Development of the lymphatic network in the muscle coat of the rat jejunum as revealed by enzyme-histochemistry

Lymphatic Vessel Network Structure and Physiology

The lymphatic system is comprised of a network of vessels interrelated with lymphoid tissue, which has the holistic function to maintain the local physiologic environment for every cell in all tissues of the body. The lymphatic system maintains extracellular fluid homeostasis favorable for optimal tissue function, removing substances that arise due to metabolism or cell death, and optimizing immunity against bacteria, viruses, parasites, and other antigens. This article provides a comprehensive review of important findings over the past century along with recent advances in the understanding of the anatomy and physiology of lymphatic vessels, including tissue/organ specificity, development, mechanisms of lymph formation and transport, lymphangiogenesis, and the roles of lymphatics in disease. © 2019 American Physiological Society. Compr Physiol 9:207-299, 2019.

Lymphatic development in mouse small intestine

Lymphatic vessels in the small intestine serve as essential conduits for the absorption and transport of lipids from the intestine to the thoracic duct. Although the morphology and function of the intestinal lymphatic vasculature are well known, little is known about the embryonic development of these vessels. In this study, we examined development of lymphatic and blood vasculatures in the intestinal tube during mouse embryonic development by immunostaining with recently discovered molecular markers for lymphatic endothelial cells: LYVE-1, VEGFR3, Prox-1, and podoplanin. Immature lymphatics became detectable in mesentery, but not in intestinal tube, around E13.5-E14.5, while organized lymphatic vessel plexuses and capillaries were observed in intestinal tube and villi around E17.5. These lymphatic plexuses and capillaries in the intestinal tube appeared to be formed through an active branching process associated with activation of VEGFR3 and involvement of LYVE-1+ macrophages. Our data also reveal that the lymphatic vessels in the intestinal tube, unlike the blood vessels, have not originated from the mesoderm of intestine. All lymphatic vessels in the intestinal tube originated by extension of mesenteric lymphatic vessels through an active branching process. Although the formation of lymphatic vessels follows the formation of blood vessels in the intestine, a mature lymphatic vasculature is formed before birth. Together, our study reveals the temporal and spatial windows of intestinal lymphatic development during embryonic development in mouse.

Abnormal recruitment of periendothelial cells to lymphatic capillaries in digestive organs of angiopoietin-2-deficient mice

The fine structure of lymphatic capillaries in the digestive organs of angiopoietin-2 (Ang2) knockout mice was studied by using both immunohistochemistry and electron microscopy. The genetic deletion of Ang2 yielded hypoplasia and disorganization of the lymphatic capillaries, with their shapes being irregular, and an aberrant recruitment of vascular periendothelial cells immunopositive for smooth muscle actin to the lymphatic capillaries. The abnormal lymphatic periendothelial cells were considered to be a type of pericyte for the lymphatic capillaries after the deletion of Ang2, because they were ultrastructurally characterized by abundant thin myofilaments in their cytoplasm and long cytoplasmic extensions similar to those shown by blood vascular pericytes. The genetic replacement of Ang2 with Ang1 rescued the defects, viz., the disorganization and disordered structure of the lymphatic capillaries. The present findings suggest that Ang2 serves the morphogenesis of lymphatic capillaries as an agonist for the receptor, Tie2, and that Ang1 can replace Ang2 in guiding lymphatic formation and development.

Lymphangiogenesis and expression of specific molecules as lymphatic endothelial cell markers

In recent years, several functional molecules specifically expressed and localized in lymphatic endothelial cells, such as 5'-nucleotidase, lymphatic vessel endothelial receptor-1, vascular endothelial growth factor receptor-3, podoplanin and Prox-1, have been identified. The discovery of the lymphatic endothelial cell markers facilitated detailed analysis of the nature and structural organization of the lymphatic vessels and their growth (lymphangiogenesis). As a result, over the past few years, advances have been made in understanding the cellular and molecular aspects of physiological lymphangiogenesis and tumor-induced lymphangiogenesis. The biology of lymphangiogenesis, particularly the mechanism of its regulation, is very important in understanding the formation of the lymphatic system as a biological regulation system transporting tissue fluid and wandering cells, including lymphocytes, and disease involving lymphangiogenesis. The understanding of the molecular mechanism of lymphangiogenesis and the elucidation of the development of normal and pathological tissues are expected to lead to the development of therapy for intractable diseases, such as malignant tumors and lymphedema.

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.