Lymphatic System

Lymphatic vessels

The central and earliest lymphatic vessels are thought to arise through outgrowth of the venous endothelium. The lymphatic vessels which develop later derive from local lymphangioblasts. Capillary networks develop which gradually coalesce to form lymph sacs.
The first lymphatic vessels appear at around week 5, which corresponds to S16. They first develop bilaterally, near the confluence of the precardinal and postcardinal veins, and enlarge to form jugular sacs (paired jugular lymph sacs). The paired axillary lymph sacs develop a few days later and fuse together with the jugular sacs. Further caudally, the primordia of the paired thoracic ducts arises as well as lumbar and iliac lymph sacs and the unpaired retroperitoneal (mesenteric) lymph sac. All of these lymph sacs develop from the surrounding veins but it is only in the jugular region that a connection remains with the veins. Due to their development and growth, at weeks 7 and 8 the lymphatic structures all come together. Caudal enlargements of the jugulo-axillary lymph sacs anastomose with the right thoracic duct, while the left thoracic duct ends blind. Caudally, the two thoracic ducts connect to the cranial parts of the lumbar lymph sacs, which develop to form the cisterna chyli. The caudal parts of the lumbar lymph sacs join together with the retroperitoneal lymph sac as well as the paired iliac lymph sacs.