Three-dimensional organization of lymphatics in the dog small intestine: a scanning electron microscopic study on corrosion casts

Comparative pathophysiology and management of protein-losing enteropathy

Protein‐losing enteropathy, or PLE, is not a disease but a syndrome that develops in numerous disease states of differing etiologies and often involving the lymphatic system, such as lymphangiectasia and lymphangitis in dogs. The pathophysiology of lymphatic disease is incompletely understood, and the disease is challenging to manage. Understanding of PLE mechanisms requires knowledge of lymphatic system structure and function, which are reviewed here. The mechanisms of enteric protein loss in PLE are identical in dogs and people, irrespective of the underlying cause. In people, PLE is usually associated with primary intestinal lymphangiectasia, suspected to arise from genetic susceptibility, or “idiopathic” lymphatic vascular obstruction. In dogs, PLE is most often a feature of inflammatory bowel disease (IBD), and less frequently intestinal lymphangiectasia, although it is not proven which process is the true driving defect. In cats, PLE is relatively rare. Review of the veterinary literature (1977‐2018) reveals that PLE was life‐ending in 54.2% of dogs compared to published disease‐associated deaths in IBD of <20%, implying that PLE is not merely a continuum of IBD spectrum pathophysiology. In people, diet is the cornerstone of management, whereas dogs are often treated with immunosuppression for causes of PLE including lymphangiectasia, lymphangitis, and crypt disease. Currently, however, there is no scientific, extrapolated, or evidence‐based support for an autoimmune or immune‐mediated mechanism. Moreover, people with PLE have disease‐associated loss of immune function, including lymphopenia, severe CD4+ T‐cell depletion, and negative vaccinal titers. Comparison of PLE in people and dogs is undertaken here, and theories in treatment of PLE are presented.

Microanatomy of the intestinal lymphatic system

The intestinal lymphatic system comprises two noncommunicating lymphatic networks: one containing the lacteals draining the villi and the connecting submucosal lymphatic network and one containing the lymphatics that drain the intestine muscular layer. These systems deliver lymph into a common network of collecting lymphatics originating near the mesenteric border. The intestinal lymphatic system serves vital functions in the regulation of tissue fluid homeostasis, immune surveillance, and the transport of nutrients; conversely, this system is affected by, and directly contributes to, disease processes within the intestine. Recent discoveries of specific lymphatic markers, factors promoting lymphangiogenesis, and factors selectively affecting the development of intestinal lymphatics, hold promise for unlocking the role of lymphatics in the pathogenesis of diseases affecting the intestine and for intestinal lymphatic selective therapies. Vital to progress in understanding how the intestinal lymphatic system functions is the integration of recent advances identifying molecular pathways for lymphatic growth and remodeling with advanced imaging modalities to observe lymphatic function and dysfunction in vivo.

A Model for Lymphatic Regeneration in Tissue Repair of the Intestinal Muscle Coat

The gastrointestinal lymphatic system, which comprises a network of thin-walled vessels, is essential for the regulation of tissue fluid volume, immune function, and transport of fatty nutrients. The identification of specific lymphatic endothelial markers has facilitated analyses of lymphatic organization and lymphangiogenesis during individual development and tissue repair. The intestinal muscle coat producing motor activity develops a dense maze-like lymphatic network by vascular sprouting consisting of thin lymphatic endothelial projections and splitting of the vessels. The lymphatic regeneration in the tissue repair of the intestinal muscle coat is essentially attributable to sprouting from preexisting lymphatics, and it progresses vigorously with vascular maturation. The regrowing lymphatic endothelial cells exhibit structural changes indicating a high migratory potential and a close association with regenerating stromal cells. The upregulation of VEGF-C, a specific lymphangiogenic molecule, in a subpopulation of the stromal cells probably contributes to lymphatic regeneration by activating its receptor, VEGFR-3, on the regrowing lymphatic endothelial cells.

Lymphatic corrosion casts in rabbit ileum: scanning electronmicroscopic studies

AIM

To investigate the three-dimensional organization and fine distribution of the lymphatics in rabbit ileum.

METHODS

Lymphatic corrosion cast with the Mercox were used for scanning electron microscopy (SEM), and semithin sections were used for light microscopy. The Mercox injected intraparenchymally into ileum wall were cut and put in a concentrated NaOH solution until the tissues were corroded away, and observed under SEM.

RESULTS

The central lacteals were found in the intestinal villi. The villi of the ileum contained two to three lacteals. The central lacteals were drained into the mucosal lymphatic capillary plexus. From the plexus, the lymphatic capillary descended into the lymphatics of submucosal layer and muscular layer. Then they were led into the serosal lymphatics and drained into the lymphatics of intestine mesentery. The cast of the lymphatics showed an appearance of a string of heads and the notch corresponding to the bicuspid valve of the lymphatics.

CONCLUSION

The three-dimensional organization of central lacteals and lymphatics in the rabbit ileum is demonstrated by lymphatic corrosion casts. Numerous impressions of the endothelial nuclei, rich central lacteals and lymphatic capillary plexus in the ileum mucosa, lymphatic capillary and lymphatics in the submucosa and muscular layer are observed on the lymphatic corrosion cast.