Embryology Notes: Lymph System Development (Year 2, Block 4)

Embryolog y Dr; Marwa Bahfein Pediatric specialist Year 2, Block 4 DEVELOPMENT OF THE  The lymphatic system begins to develop at the end of the sixth week. The precursor lymphatic endothelial cells are derived from the cardinal veins....

Embryolog y Dr; Marwa Bahfein Pediatric specialist Year 2, Block 4 DEVELOPMENT OF THE  The lymphatic system begins to develop at the end of the sixth week. The precursor lymphatic endothelial cells are derived from the cardinal veins. Lymphatic vessels develop in a manner similar to that described for blood vessels, and they make connections with the venous system.  There are six primary lymph sacs present at the end of the embryonic period: Two jugular lymph sacs near the junction of the subclavian veins with the anterior cardinal veins (internal jugular veins) Two iliac lymph sacs near the junction of the iliac veins with the posterior cardinal veins One retroperitoneal lymph sac in the root of the mesentery on the posterior abdominal wall One cisterna chyli (chyle cistern) located dorsal to the retroperitoneal lymph sac  Lymphatic vessels soon connect to the lymph sacs and pass along main veins: to the head, neck, and upper limbs from the jugular lymph sacs; to the lower trunk and lower limbs from the iliac lymph sacs; to the primordial gut from the retroperitoneal lymph sac and the cisterna chyli. Two large channels (right and left thoracic ducts) connect the jugular lymph sacs with this cistern. Soon, a large anastomosis forms between these channels.  The thoracic duct develops from: Caudal part of the right thoracic duct. Anastomosis between the thoracic ducts and the cranial part of the left thoracic duct. The right lymphatic duct is derived from the cranial part of the right thoracic duct. The thoracic duct and right lymphatic duct connect with the venous system at the venous angle between the internal jugular and subclavian veins.  Development of Lymph Nodes Except for the superior part of the cisterna chyli, the lymph sacs are transformed into groups of lymph nodes during the early fetal period. Mesenchymal cells invade each lymph sac and form a network of lymphatic channels, the primordia of the lymph sinuses. Other mesenchymal cells give rise to the capsules and connective tissue framework of the lymph nodes. The lymphocytes are derived originally from primordial stem cells in the umbilical vesicle mesenchyme and later from the liver and spleen.  The early lymphocytes eventually enter the bone marrow, where they divide to form lymphoblasts. The lymphocytes that appear in the lymph nodes before birth are derived from the thymus, a derivative of the third pair of pharyngeal pouches. Small lymphocytes leave the thymus and circulate to other lymphoid organs. Later, some mesenchymal cells in the lymph nodes also differentiate into lymphocytes. Development of Spleen and Tonsils The spleen develops from an aggregation of mesenchymal cells in the dorsal mesogastrium. The palatine tonsils develop from the endoderm of the second pair of pharyngeal pouches and nearby mesenchyme. The tubal tonsils develop from aggregations of lymph nodules around the pharyngeal openings of the pharyngotympanic tubes. The pharyngeal tonsils (adenoids) develop from an aggregation of lymph nodules in the wall of the nasopharynx. The lingual tonsil develops from an aggregation of lymph nodules in the root of the tongue. Lymph nodules also develop in the mucosa of the respiratory and alimentary systems. Thank