Histology of Lymphatic Organs

• This lecture covers the histological structure of lymphatic organs.
• The objectives are to identify the components of lymphatic organs and understand the importance of lymphatic organs in the immunological system.
• Lymphatic organs are important in the immunological system and protect the body against microorganisms.
• Lymphatic organs include the lymph node, thymus gland, spleen, and tonsils.

Lymphocytes

• Large aggregation of lymphocytes can be found in two forms: diffuse lymphocytes and lymphatic nodules.
• Diffuse lymphocytes, also known as lymphatic tissue, mainly consist of lymphocytes, tissue macrophages, plasma, and reticular cells.
• Lymphatic nodules contain lymphocytes arranged in spherical structures or nodules.
• Lymphatic nodules consist of an outer dark region and an inner light center, known as the germinal center.

Lymph Nodes

• The lymph node is surrounded by a connective tissue capsule, with septa or trabeculae, and is divided into the cortex and medulla.
• The cortex contains lymphatic nodules.
• In the medulla, lymphocytes form cords, which are branches and anastomoses separated by medullary sinuses.

Spleen

• The spleen is covered by a capsule of dense connective tissue with smooth muscles.
• The capsule sends septa, also containing smooth muscles, that divide the spleen into incomplete compartments.
• The parenchyma of the spleen is divided into two main parts: white pulp and red pulp.
• White pulp contains lymphatic nodules enclosed by a central artery.
• Red pulp is composed of blood sinusoids.

Tonsils

• Tonsils consist of epithelium (respiratory or stratified squamous) underlined by connective tissue lamina propria, which contains lymphatic tissue and lymphatic nodules.
• The epithelium invaginates deep, forming crypts.

Thymus Gland

• The thymus gland is covered by a connective tissue capsule with septa that divide it into lobules, each with an outer cortex and medulla.
• The cortex contains dark-stained lymphocytes and an extensive population of lymphocytes supported by connective tissue.
• The medulla contains light-stained lymphocytes and consists of several lobules connected together.
• The medulla contains a characteristic structure known as Hassel's or thymic corpuscles, which are acidophilic and have no known function, and no lymphatic nodules.
• The main signs of involution of the thymus gland: the cortex and medulla are destroyed, the number of lymphocytes in the cortex is depleted, the number of Hassel's corpuscles is increased, and the gland is invaded by a large amount of adipose tissue.