Lymphatic system in Organ Development, Function, and Regeneration

The term lymph, a colorless fluid circulating throughout the lymphatic system, is derived from the Greek Nymph and the Latin Lympha, both deities of clear streams and fresh water. Around 1600 BCE, the Edwin Smith Papyrus - the earliest known scientific paper, described lymphatic gland swelling in the hieroglyphs¹. In the 5th century BCE, the Hippocratic Corpus, a collection of medical texts attributed to the ancient Greek physician Hippocrates and his followers, reported the presence of the lymph nodes around the ears, the kidneys, the jugular vessels, the groin, the axilla, and the ears. The texts of the Hippocratic Corpus are notable for their emphasis on the lymphatic system comprising lymphatic vessels, chylos (chyle), lymph nodes, afferent vessels draining fluid into the lymph node, and involvement of the lymphatic system in inflammatory conditions². Around 350 BCE, Aristotle described lymphatic vessels as “fibers, which take position between blood vessels and nerves and contain colorless liquid” in Historia Animalium, a report based on the dissection of multiple types of animals³. During the 2nd and 3rd centuries CE, Galen described mesenteric lymph nodes and the transport of milk-filled mesenteric vessels into the liver and subsequently into the blood in Anatomicae administrationes^1,4.

During the renaissance period, recognition of the heart as the central organ of the circulatory system propelled the understanding of the lymphatic system. Notably, Gaspare Aselli and Fabrice de Peiresc reported venae albae aut lacteae (white lacteals carrying milky fluid) as an independent circulatory system in dogs and humans, respectively^1,5. Consistent with this, Jean Pecquet characterized that lacteals transport milky fluid into the cisterna chyli (the reservoir of the lymph), the abdominal origin of vena alba thoracis (the thoracic duct)⁶, and Bartolomeo Eustachi reported that vena alba thoracis ends into the venous system at the junction of the left subclavian and left internal jugular veins⁷. In the 17th century, Frederik Ruysch identified multiple semilunar valves within the lymphatic vessels and the direction of lymph flow in humans⁸. Dissecting over 300 corpses injected with mercury into lymphatic vessels, Paolo Mascagni meticulously characterized the entire human lymphatic network in Vasorum lymphaticorum corporis humani historia et ichnographia, published in 1787⁹. Around the same time, William Hunter, William Cruikshank, and colleagues described the lymphatic system as one great and general system that absorbs fluid all over the body, depicted in The Anatomy of the Absorbing Vessels of the Human Body¹⁰. In the 19^th century, von Recklinghausen characterized an anastomosing network of lymphatic capillaries as a single layer of endothelial cells lacking communication with blood capillaries¹¹. Consistent with this, Ernest Starling identified that the balance between hydrostatic and colloid osmotic pressures across arterial and venous capillary walls regulates the filtration of circulating blood plasma, thus generating extracellular lymph¹². Based on morphology and muscular elements of the lymphatic vessel wall, Leonetto Comparini classified lymphatic vessels into lymphatic capillaries, pre-collecting lymphatic vessels, and collecting lymphatic vessels¹³. In addition to anatomical and morphological characterization, these studies identified the functional requirement of the central lymphatic system in the body’s immune defense against pathogens, the fluid homeostasis in the extracellular regions, transport of the nutrients and signaling molecules to the blood; and in the tissue growth¹⁴.

Multiple lines of recent investigations have revealed the requirement for organ-specific lymphatic systems in organogenesis, growth, and repair¹⁵. Anatomists and physicians identified and characterized several organ-specific lymphatic systems during the last five hundred years. In the 17th century, Olaus Rudbeck and Thomas Bartholin identified a specialized lymphatic system in the canine heart^1,7. In 1706, Raymond Vieussens published detailed anatomy and absorbent functions of the cardiac lymphatic system in Nouvelles Découvertes sur le Coeur¹⁶. Approximately two hundred years later, Otto Kampmeier described the development and functions of cardiac and valvular lymphatics in neonatal human hearts¹⁷. Paul Patek demonstrated drainage of cardiac lymph from the subendocardium and the myocardium into the subepicardium and subsequently into the mediastinal lymph nodes, suggesting perfusion of all three layers of the heart¹⁸. More recently, the developmental origin of lymphatic endothelial cells lining cardiac lymphatics has been discovered to be heterogenous, including from sources as varied as the second heart field, venous endothelium, and hemogenic endothelium. Despite the tremendous progress in the structural description of lymphatic vessels of the heart, the relative understanding of the functional aspects of the cardiac lymphatic vasculature has been limited to its well-known systemic role of interstitial fluid drainage. The review by Xiaolei Liu and Guillermo Oliver discusses emerging evidence of the functional role of cardiac lymphatics in heart development, disease, and repair¹⁹. The authors discuss recent exciting and novel findings of lymphangiocrines secreted by cardiac lymphatics on embryonic heart development and touch upon emerging research involving complex crosstalk between stromal cells and lymphatic vessels that could, in turn, influence cardiac development. In line with these findings, Liu and Oliver discuss recent research describing the importance of functional lymphatics in limiting cardiac disease and how cardiac disease can reciprocally impair lymphatic function.

