Cortical Region

• Beneath the capsule and around the trabeculae we find the loose lymphoid tissue forming the subcapsular and peritrabecular sinuses
• The lymph runs inside the sub-capsular and peritrabecular sinuses
• The loose lymphoid tissue presents predominantly reticular cells, reticular fibers and fixed macrophages
• The rest of the cortical region is formed by dense lymphoid tissue with lymphatic nodules that can present a germinal center (central region that is less stained)
• The dense lymphoid tissue is formed by reticular cells, reticular fibers, fixed macrophages and mostly free cells(lymphocytes).

Lymphatic Nodules
• Formed by reticular cells, reticular fibers, fixed macrophages and free cells.
• The germinal center is formed by lymphoblasts, large, medium and small lymphocytes, and plasmacytes in various developmental stages.

Paracortical Region

• Formed by dense lymphoid tissue that is populated by T lymphocytes

Medullar Region

• Presents medullary cords and medullary sinuses
• The medullary cords are formed by dense lymphoid tissue
• • The medullary sinuses are formed by loose lymphoid tissue

Spleen
• Presents a dense connective tissue capsule that presents smooth muscle fibers.
• From this capsule, septa are sent into the organ dividing it into incomplete compartments.
• The parenchyma is divided into the White Pulp and the Red Pulp.

White Pulp

• Formed by dense lymphatic tissue that forms a supporting net of blood vessels.
• It possesses reticular fibers, mesenchymal reticular cells, lymphoblasts and plasma cells.
• In some points of the parenchyma, a network of blood vessels form nodules that contain, internally, an arteriole called the central arteriole, however it is normally observed in an excentric position in histological sections.
• The structure represented by a nodule with an excentric arteriole constitutes the characteristic elements for the histological diagnosis of the organ, the Malpighian corpuscule.
• The dense lymphatic tissue is arranged in cords that are called the Billroth cords.

Red Pulp

• Composed of loose lymphatic tissue that form splenic sinuses that are set around the entire parenchyma surrounding the Malpighian corpuscles and Billroth cords.
• There are also the sinusoid capillaries whose discontinuous walls allow free passage of the blood which will occupy all of the loose lymphatic tissue and therefore in anatomical sections a redish color is observed.
• We also find the penicillate arteries, the reticular fibers and characteristic cells of the loose lymphatic tissue
• It possesses reticular fibers, mesenchymal reticular cells, lymphoblasts, lymphocytes and plasma cells
• The Billroth cords, though formed by dense lymphatic tissue, can also be considered part of the red pulp due to its location.

Thymus
• It has 2 lobes surrounded by a capsule of dense connective tissue that emits septa dividing the organ into lobules, this septation is incomplete, and therefore the separation of the lobules is irregular.
• The lobules present a cortical zone of dense lymphatic tissue and a central zone( medullar) of diffuse lymphatic tissue with mostly young cells (lymphoblasts)
• Lymphoid nodules cannot be found in the thymus, the cortical and medullar region have the same types of cells, however in different amounts, which are: the T lymphocytes in different phases of maturation and reticular epithelial cells.

Cortical Region

• Small lymphocytes predominate.
• Epithelial reticular cells are found in small amounts.

Medullar Region

• We find lymphoblasts, young lymphocytes and epithelial reticular cells.
• We also observe epithelial reticular cells in concentric layers, forming the corpuscles of Hassal.
• We can find at the center of the Hassal Corpuscles, cell debris that are many times calcified

Bursa of Fabricius
• It is a dorsal saccular diverticulum of the proctodeum, exclusive to birds.
• Characterized by tall and thick mucous folds (plicae) filled with numerous polyhedral follicles
• Each follicle, which is composed of lymphatic tissue, is divided into a cortex and medulla.
• A layer of undifferentiated epithelial cells occupies the periphery of the medulla that is separated from the cortex by a capillary layer.
• The bursa is lined by pseudo-stratified cylindrical epithelium, except at the apex of each follicle where it is lined by simple cylindrical epithelium.