Human Anatomy - M.M. Kurepina, A.P. Ozhigova, A.A. Nikitina 2010

Cardiovascular system
Lymphatic system
Lymphoid organs

Lymphoid Organs include Lymph Nodes, Tonsils, solitary and aggregated Cytology/cytology/61.html">Lymphatic follicles of the intestine, and the Spleen.

Lymph nodes (sometimes incorrectly referred to as glands) are accumulations of lymphoid tissue situated along the course of Lymphatic vessels. On the limbs, the nodes are typically located near major joints, while on the trunk, they occur either singly—for example, near the spine or in the mesentery of the Small Intestine—or in clusters. They are particularly numerous in the neck, axilla, groin, near the intestines, etc. (Atl. Figs. 82, 8486). There are no Lymph Nodes in the Skeleton, Bone Marrow, hands, or feet. Humans have approximately 460 lymph nodes.

The nodes are round or oval structures, with a depression on one side (Atl. Fig. 87). This area is called the hilum of the node. Here, Arteries and nerves enter the node, while Veins and efferent lymphatic vessels exit. Afferent lymphatic vessels enter the node from the opposite side. Externally, the nodes are covered by a dense capsule that thickens at the hilum. The capsule is formed by Dense Connective Tissue, dominated by Collagen fibers and containing scattered bundles of smooth Muscle Cells. From the capsule, trabeculae extend deep into the organ, forming septa. Lymphoid tissue lies between them; it is separated from the capsule and trabeculae by spaces called lymphatic sinuses. These include the subcapsular, or marginal, sinus, located between the capsule and the cortex; the trabecular sinuses, between the nodules and trabeculae; and the medullary sinuses, bounded by the trabeculae and medullary cords. On a cross-section of the node, one can distinguish the peripheral cortex, consisting of lymphatic nodules, and the centrally located medulla, formed by medullary cords and sinuses. The paracortical zone lies between them. The latter is populated mainly by T-lymphocytes (T-zone). B-lymphocytes are found in most of the cortex and in the medullary cords (B-zone).

The stroma of the lymph node is composed of reticular tissue. Its fibers and cells form a complex network, in the meshes of which lie lymphocytes, lymphoblasts, macrophages, plasma cells, etc. In the central, pale zone of the cortical lymphatic nodules—the germinal center—lymphocyte proliferation occurs. During intoxication of the body, the macrophages and dendritic cells (fixed macrophages) located here phagocytose Antigens as well as dying or modified cells, and stimulate lymphocyte production. Consequently, the central zone of the nodule increases in size. As the infectious process subsides, the nodule returns to its original appearance. The appearance and disappearance of germinal centers occur within 2–3 days. B-lymphocytes formed in the nodules migrate to the medullary cords, where they differentiate into plasma cells and produce Antibodies. Some lymphocytes differentiate into memory cells and enter the bloodstream with the lymph flow or through the veins.

In the paracortical zone, T-lymphocytes proliferate and differentiate. From here, they enter the bloodstream via venules.

In the reticular tissue of the medulla, specifically within the medullary cords, B-lymphocytes differentiate into plasma cells capable of producing IMMUNOGLOBULINS (antibodies). Externally, the cords are adjacent to the endothelial Cells of the sinusoid walls, which are highly fenestrated. The lymph flowing through the cortical and medullary sinuses is enriched with lymphocytes originating from the nodules, paracortical zone, and medullary cords. Plasma cells, free macrophages, and occasional granular leukocytes may also enter the lymph. Due to the presence of macrophages within the sinus lumens, antigens that have entered the lymph nodes can be trapped here.

Thus, lymph nodes neutralize toxic substances, trap and phagocytose microbes, and filter various suspended particles carried here from the Tissues. Each lymph node or group of nodes collects and monitors lymph from a specific area of the body, serving as its biological filter. Therefore, if a purulent process, such as an abscess, develops on the arm, the cubital and axillary lymph nodes swell first. In pulmonary diseases, the bronchopulmonary lymph nodes enlarge, which can be detected during a chest X-ray.

The spleen (splen) is a reddish-blue hematopoietic organ weighing about 180 g, with a length of 10–15 cm, a width of 7–9 cm, and a thickness of 4–6 cm (Atl. Fig. 88). Its volume and mass depend on its Blood content and functional state. The spleen is located intraperitoneally to the left of The Stomach, in the left hypochondrium, with its convex surface resting against the Inferior surface of the Diaphragm. The hilum of the spleen on its concave surface faces the posterior surface of the stomach. Ligaments connecting the spleen to the stomach, diaphragm, and colon originate from the hilum. Through the hilum, 6–8 Branches of the splenic artery enter the spleen, accompanied by plexuses of the Autonomic Nervous system, while veins and lymphatic vessels exit.

