histological structure of lymphoid organs

HISTOLOGICAL STRUCTURE OF LYMPHOID ORGANSDR RANIA GABR

OBJECTIVES

Understand the location of lymphatic organs.

Discuss the microscopic features of Lymph Node.

Discuss the microscopic features of Spleen.

Discuss the microscopic features of Thymus.

Discuss the microscopic features of Tonsils

LYMPHOID TISSUE

Lymphoid tissue is connective tissue chch by rich supply of lymphocytes.

It is found either 1- Free in regular CT

2-Surrounded by capsules, forming the “lymphoid organs” Very little cytoplasm so stain dark blue with

H&E. Rich network of reticular fibrils produced by

fibroblasts.

LYMPHOID SYSTEM BASICS Two main tissue architecture types:

Diffuse: uniform appearance Follicular: consists of lymphoid follicles

Two types of lymphoid tissues:Encapsulated: connective tissue capsule

spleen, thymus, lymph nodesUnencapsulated (or partly

encapsulated)Tonsils, Peyer’s patches, lymphoid nodules in GI tract, respiratory tract, urinary & reproductive tracts

2 TYPES OF LYMPHOID ORGANS

Central (primary) lymphoid organ: where lymphoid cells undergo maturation

T cells in thymus B cells in bone marrow

Peripheral (secondary) lymphoid organ: where functional lymphocytes go including:

1- lymph nodes 2- spleen,3- Peyer’s patches, 3- lymphoid nodules of GI and other tracts

LYMPHOID FOLLICLES

Nodules of densely packed lymphocytes located in all peripheral lymphoid tissues. Most lymphocytes are B cells.

Two distinct areas:1- Mantle – darker stained, mainly small, resting lymphocytes 2- Germinal center – (defines “secondary” or “reactive” lymphoid follicles): lighter stained, larger, activated B cells

Lymph follicle:

- Mantle = cap (dark)- Germinal center

(light)

LYMPH NODES

Present throughout the body, along lymph vessels

Numerous in axilla, groin, cervical area and thoracic/abdominal mesenteries

Filter lymph before it returns to vasculature

Hilum: concave side, arteries, nerves enter; veins and efferent lymph vessels leave the organ

Afferent lymph vessels enter convex surface

• Covered by a capsule which extends to form Trabeculae.

• Divided into outer cortex and inner medulla.• OUTER CORTEX contains:

Lymphatic nodules with germinal center• INNER MEDULA contains:

Medullary Cords and Medullary Sinus

MEDULLARY CORDS

Are branched, cordlike extensions of lymphoid tissue arising from the paracortex. They contain primarily B- lymphocytes and often plasma cells and macrophages.

Medullary cords are separated by dilated spaces , frequently bridged by reticular cells and fibers , called Medullary sinuses

They contain lymph , lymphocytes, macrophages, sometimes granulocytes if the lymph node is draining an infected organ

Lymph node

CT --- Connective tissueC --- CortexM --- MedullaP --- ParacortexLN --- Lymph NodeT ---TrabeculaeMS --- Medullary SinusMC --- Medullary Cords