Eiko Ojala
I placed the cold, jade rolling pin under my cheekbone and gently glided it to the edge of my face. To be honest, I felt a bit like a piece of pastry. But my grandma assured me that I was engaging in a serious traditional Chinese practice that would make my skin glow and my face less puffy. It would be more than a decade before face rolling became the next big beauty fad on social media – and several years more before I realised its supposed benefits may hinge on the lymphatic system, a network spanning the entire body, consisting largely of thread-like vessels and bean-shaped nodes……..Continue reading…..
By Carissa Wong
Source: New Scientist
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Critics:
Unlike the circulatory system, which is a closed system, the lymphatic system is open. Lymph originates in the interstitial fluid that leaks from blood in the circulatory system into the tissues of the body. This fluid carries nutrients to the cells and collects waste products, bacteria, and damaged cells, before draining into the lymphatic vessels as lymph.
The circulatory system processes an average of 20 litres (5.3 US gal) of blood per day through capillary filtration, which removes plasma from the blood. Roughly 17 litres (4.5 US gal) of the filtered blood is reabsorbed directly into the blood vessels, while the remaining 3 litres (0.79 US gal) are left in the interstitial fluid. The lymphatic system provides an accessory return route to the blood for this remainder.
The other main function is that of immune defense. Lymph is very similar to blood plasma, in that it contains waste products and cellular debris, together with bacteria and proteins. The cells of the lymph are mostly lymphocytes. Associated lymphoid organs are composed of lymphoid tissue, and are the sites either of lymphocyte production or of lymphocyte activation. These include the lymph nodes (where the highest lymphocyte concentration is found), the spleen, the thymus, and the tonsils. Lymphocytes are initially generated in the bone marrow.
The lymphoid organs also contain other types of cells such as stromal cells for support. Lymphoid tissue is also associated with mucosas such as mucosa-associated lymphoid tissue (MALT). These vessels carry the lymph throughout the body, passing through numerous lymph nodes which filter out unwanted materials such as bacteria and damaged cells. Lymph then passes into much larger lymph vessels known as lymph ducts.
The right lymphatic duct drains the right side of the region and the much larger left lymphatic duct, known as the thoracic duct, drains the left side of the body. The ducts empty into the subclavian veins to return to the blood circulation. Lymph is moved through the system by muscle contractions. In some vertebrates, a lymph heart is present that pumps the lymph to the veins.
The lymphatic system was first described in the 17th century independently by Olaus Rudbeck and Thomas Bartholin. The lymphatic system consists of a conducting network of lymphatic vessels, lymphoid organs, lymphoid tissues, and the circulating lymph. The primary (or central) lymphoid organs, including the thymus, bone marrow, fetal liver and yolk sac, are responsible for generating lymphocytes from immature progenitor cells in the absence of antigens.
The thymus and the bone marrow constitute the primary lymphoid organs involved in the production and early clonal selection of lymphocyte tissues.Bird species’ primary lymphoid organs include the bone marrow, thymus, bursa of Fabricius, and yolk sac. Bone marrow is responsible for both the creation of T cell precursors and the production and maturation of B cells, which are important cell types of the immune system. From the bone marrow, B cells immediately join the circulatory system and travel to secondary lymphoid organs in search of pathogens.
T cells, on the other hand, travel from the bone marrow to the thymus, where they develop further and mature. Mature T cells then join B cells in search of pathogens. The other 95% of T cells begin a process of apoptosis, a form of programmed cell death: T cells that cannot interact strongly enough with self-antigens are eliminated during T cell § Positive selection, and T cells that attack the body’s own proteins are eliminated during § Negative selection.
The thymus increases in size from birth in response to postnatal antigen stimulation. It is most active during the neonatal and pre-adolescent periods. The thymus is located between the inferior neck and the superior thorax. At puberty, by the early teens, the thymus begins to atrophy and regress, with adipose tissue mostly replacing the thymic stroma. However, residual T cell lymphopoiesis continues throughout adulthood, providing some immune response.
