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Home , Red pulp, Spleen, Tonsil, Trabecular arteries, Tubal tonsil, White pulp

DEMO – I (, , & lymph nodes)

Ali Jassim Alhashli Year III – Unit VI ( and ) STATION – 1 (Embryology)

• The pharyngeal arches (mesoderm) are formed by: – Pharyngeal cleft from outside (ectoderm). – Pharyngeal pouch from inside (). • The developing embryo will have: – 4 clefts. – 6 arches. – 5 pouches (the 5th pouch is rudimentary). STATION – 1 (Embryology) STATION – 1 (Embryology) STATION – 1 (Embryology)

• During the development of the : – A connection between the stomach and the anterior abdominal wall will be formed and known as (the ventral mesogastrium). – While another connection will be formed between the stomach and the posterior abdominal wall know as (the dorsal mesogastrium). • The spleen (mesodermal in origin) will be formed between the two layers of the dorsal mesogastrium. STATION – 2 (The ) • The lymphatic system is composed of: – Lymphatic vessels. – Lymphatic organs (they can be primary such as the and thymus or secondary such as the spleen and lymph nodes). – Lymphatic tissue (Mucosal Associated Lymphoid Tissue “MALT” and tonsils). • The most important groups of lymph nodes in the body are: – Cervical lymph nodes (see the figure in next slide). – Axillary lymph nodes (including pectoral, humeral, subscapular, central, apical and supraclavicular nodes) and they drain the are above the umbilicus. – Mediastinal lymph nodes. – Superficial inguinal lymph nodes. STATION – 2 (The Lymphatic System)

Cervical lymph nodes Axillary lymph nodes STATION – 2 (The Lymphatic System)

• The tonsils are also known as the jugulodigastric nodes are composed of: – Palatine . – Lingual tonsil. – Pharyngeal tonsil (). – . Note: these are going to form a ring called Waldeyer’s ring. STATION – 2 (The Lymphatic System)

• : – It is a dilated sac at the lower end of the thoracic duct into which lymph from the intestinal trunk and two lumbar lymphatic trunks flow.

– It is a retro-peritoneal structure located posterior to the abdominal aorta on the anterior aspect of the bodies of the first and second lumbar vertebra (L1 and L2).

– It is going to form the thoracic duct which transports lymph and chyle from the via the aortic opening of the diaphragm (T12) to the junction of the left subclavian and the left internal jugular vein (left brachiocephalic vein). • Right lymphatic duct: – 1.25 cm in length formed by the right mediastinal, right cervical and right subclavian trunks. – It forms various combinations with the right subclavian vein and the right internal jugular vein (right brachiocephalic vein). STATION – 2 (The Lymphatic System)

• Spleen: – It is extending from the 9th-11th ribs. Note that the 10th rib is considered the long axis of the spleen.

– The spleen is attached to the stomach by . The short gastric are found within this ligament.

– The spleen is attached to the left by the splenorenal ligament. Within this ligament, the tail of the is found so a doctor must avoid injuring it when removing the spleen (most of the islets of Langerhan are found in the tail of the pancreas. Therefore, if it is removed the person will develop diabetes). In addition, the splenic is also found within the splenorenal ligament and it is a potential source of bleeding when injured. STATION – 2 (The Lymphatic System)

• Thymus: – It is located in the anterior superior mediastinum behind the sternum. – The stromal cells are developing from the 3rd pharyngeal pouch. – The gland is composed of 2 lobes connected to each other. – The shadow of the thymus can be seen when doing an x-ray for an or children but it is disappeared in adults because the thymus will be replaced by a fatty tissue (the involution starts at time of puberty). – Function: It is the site where immature T- proliferate and differentiate to mature T-lymphocytes: • Helper T-cells (CD4+). • Cytotoxic T-cells (CD8+). It also aids in the development of self-tolerance: which means that mature T-cells will not attack the own of the body (no autoimmunity) and it secretes thymic factors: thymocin, thymulin and thympoietin. – supply: by the inferior thyroid and the internal thoracic arteries. The venous drainage is by the inferior thyroid vein going to the left brachiocephalic vein. STATION – 2 (The Lymphatic System) STATION – 3 (Gross of The Spleen) STATION – 3 (Gross Anatomy of The Spleen)

