3. Spleen, Haemal Nodes, & Thymus

Flashcards covering the spleen, haemal nodes, and thymus, including gross anatomy, histology, and function.

Spleen Function in Fetus

Produces all types of blood cells.

Spleen Function in Adults

Produces lymphocytes, removes effete erythrocytes, stores iron from effete erythrocytes, removes debris and microorganisms from blood, provides immune defence (cell-mediated and humoral) and stores blood (especially in horse, dog and cat).

Spleen

Contains lymphoid tissue but is within the bloodstream rather than the lymph stream. It is not essential for life.

Spleen Blood Storage

Stores blood, especially in horse, dog, and cat. In horses, contraction increases packed cell volume - percentage of red blood cells (PCV) by up to 40%.

Spleen Position

Lies against the left abdominal wall, protected by ribs. Parietal surface is against diaphragm and visceral surface is against rumen/stomach.

Spleen Hilus

Dorsoventral groove (carnivores, pigs, horses) or small indentation (ruminants) on visceral surface where blood vessels, lymphatics, and nerves pass through.

Splenic Artery Origin

Branches from the Coeliac Artery.

Splenic Vein Destination

Branch of the Portal Vein, carries blood to the liver.

Carnivore & Pig Spleen Attachment

Loose gastrosplenic ligament, vulnerable to torsion.

Ruminant Spleen Attachment

Gastrosplenic ligament formed by peritoneum, attached to rumen and phrenicosplenic ligament on parietal surface attached to diaphragm.

Horse Spleen Attachment

Gastrosplenic ligament, phrenicosplenic ligament (to diaphragm), renosplenic ligament (to left kidney).

Spleen Capsule

Collagen & elastin fibers and smooth muscle cells, reticulin fibers form trabeculae.

Spleen Red Pulp (looks pink in histological samples)

Majority of pulp, meshwork of reticular connective tissue, macrophages and blood cells form splenic cords, venous sinuses (not in cat or horse).

Spleen White Pulp (looks purple in histological samples)

T lymphocytes & macrophages organized around central arterioles.

Periarteriolar Lymphoid Sheaths (PALS)

Lymphocytes (primarily T cells) organized around central arterioles in white pulp, similar to paracortex of lymph nodes.

Splenic Corpuscles/Follicles

Nodules of lymphatic tissue in PALS, visible in gross specimens, contain germinal centre (dividing B cells) and corona (non-dividing B/T cells).

Red Pulp Macrophage Function

Retain immature erythrocytes, remove erythrocyte inclusions and effete erythrocytes, remove microorganisms.

Haemal Nodes

Lymphoid structures mainly in ruminants, reddish-brown, occur near major blood vessels, function similar to spleen.

Thymus role

Production of T lymphocytes

Thymus Location

Located in ventral mediastinum, most developed in young animals, involutes with age. Consists of left and right lobes.

Thymus Lobules

Each lobe is separated by septae into lobules, consisting of cortex (densely packed dividing T lymphocytes and macrophages) and medulla (fewer mature T lymphocytes, Hassall’s corpuscles and macrophages).

Thymic Arteries

Enter at cortico-medullary junction, follow septae and branch into cortex & medulla.

Blood-Thymus Barrier

Prevents transport of potential antigens into the thymus, layers of cells including thymus epithelial cells (TECs). Prevents developing lymphocytes from being exposed to antigens before maturity.

Thymic Epithelial Cells (TECs)

Form sheets of tissue and reticular labyrinths, secrete hormones (thymosins) that regulate T cell maturation.

Hassall’s Corpuscles

In the medulla, concentric clusters of epithelial cells that contain keratohyalin, provide signalling molecules, role unclear.

T Cell Development

T lymphocytes develop from stem cells in the bone marrow and migrate to thymus to undergo maturation. Most of an individuals immature T cells are produced by puberty.

Positive Selection of T Cells

Occurs in the cortex. T cells are exposed to MHC class I & II molecules on thymic epithelial cells (TECs) to ensure that they are able to recognise self-antigen. Those that don’t bind undergo apoptosis.

Negative Selection of T Cells

T cells capable of bind with MHC Class I and II then migrate to medulla. They are exposed to self-antigen presented by macrophages and TEC’s and those that bind too strongly undergo apoptosis.