Lymphatic system The is a drainage system it includes: 1. Lymph vessel. 2. Lymphatic tissue.

Lymphatic organs Primary lymphoid Secondary lymphoid organs eg organs eg

Lymphatic nodule

• There are two types of lymphatic nodules: 1. Primary lymphatic nodules: they are aggregation of small lymphocytes without germinal centers. 2. Secondary lymphatic nodules: each consists of outer dark part and central pale part called “”. The germinal center is the site of active lymphocyte production that appears only after birth in response to antigen exposure. The germinal center contains large lymphocytes, plasmablasts, plasma cells and cells.

Capsule

Trabeculae

Reticular net work Lymph nodes

• Lymph nodes are ovoid or kidney-shaped. • They are distributed throughout the body along the course of the lymphatic vessels. • Lymph nodes have a convex side and concave depression, the hilum, through which arteries and nerves enter and veins and lymphatic vessels leave the node.

Structure: Stroma: 1. - Capsule: each lymph node is surrounded by a dense connective capsule pierced by branches of afferent lymphatic vessels. 2. - Trabeculae: they arise from the capsule and extend for variable distances into the substance of the node. 3. - Fine network of reticular fibers and reticular cells: that extends throughout the node providing a loose, supporting framework for the huge mass of lymphocytes within the lymph node.

Parenchyma: outer cortex and inner medulla. 1. Cortex The cortex is formed of lymphatic tissue. It is divided by the trabeculae into regular compartments. It contains: 1. Lymphatic nodules: They are of secondary nodules and formed mainly of B lymphocytes. Formed of two types 1ry and 2ry. 2. Subcapsular and cortical sinuses: - Subcapsular sinus is a cylindrical channel (marginal) separates the capsule from the cortical parenchyma. - They contain B-lymphocytes, plasma cells and macrophage cells. 2. Medulla 1-Medullary cords: They are cord-like, which are separated by dilated medullary sinuses. The medullary cords are formed of loose lymphatic tissue consist of lymphocytes, plasma cells and embedded in reticular tissue. 2-Medullary sinuses: They are large tortuous channels that branch and anastomose repeatedly. They resemble in structure the cortical sinuses.

• Functions of lymph node: - Filtration of lymph from foreign particles and bacteria - Storage and proliferation of B-lymphocytes and elaboration of antibodies from plasma cells. - Storage and proliferation of lymphocytes.

Lymph nodes

stroma parenchyma

Capsule Reticular network medulla trabeculae Cortex sinuses

sinuses lymphatic nodules Lymphatic cords

subcapsular cortical 2nd 1ry Spleen Structure: Stroma: - Capsule: The spleen is surrounded by a dense fibroelastic connective tissue, which is thickened at the hilum. - Trabeculae: They are thick and irregular connective tissue extend from the capsule and the hilum. - Network of reticular fibers and cells: forming the background of the spleen. Parenchyma: The parenchyma of spleen consists of white and red pulps. White pulps (Malpighian corpuscles):2 They appear white in fresh section due to the presence of lymphocytes. 1. The nodules consist mostly of B- lymphocytes with pale germinal centers. 2. periarterial sheath, which is formed of eccentric central artery surrounded with sheath of T-lymphocytes. : 2 1-Splenic sinuses: - They are irregular branching and anastomosing channels 2- Cords of Billroth or splenic cords: They are formed of lymphocytes, blood cells, macrophages and plasma cells embedded in reticular tissue.

B

T

arteriole

Filteration of blood

Formation of blood elements in the prenatal life.

Removal and destruction of damaged and abnormal erythrocytes and platelets.

Storage and concentration of blood, especially erythrocytes and platelets. Spleen

stroma parenchyma

Capsule Reticular network Red pulp trabeculae sinuses

Periarterial sheath T lymphocytes lymphatic nodules B lymphocytes Billroth cords Tonsils are organs composed of aggregations of lymphatic tissue that are partially encapsulated. According to their location they have 3 types: palatine, pharyngeal and lingual.