The thymus is where the T lymphocytes mature and become immunocompetent. The loss or lack of the thymus results in severe immunodeficiency and subsequent high susceptibility to infection. In most species, the thymus consists of lobules divided by septa, which are made up of epithelium, which is often considered an epithelial organ. T cells mature from thymocytes, proliferate, and undergo a selection process in the thymic cortex before entering the medulla to interact with epithelial cells.
Research on bony fish showed a buildup of T cells in the thymus and spleen of lymphoid tissues in salmon and showed that there are not many T cells in non-lymphoid tissues. The thymus provides an inductive environment for developing T cells from hematopoietic progenitor cells. In addition, thymic stromal cells allow for the selection of a functional and self-tolerant T cell repertoire. Therefore, one of the most important roles of the thymus is the induction of central tolerance.
However, the thymus is not where the infection is fought, as the T cells have yet to become immunocompetent. The secondary (or peripheral) lymphoid organs, which include lymph nodes and the spleen, maintain mature naive lymphocytes and initiate an adaptive immune response. The secondary lymphoid organs are the sites of lymphocyte activation by antigens. Activation leads to clonal selection and affinity maturation. Mature lymphocytes recirculate between the blood and the secondary lymphoid organs until they encounter their specific antigen.
The lymphatic vessels, also called lymph vessels, are thin-walled vessels that conduct lymph between different parts of the body. They include the tubular vessels of the lymph capillaries, and the larger collecting vessels – the right lymphatic duct and the thoracic duct (the left lymphatic duct). The lymph capillaries are mainly responsible for the absorption of interstitial fluid from the tissues, while lymph vessels propel the absorbed fluid forward into the larger collecting ducts, where it ultimately returns to the bloodstream via one of the subclavian veins.
The tissues of the lymphatic system are responsible for maintaining the balance of the body fluids. Its network of capillaries and collecting lymphatic vessels efficiently drain and transport extravasated fluid, along with proteins and antigens, back to the circulatory system. Numerous intraluminal valves in the vessels ensure a unidirectional flow of lymph without reflux. Two valve systems, a primary and a secondary valve system, are used to achieve this unidirectional flow.
The capillaries are blind-ended, and the valves at the ends of capillaries use specialised junctions together with anchoring filaments to allow a unidirectional flow to the primary vessels. When interstitial fluid increases, it causes swelling that stretches collagen fibers anchored to adjacent connective tissue, opening the unidirectional valves at the ends of these capillaries and facilitating the entry and subsequent drainage of excess lymph fluid.
The collecting lymphatics, however, propel the lymph by the combined actions of the intraluminal valves and lymphatic muscle cells. The development of the lymphatic vascular system is a highly coordinated process essential for maintaining fluid homeostasis, immune surveillance, and lipid absorption. Lymphangiogenesis, the formation of lymphatic vessels from pre-existing ones, is primarily driven by the migration and differentiation of lymphatic endothelial cells (LECs) under the influence of molecular signals such as vascular endothelial growth factor C (VEGF-C).
The maturation and specialization of lymphatic vessels involve complex interactions with surrounding tissues, ensuring the proper function of lymphatic drainage and Immune cell trafficking. Recent advances in vitro and in vivo studies have provided deeper insights into the mechanisms regulating lymphatic vascular development, as well as the processes governing its maintenance and aging. Understanding these pathways is critical, as disruptions in lymphatic development are implicated in congenital disorders, inflammatory diseases, and cancer metastasis.
Lymphatic tissues begin to develop by the end of the fifth week of embryonic development. Lymphatic vessels develop from lymph sacs that arise from developing veins, which are derived from mesoderm. The first lymph sacs to appear are the paired jugular lymph sacs at the junction of the internal jugular and subclavian veins. From the jugular lymph sacs, lymphatic capillary plexuses spread to the thorax, upper limbs, neck, and head.
Some of the plexuses enlarge and form lymphatic vessels in their respective regions. Each jugular lymph sac retains at least one connection with its jugular vein, the left one developing into the superior portion of the thoracic duct. The spleen develops from mesenchymal cells between layers of the dorsal mesentery of the stomach. The thymus arises as an outgrowth of the third pharyngeal pouch.
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