RESULTS

This experimental surgical model has been tested on two adult male pigs and three adult female pigs, and a total of 55 surgeons have been educated to perform Lichtenstein's hernia repair in these animals.

CONCLUSIONS

This new experimental surgical model for training Lichtenstein's hernia repair mimics the human inguinal anatomy enough to make it suitable as a training model. The operation facilitates the training in the positioning and fixation of the mesh and can be performed numerous times on the same pig. It is therefore a useful training method for inexperienced surgeons to obtain experience in aspects of the Lichtenstein procedure.

Anatomical particularities of the porcine immune system--a physician's view

In this article the anatomical structure of the porcine immune organs is described. The focus is on their particularities that are related to the use of pigs as an animal model. Key issues of the intrauterine development of the lymphoid organs are presented, such as the specific epithelio-chorial placenta, the appearance of the thymic tissue and the initial development of B cells. The role of the thymus for the development of alpha/beta and gamma/delta T cells and the location of tonsillar tissue in the naso-pharynx, in the oral cavity and at the basis of the tongue are described. The porcine spleen is of interest for surgical techniques to treat splenic trauma adequately. The observation of the inverted lymph node structure of pigs is puzzling and it remains unclear why only few species have this distinct morphological organisation. Based on the functional differences in lymphocyte recirculation observed in pigs, specific lymph cannulation experiments are possible in the porcine immune system. The porcine intestinal lymphoid tissue and the lymphocytes in the mucosal epithelium and lamina propria are of interest for studying the gut immune responses. For use as a model the fact that the pig is a monogastric omnivorous animal represents an advantage, although the porcine ileal Peyer's patch has no obvious anatomical equivalent in man. Based on the detailed knowledge of porcine immune morphology the pig is suitable as model animal for immunology--in addition to the various experimental approaches in physiology, pharmacology, surgery, etc. that are applicable to human medicine.

Morphology of the lymph nodes in bottlenose dolphin (Tursiops truncatus) and striped dolphin (Stenella coeruleoalba) from the Adriatic Sea

Morphology of the lymph nodes was examined in six bottlenose dolphins (Tursiops truncatus) and three striped dolphins (Stenella coeruleoalba) from the Adriatic Sea. All animals had been found dead in nature. One group of the nodes was taken from the tracheal branching area and was marked as bifurcational lymph node, and the other group was taken from the mesenteric root and was marked as mesenteric lymph node. Microscopic analysis showed that the lymph nodes in both dolphin species were surrounded by a connective tissue capsule comprising smooth muscle cells. The parenchyma of the mesenteric and bifurcational lymph nodes in bottlenose dolphin was divided into the peripherally situated cortex with the lymphatic nodules and diffuse lymphatic tissue, and the centrally situated medulla structured of the medullary cords separated by the medullary sinuses. These lymph nodes structurally correspond to the lymph nodes in the majority of terrestrial mammals. The mesenteric lymph node of striped dolphin also had a peripherally situated cortex and a centrally positioned medulla as the majority of terrestrial mammals. In the bifurcational lymph nodes of striped dolphin, there was a central dense lymphatic tissue with the lymphatic nodules and a peripheral less dense lymphatic tissue structured of the cell cords and sinuses. The bifurcational lymph node in striped dolphin resembled porcine lymph nodes and belonged to the inverse lymph nodes.

Technique for performing direct lymphangiography in pigs

RATIONALE AND OBJECTIVES

The authors developed a porcine model for direct lymphangiography to be used as a reference for lymph node harvesting.

METHODS

A pilot animal was studied first to develop the protocol, which was used successfully in 24 pigs. An indicator dye was first injected into each foot to make the lymphatic vessels visible. Then, ethiodized poppy seed oil was injected directly through cutdowns in both groins. Radiographs were obtained before and after lymph node harvesting.

RESULTS

Images were of diagnostic quality in all animals. The pilot animal developed transient respiratory distress, which was thought to be due to the oil-based contrast material. The amount injected was reduced by half, and no episodes occurred thereafter in the study group. There were no other immediate or delayed complications.

CONCLUSION

Direct lymphangiography is safe, easy, and reliable in a porcine model.

Ileal Peyer's patches in pigs: intercellular and lymphatic pathways

BACKGROUND

The lymphatics of Peyer's patches disseminate immunological information from the gut and thus play a key role in protection of the body against environmental pathogens. The aim of this project was to describe the lymphatic pathways of these Peyer's patches in pigs, and the mucosal intercellular spaces which lead to these lymphatics.

METHODS

Ileal tissue from living or freshly killed pigs was examined by light microscopy or electron microscopy, or was injected with Mercox (CL-2B, Japan Vilene Hospital, Tokyo) for scanning electron microscopy of corrosion casts.

RESULTS

Intercellular fluid between intestinal epithelial cells passes through pores in the basal lamina to mix with that in the intercellular spaces and prelymphatic intercellular channels of the lamina propria and follicle domes. From there, lymph enters lacteals in the villi, or a branching network of vessels within the lamina propria. Small lymphatics penetrate the muscularis mucosae and are continuous with (1) lymphatic vessels which pass directly to the deep submucosa between follicles, or (2) lymphatic sinuses which lie adjacent to the follicles. This differs from the situation in sheep and rabbits. Basal lymphatics beneath the follicles convey lymph to vessels which leave the surface at the serosa.

