1. THYMUS

The thymus is surrounded by a capsule of connective tissue continuous with thin septa that subdivide the organ into separated lobules. The thymic cortex consists mainly of an epithelial reticulum and lymphocytes. Because it contains a greater number of lymphocytes than the medulla, the cortex always stains much darker. There are not lymphoid follicles. Hassall’s corpuscles are present in the thymic medulla: consist typically of one or several central calcified or degenerate large cells, surrounded by flat keratinized cells in a concentric arrangement.

Thymic lobules



Thymic cortex and medulla



Hassall's corpuscle

2. CLOACAL BURSA OF FABRICIUS

The cloacal bursa is an oval sacculated organ dorsal to the cloaca. The mucosal wall is thrown into folds covered by a pseudostratified columnar epithelium continuous with that of the cloaca. Each fold consists of numerous lymphoid follicles composed by B- lymphocytes.

3. TONSILS

Tonsils are aggregates of lymphoid nodules in the subepithelial connective tissue of the mucous membrane of the tongue, pharynx and the caudal oral cavity. Tonsils have a relatively smooth surface or deep surface invaginations called tonsillar fossulae or crypts. Primary and secondary (showing germinal center) lymphoid follicles are present depending upon the activity of the immune response. Glands associated with the tonsils are typically mucous (seromucous in carnivores).

4. PEYER’S PATCHES

Small intestine
The intestinal wall contains aggregated lymphoid follicles and diffuse lymphoid tissue that tend to be most conspicuous in the ileum. Germinal centers (in which there is a marked lymphoblastic mitotic activity) may be present in the lymphoid follicles depending upon the activity of the immune response. Diffuse lymphoid tissue consists of T-lymphocytes.

5. LYMPH NODE

Lymph nodes are surrounded by a connective capsule continuous with trabeculae into the parenchyma forming irregular septa. The outer cortex consists of primary and secondary lymphoid follicles. The deeper cortex (paracortical area) is a thymus-dependent zone (T-lymphocytes). The medulla consists of medullary cords (composed of plasma cells, B-lymphocytes, and macrophages), and medullary sinuses lined with endothelial cells on a reticular framework with attached macrophages.

Cortex




Medulla

6. LYMPH NODE (PIG)

Pig lymph nodes are markedly different from those of domestic mammals. The cortical and medullary tissues are reversed, with most nodules occupying a deep position along trabecular sinuses. The periphery is occupied primarily by a loose unorganized tissue containing macrophages and plasma cells.

7. SPLEEN

The spleen is surrounded by a thick connective tissue capsule with variable (species dependent) amount of smooth muscle. Trabeculae composed of collagen and elastic fibers and smooth muscle cells extend from the capsule. The capsule, trabeculae, and reticular fibers support the splenic parenchyma composed of red and white pulp.

White pulp is the lymphoid tissue distributed throughout the spleen as lymphoid follicles (B-lymphocytes) and periarterial lymphoid sheaths (T-lymphocytes). A central arteriole (usually eccentric) is always identified. The red pulp is composed primarily of splenic sinuses and splenic cords.

A network of fine reticular fibers provides a supportive framework for the diffuse and nodular lymphoid tissue.