Alderfer et al. Journal of Biological Engineering (2018) 12:32 https://doi.org/10.1186/s13036-018-0122-7 REVIEW Open Access Lymphatic Tissue Engineering and Regeneration Laura Alderfer1, Alicia Wei1 and Donny Hanjaya-Putra1,2,3,4,5,6* Abstract The lymphatic system is a major circulatory system within the body, responsible for the transport of interstitial fluid, waste products, immune cells, and proteins. Compared to other physiological systems, the molecular mechanisms and underlying disease pathology largely remain to be understood which has hindered advancements in therapeutic options for lymphatic disorders. Dysfunction of the lymphatic system is associated with a wide range of disease phenotypes and has also been speculated as a route to rescue healthy phenotypes in areas including cardiovascular disease, metabolic syndrome, and neurological conditions. This review will discuss lymphatic system functions and structure, cell sources for regenerating lymphatic vessels, current approaches for engineering lymphatic vessels, and specific therapeutic areas that would benefit from advances in lymphatic tissue engineering and regeneration. Keywords: Lymphangiogenesis, Tissue Engineering, Disease Modeling, Wound Healing, Lymphedema, Stem Cells, Biomaterials, Interstitial Fluid, Regeneration I. Introduction to the Lymphatic System and its role Interstitial fluid (IF) is a plasma filtrate that is generated Function by transcapillary filtration and is governed by Starling The lymphatic system is nearly ubiquitous in the human forces, the net difference between hydrostatic and body, present in all tissues except the epidermis, cartil- osmotic pressures, at the microcirculatory level [9]. In age, eye lens, cornea, retina, and bone marrow [1, 2]. order to maintain fluid homeostasis, lymph formation in The main functions of the lymphatic system include the initial lymphatic vessels must be balanced by the net fluid homeostasis and interstitial fluid drainage, immune flux of plasma being filtered out [4]. Transport of IF cell surveillance and trafficking, and lipid absorption [1, from the initial capillaries to the collecting vessels is 3–6]. Lymphangiogenesis, the process of forming new facilitated by IF pressure and systemic forces, including lymphatic vessels from pre-existing vessels, not only blood pressure, respiratory motion massage, peristaltic occurs during development but also in adults during movement, and contractility of surrounding skeletal wound healing, inflammatory responses, and the cancer muscle [10–14]. As a result of constantly clearing IF, the microenvironment [1, 7]. lymphatic system is chronically exposed to and stimu- The lymphatic system includes bone marrow and the lated by fluid flow and pressure [5]. thymus, classified as central or primary lymphoid IF is transported via lymph vessels to lymph nodes and organs, as well as lymphatic vessels, lymph nodes, then returned back to the blood circulation. Properties spleen, adenoids, Peyer’s patches, appendix, and lymph- of the lymphatic capillary wall, hydrostatic pressure, and oid tissue, classified as peripheral or secondary lymphoid protein concentrations in the blood and interstitium are organs [8]. Within the cellular microenvironment in tis- determining factors in the formation of IF [4]. Contained sues, the fluid, proteins, solutes, and extracellular matrix within IF are macromolecules, dissolved solutes, viruses, (ECM) are collectively termed the interstitium [4]. bacteria, certain leukocytes, and cell debris [1]. IF facili- tates the transportation of various molecules between * Correspondence: [email protected] local sites and tissues, including nutrients, waste prod- 1 Department of Aerospace and Mechanical Engineering, Bioengineering ucts, signaling molecules, antigens, and cytokines. The Graduate Program, University of Notre Dame, Notre Dame, IN 46556, USA 2Department of Chemical and Biomolecular Engineering, University of Notre specific composition of IF depends on pathogenesis, in- Dame, Notre Dame, IN 46656, USA flammatory responses, and the nearby organs or tissues Full list of author information is available at the end of the article © The Author(s). 