thymus, tonsils & macrophages

Thymus Gland Definition

Primary lymphoid organ with endocrine function located in the anterior mediastinum.

Thymus Development Origin

Lymphocytes are Mesodermal

Thymus Structure Differences

No lymph follicles, no B-lymphocytes, no plasma cells, no afferent lymphatics.

Thymus Capsule

Thin connective tissue sending trabeculae that divide lobes into incomplete lobules.

Thymus Cortex

Outer dark zone densely packed with young T-lymphocytes (thymocytes) and macrophages.

Thymus Medulla

Inner pale zone with fewer lymphocytes and more reticular epithelial cells.

Reticular Epithelial Cells (REC)

Large branching cells attached by desmosomes forming the framework of the thymus (instead of reticular fibers).

REC Function

Secrete thymic hormones (thymosin) and create a microenvironment for T-cell maturation.

Hassall's Corpuscles

Concentric acidophilic lamellae of degenerating reticular epithelial cells found only in the Thymus Medulla.

Blood-Thymic Barrier Function

Prevents circulating antigens from reaching developing T-lymphocytes in the cortex.

Blood-Thymic Barrier Layers

Capillary endothelium -> Thick basement membrane -> Perivascular space (macrophages) -> REC basement membrane -> REC sheath.

Thymic Education

Process where T-cells develop markers and undergo selection

DiGeorge Syndrome

Congenital failure of the 3rd/4th pharyngeal pouches leading to thymic hypoplasia and depressed cell-mediated immunity.

Tonsils Definition

Partially encapsulated masses of lymphoid tissue beneath the epithelium of the aerodigestive tract.

Palatine Tonsils Site

Lateral walls of the oropharynx.

Palatine Tonsils Epithelium

Stratified squamous non-keratinized epithelium.

Palatine Tonsils Crypts

Deep invaginations of epithelium (10-20) into the lymphoid tissue.

Palatine Tonsils Capsule

Incomplete fibrous capsule separating the tonsil from underlying muscle.

Lingual Tonsils Site

Base of the tongue (posterior 1/3).

Lingual Tonsils Epithelium

Stratified squamous non-keratinized epithelium.

Lingual Tonsils Crypts

Each nodule has a single, non-branched crypt.

Salivary Glands in Lingual Tonsil

Ducts of mucous glands open directly into the base of the crypts to flush them.

Pharyngeal Tonsil (Adenoid) Site

Roof/Posterior wall of the nasopharynx.

Pharyngeal Tonsil Epithelium

Pseudostratified ciliated columnar with goblet cells (Respiratory epithelium).

Pharyngeal Tonsil Crypts

No true crypts

Pharyngeal Tonsil Capsule

Incomplete thin capsule

Macrophage System (MPS)

Group of phagocytic cells widely distributed in the body derived from blood monocytes.

Macrophage Morphology

Irregular outline (pseudopodia), kidney-shaped eccentric nucleus, rich in lysosomes.

Vital Stain for Macrophages

Trypan Blue or Indian Ink (taken up by phagocytosis).

Histochemical Stain for Macrophages

Acid Phosphatase (stains lysosomes black).

Macrophage Function - Phagocytosis

Engulfment of dead cells, bacteria, and foreign particles.

Macrophage Function - Immunity

Antigen processing and presentation to T-lymphocytes.

Macrophage Function - Secretion

Secrete Interleukin-1 to regulate lymphocyte proliferation.

Histiocytes

Macrophages found in loose connective tissue.

Von Kupffer Cells

Macrophages found in the Liver.

Littoral Cells

Macrophages found in the Spleen and Bone Marrow sinuses.

Langerhans Cells

Antigen-presenting cells found in the Skin.

Microglia

Macrophages found in the Central Nervous System (CNS).

Osteoclasts

Multinucleated macrophages found in Bone (resorption).

Dust Cells

Alveolar macrophages found in the Lung.

Heart Failure Cells

Hemosiderin-laden macrophages in the lung due to pulmonary congestion.

Monocytes

Precursors of the macrophage system found in circulating blood.

Thymic Involution

Process where thymus decreases in size and becomes fibrotic/fatty after puberty.

Primary Lymphoid Organ Function

Production and maturation of lymphocytes (antigen-independent).

Secondary Lymphoid Organ Function

Activation of lymphocytes by antigens (antigen-dependent).

Diffuse Lymphatic Tissue

Random arrangement of lymphocytes in mucosa (MALT).

Lymphatic Nodules

Spherical aggregations of lymphocytes found in tonsils and lymph nodes.

Epithelial Reticular Cells vs Reticular Fibers

Thymus has cellular reticulum (REC)

Thymus-Dependent Zone (Node)

Paracortex.

Thymus-Dependent Zone (Spleen)

Peri-Arteriolar Lymphatic Sheath (PALS).