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Lymphoid tissue


The lymphatic system forms the immunological fundament for the body's defence system. A circulating pool of lymphocytes ensures a continuous immunologic surveillance of the entire body in order to protect against harmful invasion. These cells circulate between various lymphoid tissues via lymph and bloodstream. When certain lymphoid cells encounter their determined target antigens, these cells return to relevant lymphatic organ or tissue to differentiate (specialise) and proliferate (multiply).

The lymphoid tissues are here represented by:

  • The Lymph node structure in pigs is inverted compared to most mammals, with loose medullary tissue surrouding the germinal centers that are in general located further down in the structure.
  • Tonsil looks similar in pig and human, covered by stratified squamous epithelium which forms deep irregular invaginations into the tonsils.
  • Thymus is proportionally larger in pig than in human
  • Spleen looks slighlty different, with more connective tissue in pig compared to human spleen.

Primary lymph follicles appear as homogeneous aggregations of darkly stained small lymphocytes, primarely B-cells. What separates the secondary lymph follicles from the primary is the presence of a lighter spherical area inside the aggregated darkly stained lymphocyte sphere. This area is called a germinal center and represents proliferating B-cells. Secondary lymph follicle germinal centers are surrounded by whats known as mantle zone, where the primary follicles memory and resting B-cells are found. Surrounding the mantle zone is the marginal zone which is where you find most antigen-presenting cells (APCs), such as dendritic cells and macrophages.

The thymus is located beneath the sternum and consists of two lobes surrounded by a fibrous capsule. The lobes are divided by a fine septa into many lobules, with an outer cortex with high cellular density, and an inner medulla with lower cellular density. Hassall’s corpuscles are structures only found in the thymic medulla which increase in number throughout life. These are non-secreting flattened thymic epithelial cells in a whorl-like formation arranged in concentric layers.

The spleen consists of a red pulp and a white pulp within a meshwork of fibers enclosed by a dense connective tissue capsule. The splenic lymphoid follicles have an appearance similar to lymphoid follicles in other lymphoid tissues, except for the presence of a central artery.The main task of the red pulp is to filter blood to find defective and old erythrocytes as well as select them for breakdown. The red pulp also acts as a reservoir for monocytes, stored inside the splenic cords, which can rapidly be mobilized to leave the spleen and assist in an immunological response. Blood vessels, lymphatic vessels and nerves enter and leave the spleen through the hilum, which is an opening in the capsule. From the capsule, connective tissue called trabeculae extends into the organ and acts as a support for blood vessels. The trabeculae form a path for branches of the splenic artery, from which smaller arteries (central arteries) branch off to enter the pulp.