2018 Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated. Alderfer et al. Journal of Biological Engineering (2018) 12:32 Page 2 of 26 [4]. Under healthy conditions, IF will comprise approxi- the surrounding tissues as well as blood pressure also as- mately 20% of the body’s weight and 2-4 liters of IF will sist in creating this unidirectional propulsion [14, 25]. be returned to the venous vasculature from the lymph- In addition to these general characteristics of the atic system daily [1, 15]. IF volume is constantly main- lymphatic system that can be found throughout the tained by interstitial buffering mechanisms [8], which body, there are also several specialized functions or not- include structural alterations, differences in forces acting able lymphatic features within organ systems. In the case across the capillary wall, and lymph flow [4]. of regulating lipid uptake in the gastric lymphatic sys- tem, lacteals, specialized lymphatic vessels, are posi- tioned in the villi of lumen next to blood capillaries [26]. Structure Endothelial cells, along with keratinocytes, fibroblasts, Despite the lymphatic system being so extensive, the macrophages, and platelets are involved in the wound field of lymphatic research is very young due to lymph- healing process [27]. In the case of inflammation, the atic specific markers being discovered only 20 years ago. gene expression of LECs is altered and leads to the Since the identification of lymphatic specific markers lymphatic network expanding, along with increased fluid and isolation of lymphatic endothelial cells, key differ- drainage both to and from the site of inflammation [28]. ences between the vascular and lymphatic systems have LVs also contribute to the inflammatory response by been identified, allowing for specific research efforts into draining cytokines and chemokines [26]. The heart con- the lymphatic system without results being confounded tains an extensive lymphatic network, and combined by the inclusion of the vascular system [4]. with the role of the lymphatic system in inflammation, Several key differences exist between blood vessels and targeting lymphangiogenesis in the heart after myocar- lymphatic vessels. Composed of blood endothelial cells dial infarctions to improve recovery has become an area (BECs), blood vessels exhibit tight junctions and a con- of interest [29, 30]. tinuous basal lamina. Conversely, lymphatic vessel (LVs), composed of a single layer of lymphatic endothelial cells II. Complications Associated with the Lymphatic (LECs), have a discontinuous basal lamina as a result of System overlapping and interdigitated endothelial cells [4, 16], Complications associated with the lymphatic system blind ended sacs [16], and a wide lumen [2]. Addition- span a wide spectrum, including congenital disorders, ally, lymphatic capillaries lack pericytes, smooth muscle cancer and side-effects of cancer treatments, cardiovas- cells (SMCs), and mural cell coverage [3, 17]. The ECM cular disease, diabetes, and parasitic infections [25, 31]. and lymphatic capillaries are connected with anchoring While some lymphatic disorders are genetically related, filaments and when the interstitial volume increases, lymphatic complications most often arise as a secondary these anchoring filaments are pulled apart which causes complication following cancer, cardiovascular disease, lymphatic valves to open [18, 19]. These anchoring and immunological diseases [32]. Specific pathologies filaments are composed of collagen VII [20, 21], trans- and areas that could benefit from improved lymphatic membrane integrins, and focal adhesion kinase [17]. function or engineered lymphatic tissue are summarized VE-cadherin joins discontinuous and overlapping endo- in Fig. 1. thelial cells together into buttonlike patterns [22, 23] which are postulated to serve as one-way flaps that fa- Lymphedema cilitate the absorption of cells, fluid, and proteins [4]. IF Lymphedema, characterized by chronic swelling of an enters LVs through these button-like junctions and is fa- extremity, results from local accumulation of interstitial cilitated by the pressure gradient [22]. fluid due to insufficient lymph drainage [4]andisone Unlike the circulatory system, the lymphatic system is of the most prevalent lymphatic-dysfunction conditions a one-way drainage system that originates in tissues and [24]. Globally, up to 250 million people are affected by organs, is funneled through a series of many small ves- lymphedema with the most prevalent cause being the sels emptying into fewer larger vessels, and empties into parasitic disease filariasis [33]. In developed countries, the circulatory system [5]. Continuous fluid flow the most common cause of lymphedema is disruption between blood capillaries and tissues is achieved by of lymphatic pathways, typically from