Human Anatomy - Kotsan I. Y. 2009

The Lymphatic System
Lymphatic vessels and regional lymph nodes of specific body regions
The lymphatic system in phylo- and ontogenesis

The Development of The Lymphatic system in phylogeny is closely intertwined with the Evolution of the Circulatory system, which is largely driven by the adaptation of respiratory Organs to the environment.

In aquatic animals (such as fish) that breathe through gills and possess a two-chambered venous heart, Lymph is propelled by a lymphatic heart—a pulsating dilation of a lymphatic vessel that pumps lymph into the Venous system. Lymph Nodes are not yet present, and the lymphatic tissue is diffuse in nature.

In amphibians, the number of lymphatic hearts increases, and they are located in pairs at the junction of the trunk and limbs (anterior and posterior pairs).

In reptiles, as gills are completely replaced by Lungs and a pulmonary circuit develops alongside the systemic Circulation, new mechanical factors facilitating lymph flow emerge (namely, Cardiac Activity). As a result, the functional importance of lymphatic hearts diminishes and they begin to regress, leaving only a single (posterior) pair intact. Concurrently, the overall number of Lymphatic vessels increases.

In birds, the regression of lymphatic hearts continues while the network of lymphatic vessels expands further, and several discrete lymph nodes begin to form.

In mammals, the appearance of the muscular Diaphragm, coupled with the further Development of the heart, Blood Vessels, and skeletal musculature, facilitates lymph flow even more. The Need for lymphatic hearts disappears entirely, leading to their complete reduction. At the same time, the number of lymphatic vessels increases, developing numerous Valves. Lymphatic pathways along the aorta converge into a large unpaired trunk known as the Thoracic duct. The number of lymph nodes increases sharply, especially in primates.

In humans, bipedalism has led to an increased number of valves in the lymphatic Vessels of the limbs—particularly the lower extremities—as well as the highest concentration of lymph nodes. This highlights the growing Significance of the lymphatic system's barrier function in limiting the spread of pathological processes.

Thus, the major evolutionary transformations of the lymphatic system boil down to two main trends: on the one hand, the disappearance of lymphatic hearts, and on the other, the emergence and proliferation of lymph nodes.

Regarding the embryonic development of the lymphatic system, the majority of authors support the theory that it develops independently of the circulatory system, establishing its connection with the venous system secondarily. The lymphatic system originates as isolated primordia that grow, branch, and form channels—namely, lymphatic vessels. By expanding and fusing, these vessels form six lymph sacs at specific sites during the second month of embryonic development: two near the jugular Veins, one retroperitoneal (located near the Base of the mesentery and the Adrenal Glands), one adjacent to the latter (the cisterna chyli), and two near the iliac veins.

The jugular sacs give rise to the lymphatic vessels of the HEAD, neck, and upper limb (with the latter originating from secondary sacs that form near the subclavian veins). The retroperitoneal sac produces the mesenteric vessels that drain the intestines, while the iliac sacs form the vessels of the lower limbs and pelvis. Additionally, the jugular sacs expand toward the thoracic cavity and fuse into a single trunk, which meets the expanding cisterna chyli. This union forms the thoracic duct, which links the systems of the iliac, retroperitoneal, and jugular sacs into a unified whole. Thus, a single system of lymphatic vessels is established, connecting with the veins exclusively in the region of the jugular sacs, near the junction of the jugular and subclavian veins on both sides of the body.

Later on, the initial symmetrical Structure OF THE lymphatic system is disrupted because the left duct (the thoracic duct) achieves greater development. The prominent development of the left lymphatic duct compared to the right is explained by the asymmetrical position of The Heart and large veins. As a result, on the left side in the region of the left venous angle, more favorable hemodynamic conditions are created for the flow of both lymph and blood. Conversely, on the right side, due to the proximity of the venous half of the heart, the periodic pressure surges in the SUPERIOR VENA CAVA—driven by cardiac contractions—are felt more strongly, which impedes the smooth entry of the lymph stream into the venous blood flow. This functional disparity in lymph circulation conditions for the right and left main lymphatic trunks also accounts for their asymmetric development. As an anatomical variant, a double thoracic duct sometimes persists, which is the rule in lower vertebrates.

Lymph nodes develop from the mesenchyme surrounding the forming plexuses of Blood and Lymphatic vessels, starting around the 5th to 6th week of embryonic life. The primordia of Lymph Nodes in various parts of The Human Body form at different times, continuing right up to birth and even postnatally.

During node development, clusters of mesenchymal Cells invaginate into the lumen of the adjacent lymphatic vessel. The lumen of this vessel later transforms into the subcapsular sinus. Intermediate sinuses develop on The basis of a branching lymphatic plexus, between the vessels of which cords of embryonic Connective Tissue grow. Starting from the 19th week, a distinct boundary between the cortex and medulla can be observed in individual lymph nodes. Lymphoid follicles within the lymph nodes begin to form during the prenatal period. The primary age-related morphogenetic processes in lymph nodes are completed by 10–12 years of age. Involutionary age-related changes (such as a decrease in lymphoid tissue and proliferation of adipose tissue) in the lymph nodes are observed as early as adolescence. Connective tissue proliferates within the stroma and parenchyma of the nodes, and clusters of fat cells appear, while The amount of lymphoid parenchyma concurrently decreases. With age, the number of lymph nodes in regional groups also declines. Many small lymph nodes are replaced by connective tissue and cease to exist as organs of The Immune System. Adjacent lymph nodes, most frequently medium-sized ones, fuse together to form larger segmental or ribbon-shaped nodes.