CONCLUSION

The differences in the structure and arrangement of the lymphatics of Peyer's patches between pigs, sheep, and rabbits will require further investigation to determine if such variation between species has an effect on the distribution of immune products to effector sites.

The unique ultrastructure of high-endothelial venules in inguinal lymph nodes of the pig

Lymph nodes in pigs are unique in their inverted structure, with the medulla in the periphery and the cortex in central areas. Furthermore, in this species most migrating lymphocytes do not use the classical route via efferent lymphatics to leave the lymph node. High-endothelial venules (HEV) are the entry sites for lymphocytes and in pigs probably also the exit site for recirculating lymphocytes. Therefore, the blood vessels and especially the HEV of the pig superficial inguinal lymph node were investigated as to whether morphological peculiarities could be found in the vascular system, using vascular casting, transmission- and scanning electron microscopy. A thin layer of capillary network surrounded the periphery of the lymph node and HEV branched acutely. The endothelial cells of HEV possessed well developed cytoplasmic organelles, interdigitated with each other, and demonstrated local cell-cell contacts. There were unusual cells bridging the adluminal wall of HEV. These cells were called intravascular bridging cells. They were characterized by an often invaginated nucleus, few pinocytotic vesicles, many microvilli on the surface, wide, flat, cytoplasmic processes like a pseudopod, Weibel-Palade bodies and local cell-cell contacts with endothelial cells. The pseudopod-like processes ramified over the endothelial junctions and covered lymphocytes. Lymphocytes were seen in different phases of migration between endothelial cells and in the intercellular junctions. The previous functional studies on the peculiar route of lymphocyte recirculation in pig lymph nodes are extended by these morphological data, showing a unique structure of HEV in pigs.

Pathways of lymph flow through intestinal lymph nodes in the horse

In the horse, several thousand lymph nodes receive lymph from the intestine, part of which is very large and contains microorganisms that enable the animal to utilize refractory dietary constituents such as cellulose. The aim of this study was to describe the pathways by which lymph is delivered into, traverses, and is drained from these lymph nodes. These pathways were studied with either Microfil or methacrylate casting materials and with light and electron microscopy. The afferent lymphatic vessel delivering lymph into one of the nodes divides over the capsular surface and within trabeculae into terminal branches, and these are continuous with the subcapsular and trabecular sinuses through rounded holes up to 30 microns across. Lymph is conveyed from the subcapsular and trabecular sinuses through the cortex by four types of sinuses: trabecular sinuses, cortical tubular sinuses, tubule-like sinuses with a network of stellate cell processes, and sinuses between cortical cords. It is conveyed through the medulla by sinuses both within and between medullary cords. Lymph is drained from these sinuses by initial efferent lymphatics of three types: those between medullary cords, those within the subcapsular sinus overlying medullary or cortical cords, and those within trabeculae. All three types are continuous with surrounding sinuses through holes 5-30 microns across. These three alternative routes for lymph drainage may ensure adequate lymph flow during different intranodal conditions that may exist when the node is responding to microorganisms or other foreign materials.

Pathways between lymph vessels and sinuses in lymph nodes: a study in horses

The pathways through which lymph flows from terminal afferent lymphatics to the lymph sinuses, and from the sinuses to initial efferent lymphatics, were studied in horse lymph nodes by using Microfil casts and electron microscopy. Terminal afferent lymphatics are continuous with the subcapsular sinus through oval holes, about 3 microns wide, along their length, and through larger openings near their end. Other terminal afferents penetrate into the node within trabeculae and are supported by processes across the lumen. They connect with trabecular and medullary sinuses through oval or round holes, generally 20-30 microns across. Some of these have processes, similar to the luminal strands of the adjacent sinuses, extending across them, apparently providing reinforcement. Initial efferent lymphatics receive lymph from medullary sinuses through holes which are similar to those in afferent lymphatics, though more common, especially near the origin of the vessel. Initial efferents within the medulla appear to be supported by the luminal strands of the surrounding medullary sinuses. It seems likely that these strands, like the trabeculae which surround the penetrating afferent lymphatics, may help to keep the lymphatic vessels open as the node swells while draining an area of infection.

An electron microscopic study of cells with steroid-secreting morphology in the paraaortic lymph node of the hamster

Clustered cells with steroid-secreting morphology (SH), located within the paraaortic lymph node (PLN) capsules of normal and pregnant female golden hamsters, were examined by light and electron microscopy. Hydroxysteroid dehydrogenase (HD) activity of the SH cell cluster also was examined by histochemical techniques. The cluster was composed of mostly packed SH cells and surrounding mesenchymal cells. Individual SH cells possessed prominent smooth endoplasmic reticulum, well-developed Golgi complexes, some lipid droplets, and numerous mitochondria containing tubulovesicular cristae common to mammalian SH cells. Intermediate-type junctions were often observed between SH cells. The same SH cells were rarely detected in the subcapsular sinus and in the cortical parenchyma of the PLN. Early oophorectomy of the hamster resulted in cytoplasmic degeneration of the SH cell at day 5 after the operation. On the other hand, normal adult male hamsters possessed similar PLNs, but no SH cells were recognized in the nodes. The present histochemical preparation of normal female PLNs revealed moderate-to-strong HD activity, probably associated with SH cell clusters, in the limited regions of the capsules. Based on the present ultrastructural and histochemical findings, it is proposed that the SH cell cluster consists of steroid hormone producing cells.