Notice the gastric, colic and renal impressions in the spleen STATION – 3 (Gross Anatomy of The Spleen)

Vasculature of the spleen

Arterial supply: by the splenic artery (the largest branch of the celiac trunk). Venous drainage: by splenic vein which will merge with SMV to form the portal vein draining to the . STATION – 4 (Histology of Spleen and Lymph Nodes)

• Spleen: – The spleen is covered with a capsule which is extending to the parenchyma forming septa known as trabeculae (containing & ). – : it is a lymphatic nodule with an outer dark staining area (PALS: Peri-Arteriolar Lymphatic Sheath) which contains T-lymphocytes and an inner light staining area () containing B-lymphocytes. Note: the white pulp is characterized by the presence of a central artery (although the name is central but it is placed toward the periphery of the pulp). – : it is the reddish-brownish substance found between the white pulp. It is composed of: • Sinusoids: which are dilated blood vessels with modified elongated endothelial cells. • Splenic cords: which contains lymphocytes, , , RBCs, plasma cells and . STATION – 4 (Histology of Spleen and Lymph Nodes) STATION – 4 (Histology of Spleen and Lymph Nodes)

• Lymph nodes: – They are bean-shaped structures with a concave side know as the (into which arteries, veins and efferent lymphatic vessels enter). – They aid in the filtration of lymph. – A is covered with a capsule which is penetrated by afferent lymphatic vessels. – The interior is divided into: • Cortex: which is made up of lymphatic nodules (with outer dark staining area and inner light staining area) containing B-lymphocytes. • Para-cortex (deep cortex): which is composed of T-lymphocytes. • Medulla: which is composed of medullary cords (containing lymphocytes, macrophages and plasma cells) and medullary sinuses (which empty their lymph into the efferent lymphatics). – Note: most lymphocytes enter the lymph node through specialized (HEVs). Only 10% of lymphocytes will enter the lymph node through afferent lymphatic vessels. STATION – 4 (Histology of Spleen and Lymph Nodes) STATION – 4 (Histology of Spleen and Lymph Nodes) STATION – 5 (Histology of Thymus and Tonsils)

• Thymus: – It is surrounded by a capsule which will send trabeculae dividing the parenchyma into incomplete lobules. Note: the trabeculae contain blood vessels and adipose cells. – Each lobule is composed of: • A cortex: which is having densely packed small (immature) lymphocytes and it is the site for proliferation and terminal differentiation of T-cells. The differentiating T-cells in the cortex are protected by the blood-thymus barrier which is consisting of (Epithelial “ECR”, perivascular connective tissue and wall). • A medulla: lymphocytes in the medulla are large mature T-cells. Macrophages, fibroblasts and mast cells can also be seen in the medulla. The unique feature which is characterizing the thymus is the presence of thymic (Hassall’s) corpuscles which are concentric, flat, degenerated type VI-ERCs.These corpuscles produce IL-4 and IL-7 which function in thymic differentiation & education of T- lymphocytes. Note that the medulla has no blood-thymus barrier. STATION – 5 (Histology of Thymus and Tonsils) STATION – 5 (Histology of Thymus and Tonsils) STATION – 5 (Histology of Thymus and Tonsils)

• Palatine tonsils: – : stratified squamous non-keratinized.

– There are folds forming .

– In these folds, there are lymphatic nodules with germinal centers in the middle (light staining). STATION – 5 (Histology of Thymus and Tonsils) STATION – 5 (Histology of Thymus and Tonsils)

• Peyer’s patches: – Found in the ileum. – Also known as Mucosal- Associated Lymphoid Tissue (MALT). – Lymphatic nodules in Peyer’s patches are dark staining (with no pale centers).

GOOD LUCK!