Special Histology and Embryology: Practical Course - V. K. Napkhanyuk 2001

Organs of Hematopoiesis and Immune Defense
Lymph Nodes

Lymph Nodes (nodi lymphatici) are bean-shaped structures 0.5-1 cm in size, located along the course of Lymphatic vessels. They serve as Organs of lymphocytopoiesis, immune defense, and the pooling of circulating Blood.

The area where Arteries and nerves enter the lymph node and efferent lymphatic vessels exit is called the hilum of the lymph node. Afferent lymphatic vessels enter from the opposite, convex side of the node.

Externally, the node is covered by a Connective Tissue capsule that is denser in the region of the hilum. The capsule contains numerous Collagen fibers and few elastic fibers; additionally, isolated bundles of smooth Muscle fibers are located in the hilar region.

Inward from the capsule, at relatively regular intervals, thin connective tissue septa—trabeculae—extend and interconnect in the deeper PARTS OF THE node.

A lymph node consists of a denser cortex containing lymphoid nodules; a paracortical (diffuse) zone; and a lighter medulla formed by medullary cords.

The cortex is formed by lymphatic follicles (nodules) located beneath the capsule. These are rounded clusters of B-lymphocytes, lymphoblasts, macrophages, and their variant—dendritic Cells, which are capable of fixing antibody-antigen complexes on their surface.

The framework of the follicle consists of reticular connective tissue (a delicate stroma). Externally, the follicle is surrounded by reticuloendotheliocytes, which are reticular cells in Structure, but function

—as endothelial cells, because they line the sinuses of the lymph nodes. Among the reticuloendotheliocytes, There is a significant number of fixed macrophages, the so-called littoral cells.

Each follicle features a light reactive, or germinal, center where lymphocyte proliferation takes place and which contains predominantly B-lymphoblasts, surrounded by a dark peripheral zone densely packed with small and medium lymphocytes.

The paracortical zone lies between the cortex and the medulla. It is a diffuse accumulation of T-lymphocytes and macrophages. Macrophages within the paracortical zone are represented by their specialized variant—interdigitating cells—which contact one another via finger-like processes. They produce BIOLOGICALLY ACTIVE SUBSTANCES that stimulate the proliferation of T-lymphocytes.

The medulla of the lymph node is formed by medullary cords (chordae medullaria) along with surrounding trabeculae and sinuses, as well as band-like clusters of B-lymphocytes, plasma cells, and macrophages extending from the cortex to the hilum of the node.

Externally, the medullary cords are covered by endothelial-like reticular cells that lie on bundles of reticular fibrils, forming the wall of the sinuses.

Sinuses are slit-like spaces bounded by the capsule and trabeculae on one side, and by nodules and medullary cords on the other.

Sinuses essentially act as continuations of the afferent lymphatic vessels. Depending on their localization, several types of sinuses are distinguished:

— subcapsular, or marginal, sinus (sinus subcapsularis), located between the capsule and the nodules;

— perinodular, or perifollicular, sinus (sinus corticalis perinodullaris), situated between the nodules and trabeculae;

— medullary sinus (sinus medullaris), bounded by trabeculae and medullary cords;

— hilar sinus (sinus portae), located in the region of the hilum.

The sinus system facilitates the Circulation of lymph from the marginal sinus, where afferent lymphatic vessels empty, to the hilar sinus, from which lymph drains via the efferent lymphatic vessel system. This process ensures the purification of lymph through the phagocytosis of foreign particles by littoral macrophages, as well as the enrichment of lymph with immunocompetent T- and B-lymphocytes, memory cells, and IMMUNOGLOBULINS.

Slide 3. Cross-section of a lymph node (Figs. 31, 32).

Low magnification. Examine the slide.

Locate the pink-stained capsule of the node, which consists of Fibrous connective tissue. Beneath the capsule, the cortex is visible, containing lymphoid nodules (follicles). These are rounded blue structures. In the center of the nodule lies a lighter germinal center. Connective tissue trabeculae extend from the capsule into the parenchyma. The central part of the node is occupied by its medulla, which contains blue medullary cords.

High magnification. It is clearly visible that the lymphoid nodules and medullary cords contain numerous lymphocytes that mask the reticular tissue, making it invisible within the lymphoid nodules and cords. The reticular tissue is clearly noticeable in the sinuses of the node. The subcapsular sinus is located between the capsule and the nodules, while perinodular sinuses lie between the trabeculae and the nodules. In the medulla, wide medullary sinuses are visible between the medullary cords.

Label the following on the diagram: 1) capsule of the lymph node; 2) trabecula of the lymph node; 3) marginal sinus; 4) lymphatic nodule (follicle); 5) germinal center; 6) Blood Vessels; 7) medullary cords; 8) medullary sinus; 9) reticular tissue.

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Fig. 31. Section of a lymph node. Cortex of the lymph node. Hematoxylin and eosin staining. x 120:

1 — lymph node capsule; 2 — lymph node trabecula; 3 — marginal sinus; 4 — lymphatic nodule (follicle); 5 — germinal center

Fig. 32. Medulla of the lymph node. Fragment of the previous slide. Hematoxylin and eosin staining. x 400:

1 — trabeculae of the lymph node; 2 — blood vessel; 3 — medullary cord; 4 — medullary sinus; 5 — reticular tissue