In 1856, Rudolf Virchow reported abnormally distended lymphatic vessels, known as lymphangiectasia, in neonatal human lungs²⁰. Subsequently, William Councilman described the microscopic structure of the pulmonary lymphatics and their relationship to the pleura, the lobules, and the alveoli in children²¹. In pathological pediatric lungs, William Councilman also observed pulmonary lymphangiectasia. In 1975, Mary Engle reported pulmonary lymphangiectasia associated with chylothorax in several neonatal cases with Noonan syndrome, which is caused by gain-of-function mutations in components of MAPK signaling pathways, including PTPN11, SOS1, RAF1, and KRAS. Our recent studies demonstrated that gain-of-function mutation KRASG12D and subsequent pathological activation of MAPK signaling disrupt lymphatic basement membrane composition and assembly, causing pulmonary lymphangiectasia, which promotes chylothorax limiting gas exchange in alveoli and neonatal survival (Figure)²².

Figure:

Pathological activation of KRAS^G12D/MAPK signaling causes lymphangiectasia that is treatable with Ravoxertinib. KRAS^G12D-mediated activation of MAPK pathway represses transcription of basement membrane genes, leading to defective development of lymphatic basement membrane, which in turn promotes pulmonary and intercostal lymphangiectasia, as well as chyle effusion, in the pleural space. Adapted from Janardhan et al with CC BY 4.0 license. Copyright ©2022, the American Society for Clinical Investigation.

In 1787, Paolo Mascagni provided detailed illustrations of mesenteric and renal lymphatics in human cadavers⁹. Over the last hundred years, multiple investigations have identified and characterized the integrated nature of lymphatic function or dysfunction in various disease states including cardiac and renal diseases. In this review series, Zhong et al. illustrate this interconnected complexity as they discuss recent research highlighting the interplay between the renal system, the intestinal lymphatics, and the gastrointestinal system and the consequence of this interaction in mediating a wide variety of local and systemic pathophysiological effects²³. The authors specifically focus on how kidney disease alters or disrupts intestinal lymphatic structure and function, contributing to various diseases commonly associated with kidney dysfunction, including cardiovascular disease, osteoporosis, and cognitive dysfunction. Zhong et al. further synthesize and delineate how kidney disease affects myriad factors such as intestinal barrier disruption, dysbiosis, dyslipidemia, inflammation, and sodium accumulation and how these factors mechanistically alter intestinal lymphatic function. Finally, the authors discuss the potential clinical implications and therapeutic opportunities derived from deciphering the abovementioned kidney-intestinal lymphatic-organ crosstalk, ultimately helping reduce and resolve kidney disease and its complications.

Approximately 500–600 lymph nodes, first described in the Edwin Smith Papyrus around 1600 BCE, are an integral component of the lymphatic and immune systems in the human body. Lymph nodes perform diverse functions, including filtration of lymphatic fluid, generation of immune responses, and trapping pathogens, as an independent functional unit of the lymphatic system. Recent advances in genetics and molecular profiling have revealed the distinct cellular composition of the lymph node outer cortex, paracortex and medulla under normal and disease conditions. In the review, Arroz-Madeira and colleagues provide in-depth analyses of the heterogeneity of lymphatic endothelial cells within the lymph node and their specialized roles in lymph node function and development²⁴. Lymphatic endothelial cells occupy various niches of the lymph node, including the subcapsular sinus, paracortical/cortical sinuses, and medullary sinuses. Their specialization is critical to lymph node structure, maintenance, and function, particularly for immune cell passage and antigen storage. Arroz-Madeira and colleagues review the molecular phenotypes of five distinct subsets of lymphatic endothelial cells within lymph nodes and their specialized roles in immune cell homeostasis, tolerance, and active inflammation. In addition, inflammation-induced changes in lymphatic endothelia, molecular plasticity, and the impact of circadian rhythms on lymphatic vessels and lymphoid organs are also discussed. The involvement of lymphatic vessels on the lymph node, the spleen, and Peyer’s patch development is discussed. The authors end with a clinically relevant perspective on lymph node fibrosis and lipomatosis in aging.

Lymphatic system facilitates the transport and distribution of indispensable molecules, including glucose, lipids, growth factors, hormones, and signaling ligands. Adrenomedullin, a 52 amino acid peptide secreted by vascular cells – including lymphatic endothelial cells, functions as a hormone to regulate lymphatic vessel function, growth, and heart development. Adrenomedullin binds to the calcitonin-receptor-like receptor to activate eNOS, which increases NO levels and causes subsequent vasodilation. Balint and colleagues review the current knowledge on Adrenomedullin signaling and its effects on lymphatic vessel development, growth, and function²⁵. In this review, the authors discuss Adrenomedullin’s varied pathophysiological roles and therapeutic applications in cardiovascular diseases, including heart failure, myocardial infarction, and pulmonary hypertension, as well as its potential diagnostic uses in stroke, cancer, and lymphedema, among others. The authors address the advantages and caveats of using Adrenomedullin as a target for therapeutics and as a biomarker, covering several recent advances that could boost the clinical potency of the Adrenomedullin-targeting method. As a G protein-coupled receptor agonist, Adrenomedullin can be used in clinics to treat a broad range of diseases. Future work on Adrenomedullin will enhance its use as a biomarker and therapeutic target.

The lymphatic system is essential in organ development, growth, function, and regeneration²⁶. Future studies utilizing state-of-the-art genomic, proteomic, and imaging approaches at a single-cell level will likely advance our understanding of the lymphatic system in health and in disease. In the 21st century, new diagnostic and therapeutic approaches are warranted to treat organ-specific lymphatic diseases.