The spleen is covered by a capsule composed of collagen and elastic fibers, containing smooth muscle cells. Trabeculae (septa) extend from the capsule deep into the organ, forming a reticular framework together with the trabeculae originating from the hilum. Branches of the splenic artery run through these connective tissue septa, branching repeatedly and ending in penicillar arterioles equipped with sphincters. The arterioles split into a network of sinusoidal capillaries (see Atl.). Sphincters are located at the junctions where they transition into venules. The degree of splenic blood engorgement depends on the state of the arteriolar and sinusoidal sphincters: when the arterial sphincters relax and the sinusoidal ones contract, the spleen fills with blood, whereas the relaxation of sinusoidal and contraction of arterial sphincters leads to its emptying. This is also facilitated by the contraction of the smooth Muscles of the capsule and trabeculae. Thus, the spleen serves as a blood reservoir during rest. It also stores Blood Cells and platelets, which can enter the bloodstream when needed.

Between the blood capillaries and sinusoids lies the red and white pulp. The white pulp consists of accumulations of lymphocytes in the form of sheaths and lymphoid nodules scattered around small arteries. They serve as sites for lymphocyte development, which enter the bloodstream as they mature. The arteries emerging from the nodules enter the red pulp, where they branch like small brushes. The red pulp occupies all the space between the nodules, sinusoids, and trabeculae. It consists of reticular tissue containing scattered free blood cells and connective tissue cells. Its function is to destroy worn-out blood cells, while antibodies produced by lymphocytes neutralize antigens that have entered the blood. The red pulp also participates in blood storage. In certain diseases, hematopoiesis can be reactivated in the spleen of an adult.

The Thymus gland (thymus) is located behind the Sternum, in the upper part of the anterior Mediastinum, overlying the Trachea, Pericardium, and great vessels. The lateral portions of the gland border the mediastinal Pleura.

The gland consists of asymmetric right and left lobes connected by loose connective tissue. The lobes are covered by a connective tissue capsule and are surrounded externally by adipose and loose connective tissue, which anchor the gland to adjacent organs. Anterior mediastinal lymph nodes are embedded within the gland. Connective tissue septa extend from the capsule deep into the gland, dividing it into lobules. Each lobule features a lighter, centrally located medulla surrounded by an outer cortex.

The stroma of the gland is composed of modified Epithelial Tissue; its cells (epithelial reticular cells) are interconnected by processes to form a network, within the meshes of which lymphocytes reside. Stromal cells of the thymic cortex secrete thymosin, which stimulates the division of lymphoblasts—the precursors of T-lymphocytes. The final maturation of T-lymphocytes occurs in the peripheral lymphoid organs, where they are delivered by the bloodstream. T-lymphocytes differentiating in the cortex are separated from the blood by a specialized barrier. It is formed by endothelial cells surrounded by a basement membrane, perivascular space, and epithelial reticular cells.

The thymic medulla contains fewer lymphocytes than the cortex. It features unique thymic corpuscles, formed by concentrically arranged epithelial reticular cells containing numerous vacuoles and keratin granules in their Cytoplasm. Their number increases with age. No blood-thymus barrier is formed around the capillaries in the medulla.

The Blood supply to the thymus is provided by branches of the internal thoracic, thyroid, and pericardial arteries. Beneath the capsule, the arteries branch into interlobular and intralobular vessels. Blood capillaries form a dense network, especially in the cortex, and then drain into subcapsular venules. Lymphatic capillaries lie beneath the capsule and in the deep PARTS OF THE thymus. Their network is particularly well-developed in the cortex. Lymphatic vessels are located within the interlobular septa.

Lymphatic vessels appear during Embryogenesis when large Blood Vessels are already formed—at the end of the 6th week of intrauterine development. Initially, blind-ended sacs are formed, which expand, produce numerous outgrowths, and open into the veins. These outgrowths subsequently form lymphatic vessels, along the course of which lymph nodes develop at the 16th–20th week of intrauterine development. In newborns, lymph nodes in the pharyngeal region are poorly developed but are already capable of performing protective Functions against harmful substances entering with food and air. During childhood and adolescence, enhanced differentiation of lymphoid tissue occurs under the Influence of Environmental factors: The Immune System and lymphatic organs develop intensively, primarily the lymph nodes in the walls of the digestive tract, respiratory and urinary tracts, and the tonsils.

The gland develops from the outgrowths of the III and IV pharyngeal pouches at the end of the 1st month of intrauterine development. Subsequently, the thymus detaches from the Pharynx, and its right and left primordia approach each other and fuse. Initially, the thymus is composed of epithelial tissue of endodermal origin. Later, it begins to be colonized by mesenchymal and lymphoid cells, which proliferate rapidly. During the 3rd month, the thymus differentiates into medullary and cortical parts, developing a Structure characteristic of lymphoid organs. The epithelial cells separate but remain interconnected by their processes, forming a reticular network. Hassall's corpuscles appear in the medulla. T-lymphocytes formed in the thymus colonize peripheral lymphoid organs. In newborns, the thymus has its greatest relative weight and size. The gland reaches its maximum absolute weight (25–30 g) by 13–14 years of age, after which its development ceases. With age, it undergoes significant involution, being replaced by loose connective and adipose tissue. The cortex undergoes the most dramatic changes.

Review Questions

1. What are the general structural patterns of The Lymphatic System?

2. How does Lymph formation occur?

3. Describe The structure of the wall of lymphatic capillaries, vessels, and ducts.

4. Lymph nodes: their structure and distribution in The Human Body.

5. The spleen: its structure, topography, blood supply, and functions.

6. The thymus: topography and structural features.