Histology and Embryology of the Hematopoietic and Lymphoid Organs

Vocabulary flashcards covering the anatomical organization, embryological development, cell lineages, histology, and physiological mechanisms of the primary and secondary hematopoietic organs (thymus, bone marrow, lymph nodes, MALT, and spleen).

Thymus

A primary (central) lymphoepithelial organ located in the upper anterior mediastinum, weighing $10\text{ to }15\,\text{g}$ at birth and $30\text{ to }40\,\text{g}$ at puberty, where immature prothymocytes and naive T lymphocytes differentiate into mature immunocompetent cells.

Thymic Nurse Cells

Specialized cortical reticular epithelial cells whose plasma membrane extensions form pockets that envelop clusters of dividing thymocytes to support their maturation.

Hassall's Corpuscles

Concentric, onion-like structures located in the thymic medulla formed by degenerating epithelial cells in three stages (nucleated squamous, flattened with pyknotic nuclei, and keratinized) surrounding central cellular debris, representing the final stage and cemetery of epithelial cells.

Thymosine

A thymic polypeptide hormone (including fractions TF5, $\alpha 1$, and $\beta 4$) that induces T-cell differentiation, stimulates immune functions, promotes the release of hormones (ACTH, GH, PRL, glucocorticoids, LH-RH), and in the $\alpha 1$ form prolongs survival in lung cancer when combined with radiotherapy.

Thymuline

A thymic peptide composed of $9$ amino acids that induces lymphocyte proliferation and acts on the thyroid axis.

Humoral Thymic Factor

A thymic polypeptide hormone responsible for inducing the graft-versus-host reaction.

Thymopoietin

A thymic polypeptide hormone produced by reticular cells that acts directly on neuromuscular transmission.

Thymic Involution

The progressive physiological regression of the thymus following puberty, characterized by replacement of the lymphoid parenchyma with adipose tissue while remnants of lymphoid tissue and Hassall's corpuscles persist in the medulla.

Hematopoiesis

The physiological process encompassing the successive stages of differentiation and maturation of the formed elements of blood (erythrocytes, leukocytes, and platelets) from pluripotent stem cells.

Blood Islands of Wolff and Pander

Cellular clusters appearing within the extra-embryonic mesenchyme near the yolk sac during the mesoblastic (pre-hepatic) period between the 3rd week and the end of the 2nd month of intrauterine life.

Hepatosplenic Period

The second embryonic stage of hematopoiesis occurring from the 3rd to the 8th month of intrauterine life, during which blood cell production takes place predominantly in the liver and spleen.

Red Hematogenous Bone Marrow

Active hematopoietic tissue located in spongy bone cavities (sternum, vertebrae, ribs, ilia, skull, proximal epiphyses of the femur and humerus) that serves as the exclusive site of physiological hematopoiesis after birth.

Yellow Bone Marrow

Inactive marrow located within the medullary cavities of adult long bones where hematopoietic tissue has been replaced by adipocytes.

Gray (Gelatinous) Bone Marrow

A degenerate fibrous marrow observed in elderly adults, where yellow marrow is replaced by fibrosis.

Megakaryocyte

A giant bone marrow cell characterized by a large, multilobulated (budding) nucleus that undergoes fragmentation to produce blood platelets (thrombocytes).

Bone Marrow Sinusoids

Specialized capillary vessels within the medullary parenchyma that feature a discontinuous endothelial lining and lack a basement membrane, facilitating the passage of mature blood cells into the bloodstream.

Leukemia

A cancer of the bone marrow cells characterized by abnormal and excessive proliferation of white blood cell precursors arrested at an immature stage of differentiation, which infiltrate the bone marrow and enter systemic circulation.

Secondary Lymphoid Nodule

A well-delimited lymphoid formation containing a central pale Flemming center (germinal center) composed of activated, proliferating B lymphocytes.

Lymph Node

A rounded or ovoid secondary lymphoid organ measuring $0.5\text{ to }1\,\text{cm}$, enclosed by a fibroelastic capsule, that receives lymph via afferent vessels on its convex surface and filters it before drainage into an efferent vessel at the hilum.

Subcapsular (Marginal) Sinus

The lymphatic channel situated immediately beneath the lymph node capsule that directly receives lymph entering from the afferent lymphatic vessels.

Paracortex (Deep Cortex)

The intermediate, T-lymphocyte-dependent cortical zone of a lymph node situated between the superficial nodular cortex and the medulla, containing high endothelial venules.

High Endothelial Venules (HEVs)

Specialized post-capillary venules in the lymph node paracortex lined by plump endothelial cells that enable lymphocyte recirculation and homing from the bloodstream into the node parenchyma.

Lymphadenopathy

The hypertrophy or abnormal enlargement of a lymph node in response to local or systemic antigenic stimulation, the accumulation of pathological cells, or a primary lymphoma.

Mucosa-Associated Lymphoid Tissue (MALT)

Diffuse and organized non-encapsulated lymphoid aggregates located within the walls of hollow organs in contact with external environments (including NALT, BALT, Peyer's patches, and urogenital tracts) that act as protective immunological barriers.

Spleen

A secondary lymphoid organ weighing approximately $200\,\text{g}$, located in the left hypochondrium along blood vessels, connected directly to the blood circulation rather than the lymphatic circulation.

Dorsal Mesogastrium

The mesodermal mesentery in which the spleen primordium originates around the 5th week of embryonic development through the proliferation of coelomic epithelium.

White Pulp

The lymphoid compartment of the spleen organized around arterial branches, consisting of the periarteriolar lymphoid sheath (PALS) and Malpighian corpuscles.

Periarteriolar Lymphoid Sheath (PALS)

A cylindrical cuff of lymphoid tissue surrounding the central arteriole of the splenic white pulp, populated predominantly by T lymphocytes in its deep layer.

Malpighian Corpuscles (Splenic)

Spherical lymphoid nodules with germinal centers located within the splenic white pulp along central arterioles, containing primarily B lymphocytes.

Red Pulp

The splenic parenchymal compartment dedicated to blood filtration and the destruction of aged erythrocytes, composed of venous sinuses and cellular cords of Billroth.

Cords of Billroth

Cellular columns located between splenic venous sinuses composed of a reticular framework populated by macrophages, erythrocytes, B lymphocytes, plasma cells, platelets, and mast cells.

Littoral Cells

Elongated, spindle-shaped endothelial cells lining the venous sinuses of the splenic red pulp, separated from one another by slit-like gaps that allow passage of blood cells.

Henle's Fibers

Circularly oriented reticular fibers arranged around the endothelial littoral cells of the splenic venous sinuses within the perisinusoidal space.

Schweigger-Seidel Sheath

A contractile sheath composed of reticular cells and macrophages surrounding the terminal ends of penicillar arterioles in the spleen.

Open Circulation (Spleen)

A splenic vascular route in which terminal arterial blood discharges directly into the meshes of the cords of Billroth before passing through the walls of venous sinuses.

Closed Circulation (Spleen)

A splenic vascular route in which terminal penicillar capillaries connect directly into the lumen of the venous sinuses without initial extravasation into the cords.

Hemosiderin

A yellowish-brown iron-containing pigment accumulated within splenic macrophages following the phagocytosis and breakdown of senescent erythrocytes, demonstrated histochemically by Perls' staining.

Mononuclear Phagocyte System

A body-wide family of mobile, plastic, lysosome-rich phagocytic cells derived from blood monocytes, comprising connective tissue histiocytes, splenic littoral macrophages, Kupffer cells, alveolar macrophages, Langerhans cells, and microglia.