The late anatomical discoveries of the motion of the chyle and lymphatick liquor . . . hath yet made men cure diseases much better than before. BOYLE Usef. Exp. Nat. Philos. II. v. x. 224, 1663
Development of the lymphatic system
Lymph sacs
Early in development, clefts lined with endothelium appear in the mesenchyme and form capillary plexuses. Six lymph sacs arise from the plexuses: lymph sacs in the neck, the paired iliac lymph sacs at the junction of the iliac and postcardinal veins, and the retroperitoneal lymph sac near the adrenals and the cisterna chyli at the L3 and L4 vertebral level ( Fig. 3-1 ). Channels that will form the future right and left thoracic ducts will ascend from the cisterna. The iliac lymph sacs drain the legs, and the retroperitoneal sac drains the abdominal viscera.
The lymph vessels are formed as branches from the sacs and follow the course of the primitive veins. Alternatively, they are formed directly in the mesenchyme and become connected secondarily.
The sacs become divided by the formation of septa by encroaching mesenchymal cells and are invaded by lymphocytes early in fetal life to become groups of lymph nodes. The sinuses within a node ( Figs. 3-3 and 3-4 ) represent the cavity of the original lymph sac. The exception is the upper portion of the cisterna chyli, which does not divide but may become plexiform. From each of these sacs, lymphatic vessels follow the main veins to the structure to be drained.
Thoracic duct
Originally, the two jugular lymph sacs are connected to the cisterna chyli by the right and left thoracic ducts , between which an anastomotic channel forms. The left duct and part of the right thoracic duct regress, so that the final duct is formed from the caudal part of the right duct, the anastomotic channel, and the cranial part of the left duct ( Fig. 3-2 ).
Structure and function of the lymphatic system
The superficial lymphatic vessels are associated with the superficial veins and are a system distinct from the deep lymphatics, which are associated with named arteries or veins. All of the lymph except for a small portion from the neck eventually reaches the thoracic duct.
Lymphatic vessels and lymph nodes
Blind-ending lymph capillaries lying in the tissue spaces collect lymph through their permeable walls and channel it through larger trunks to collections of lymphoid tissue, the lymph nodes. Groups of lymph nodes drain particular regions, but connections between the individual nodes in a group are common and lymph may pass consecutively through several nodes before reaching a major collector.
Lymph nodes are small, somewhat flattened bodies that receive lymph from the several valved afferent lymphatic vessels enteri ng around the periphery ( Figs. 3-3 and 3-4 ). The lymph first passes through the subcapsular sinuses , then into the cortical (trabecular) sinuses , and finally into medullary sinuses near the hilum . A capsule composed of dense connective tissue surrounds the node, and from the capsule extend trabeculae, which are surrounded by cortical sinuses and separate the lymph follicles. The trabeculae support a fine reticulum that fills the node and serves as a framework for the attachment of several types of cells. The reticulum provides for maximal contact between the cells and the circulating lymph. In the cortex , the entangled cells form dense aggregates, which are the lymph follicles that surround the germinal centers . The germinal centers contain lymphoblasts, which mature to small lymphocytes. These reach the marginal zone of the germinal center and the paracortex that surround the follicle before passing into the lymph sinuses. In the medulla , the lymphocytes (including plasma cells), macrophages, and granulocytes are less closely packed and form medullary cords .
