Lymph sacs are a part of the development of the lymphatic system, known as lymphangiogenesis.The lymph sacs are precursors of the lymph vessels.These sacs develop through the processes of vasculogenesis and angiogenesis. However, there is evidence of both of these processes in different organisms. In mice, it is thought that the lymphatic components form through an angiogenic process. But, there is evidence from bird embryos that gives rise to the idea that lymphatic vessels arise in the embryos through a vasculogenesis-like process from the lymphangioblastic endothelial precursor cells.

The development of the lymphatic system has been a highly debated topic in developmental biology for a long time. Previously, it was debated whether the lymph sacs developed from the venous system, or if they came from spaces in the mesenchyme, which come together in a centripetal direction and secondarily opened into the veins.However, more recent research has shown that the formation of the lymphatic system begins when a subset of endothelial cells from the previously formed jugular vein sprout off to form the lymphatic sacs.Because lymph sacs form from the venous system, they typically contain red blood cells.It is believed that the lymph sacs are directly connected to the venous system and that the venous components and lymphatic components communicate through a small hole.Studies have shown that the development of lymph sacs occurs through swelling and outgrowth of pre-lymphatic clusters from the cardinal vein, in a process termed ballooning. Following ballooning, there is the process of pinching, which separates the lymph sacs from the venous system.

These processes begin forming the lymph sacs during the 5th week of fetal development. At this time, the jugular lymph sacs develop. These are a pair of enlargements that function in collecting fluid from the lymphatics of the upper limbs, upper trunk, head, and neck. The lymph nodes eventually develop at the place of the jugular lymphatic sacs.From the left jugular lymphatic sac, the cervical part of the thoracic duct forms. From the right jugular lymphatic sac, the right lymphatic duct and the jugular and the subclavicular lymphatic trunks form.One week later, during the 6th week of fetal development, four more lymph sacs form. These are the retroperitoneal lymph sac, the cysterna chyli, and paired posterior lymph sacs.The posterior lymph sacs are associated with the junctions of the external and internal iliac veins. These four new lymph sacs function in the collection of lymph from the trunk and lower extremities of the body. The cysterna chyli drains into a pair of thoracic lymphatic ducts initially. These ducts drain into the venous junctions of the internal jugular and subclavian veins. However, these ducts eventually become one thoracic duct that is derived from the caudal portion of the right duct, the cranial portion of the left duct, and median anastomosis.

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Journal of Phlebology and Lymphology
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