Lymphatic Organs: Microscopic Structure

Lymphatic Organs: Microscopic Structure

• Objectives: Understand the location of lymphatic organs, and the microscopic features of lymph nodes, spleen, thymus, and tonsils.

Lymph Node

• Stroma: Covered by a capsule, which extends inward to form trabeculae.
• Parenchyma: Divided into outer cortex, paracortex, and inner medulla.
• Cellular Components: Reticular cells, plasma cells, dendritic cells, macrophages, and lymphocytes of all types are abundant in lymph nodes.

Lymph Node: Outer Cortex

• Subcapsular Sinus: A sinus running between lymphoid nodules, arising from the subcapsular sinuses.
• Lymphoid Nodules: Can have or lack a germinal center, composed mostly of developing B lymphocytes and few helper T lymphocytes.

Lymph Node: Paracortex

• Lymphoid Nodules: Lacking lymphoid nodules, and an accumulation of T cells.

Lymph Node: Inner Medulla

• Medullary Cords: Composed of B lymphocytes, plasma cells, and macrophages.
• Medullary Sinus: Containing lymph, lymphocytes, and often many macrophages.

Spleen

• Stroma (Connective Tissue): Covered by a capsule with trabeculae, which partially subdivide the parenchyma. Trabeculae include trabecular arteries and veins.
• Parenchyma (Splenic Pulp): Lacks cortex and medulla, instead composed of white pulp and red pulp.

Spleen: White Pulp

• Lymphatic Nodules: Consists of lymphatic nodules with germinal centers.
• Central Artery: Passes through the lymphatic nodules, surrounded by PALS (periarteriolar lymphoid sheaths) consisting mostly of T cells.

Spleen: Red Pulp

• Splenic Cords of Billroth: Composed of reticular tissue with T and B lymphocytes, macrophages and red blood cells.
• Splenic Sinusoids: Blood vessels, where blood cells—of all types—are located within both cords and sinuses.

Thymus

• Stroma: Covered by a capsule which extends into trabeculae, creating incomplete lobules.
• Parenchyma: Each lobule is composed of an outer cortex and an inner medulla.

Thymus: Cortex

• Cellular Components: Contains more densely packed T lymphoblasts (thymocytes) and macrophages.
• Reticular Cells: Fewer epithelial reticular cells forms a cytoreticulum.
• Lymphatic Nodules: No lymphatic nodules.

Thymus: Medulla

• Cellular Components: Contains epithelial reticular cells.
• T Lymphocytes: Contains fewer densely packed differentiated T lymphocytes.
• Epithelial Reticular Cells: More epithelial reticular cells.
• Thymic Corpuscles (Hassall's Corpuscles): Oval structures made up of round whorls of flattened epithelial reticular cells.

Tonsils

• Structure: Masses of lymphoid nodules in pharynx:
  • Palatine tonsils
  • Lingual tonsils
  • Pharyngeal tonsils (adenoids)
• Palatine and Lingual Tonsils: Covered with Stratified squamous nonkeratinized epithelium containing crypts.
• Pharyngeal Tonsils: Covered by ciliated pseudostratified columnar epithelium, lacking crypts.
• Cellular Composition: All tonsillar epithelia have intraepithelial lymphocytes and dendritic cells.
• Additional Palatine Tonsil Features: The tonsils are invaginated by deep grooves called tonsillar crypts, and lymphoid tissue below the epithelium contains lymphocytes and secondary lymphoid nodules.
• Additional Lingual Tonsil Features: Stratified squamous epithelium and crypts; similar lymphoid tissue makeup to palatine tonsils.