Lymphatic Organs

• Splenic artery enters at the hilus of the spleen via splenic capsule and breaks up into capsular and trabecular arteries.
• Parenchymatous organs that produce lymphocytes include the thymus and Bursa of Fabricius.
• The thymus is a primary lymphatic organ in mammals, a lobulated lymphoepithelial organ with a loose connective tissue capsule and incomplete loose connective tissue septa that divide it into lobules.
• Each thymic lobule presents an outer cortex and central medulla, with the staining difference due to the much higher density of T-cells in the cortex.
• The medulla of the thymus presents distinct acidophilic bodies called Thymic corpuscles, which appear as eosinophilic concentric whorls of exhausted degenerating epithelia-reticular cells joined by desmosomes.
• Thymic corpuscles are found only in mammals.
• The thymus has no afferent lymphatic vessels but does have efferent lymphatic vessels.
• The thymus produces the first generation of T-lymphocytes, known as thymocytes, along with macrophages and epithelia-reticular cells.
• The thymus is relatively largest (in relation to body weight) very early in life, and actually continues to grow in absolute size for a while after birth.
• The thymus undergoes natural involution with age, reducing the total volume of active tissue and replacing it with Connective tissue and Adipose tissue, but some functional tissue remains throughout life.
• Natural involution of the thymus is irreversible.
• Many stimuli, such as overexposure to radiation and some chemicals, can cause accidental involution of the thymus, and in these cases the organ is usually capable of regeneration if the offending stimulus is removed.
• Accidental involution of the thymus is reversible.
• In birds, the Bursa of Fabricius completely involutes in 13-17 months of age.
• Spleen is involved in hematopoiesis, blood filtration and destruction of old RBCs.
• Spleen is a flaccid bag that serves as a storage site for blood, and a processing station for the scavenging of aged erythrocytes.
• Spleen is part of the circulatory system and is always described with lymphatic organs because of the very large population of lymphocytes found in it.
• Splenic corpuscle is made up of a pale centrally located germinal center and dark peripheral mantle layer or zone.
• Splenic arteriole, inappropriately called central artery because it is not an artery but an arteriole, occupies a central or paracentral position in the splenic corpuscle and is surrounded by the periarterial sheath.
• Red Pulp of spleen gets its appearance from the formed elements of the blood (mostly erythrocytes) it contains and fills the bulk of the spleen’s volume.
• Splenic cords, also known as Billroth cords, are anastomosing cords of erythrocytes, lymphocytes and macrophages separated by sinuses; corresponds to medullary cords of lymph node.
• Tongue-shaped or leaf-shaped in mammals and small and round in birds, spleen is enclosed by a capsule of Dense White Fibrous Connective Tissue (DWFCT) with few smooth mucle cells and elastic fibers to allow the spleen to contract and eject stored erythrocytes when needed.
• Periarterial sheath is a sleeve of lymphocytes arranged along the splenic arteriole and blends with a loose stromal framework of reticular fibers.
• Blood sinuses are tiny venous spaces between splenic cords and contain a few erythrocytes and the normal circulating white blood cells.
• Splenic circulation is the circulation of blood through the spleen.
• Hemorrhagic lymph node is a type of lymph node that is only present in ruminant animals and is similar in structure to the lymph node except that its medullary sinuses contain blood and lymph.
• Connective tissue trabeculae in the spleen’s interior volume arise from and are continuous with the capsule.
• Spleen has no afferent lymphatic vessels: only efferent lymphatic vessels are present.
• Red Pulp of spleen is supported by fibrous (reticular fibers) and cellular (reticulocytes) tissue.
• White pulp of spleen consists almost entirely of lymphocytes, in association with the arterial blood supply.
• Parenchyma of spleen consists of white pulp and red pulp.
• Plicated (folds) luminal surface (not all)
• Porcine lymph node: Arrangement of the cortex and medulla is reversed.
• Hemal node is derived from muscular arteries during embryonic development, as cul-de-sacs, blind pouches.
• Lymph node is the most organized of the lymphatic organs.
• Function: produces B lymphocytes
• Duck – elongated
• Parenchyma of lymph node is divided into two regions: Outer cortex and Central medulla.
• Hemal node does not present afferent or efferent lymphatic vessels.
• Hemal node is considered as accessory spleen because it contains large numbers of erythrocytes and while they develop internal capillary beds to serve germinal centers, they don’t have their own internal circulatory subsystem.