Lymph Nodes

Lymph nodes are small encapsulated organs located along the pathway of lymphatic vessels

Lymph nodes are small, bean-shaped, encapsulated lymphatic organs. They range in size from about 1 mm (barely visible with the unaided eye) to about 1 to 2 cm in their longest dimension. Lymph nodes are interposed along lymphatic vessels (Fig. 13.18) and serve as filters through which lymph percolates on its way to the blood vascular system. Although widely distributed throughout the body, they are concentrated in certain regions such as the axilla, groin, and mesenteries.

Two types of lymphatic vessels serve the lymph node:

• Afferent lymphatic vessels convey lymph toward the node and enter it at various points on the convex surface of the capsule.

• Efferent lymphatic vessels convey lymph away from the node and leave at the hilum, a depression on the concave surface of the node that also serves as the entrance and exit for blood vessels and nerves.

Note that activated lymphocytes, which remain in the lymph node to proliferate and differentiate, are carried to the node primarily by blood vessels.

The supporting elements of the lymph node are

• Capsule, composed of dense connective tissue that surrounds the node

• Trabeculae, also composed of dense connective tissue, which extend from the capsule into the substance of the node, forming a gross framework

• Reticular tissue, composed of reticular cells and reticular fibers that form a fine supporting meshwork throughout the remainder of the organ (Fig. 13.19)

The reticular meshwork of lymphatic tissues and organs (except the thymus) consists of cells of mesenchymal origin and reticular fibers and ground substance produced by those cells. The cells of the reticular meshwork appear as stellate or elongated cells with an oval euchromatic nucleus and a small amount of acidophilic cytoplasm. These cells can take up dyes and colloidal materials. Transmission electron microscopy, immuno-cytochemistry, and autoradiography indicate two populations of these cells:

• Reticular cells, indistinguishable from typical fibroblasts. These cells synthesize and secrete collagen (reticular fibers) and the associated ground substance that forms the stroma observed with the light microscope. Elongated cytoplasmic processes of these cells wrap around the bundles of reticular fibers, effectively isolating these structural components from the parenchyma of the lymphatic tissue and organs (Fig. 13.20).

afferent lymphatic vessel capsule deep cortex cortex

germinal center lymphatic nodules medullary sinus subcapsular sinus

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