The development of lymphatic tissues and vessels in embryonic life begins around the fifth week. These structures originate from the mesoderm layer, with lymph sacs emerging from developing veins.
The first lymph sacs to form are the paired jugular lymph sacs located at the junction of the internal jugular and subclavian veins. From these sacs, lymphatic capillary plexuses extend to the thorax, upper limbs, neck, and head, eventually forming lymphatic vessels. Each jugular lymph sac maintains a connection with its respective jugular vein, with the left sac becoming the superior part of the thoracic duct.
The retroperitoneal lymph sac, situated at the root of the mesentery of the intestine, develops next. It derives from primitive vena cava and mesonephric veins. Lymphatic capillary plexuses and vessels spread to the abdominal viscera and diaphragm. While it connects to the cisterna chyli, it loses connections with neighboring veins. Simultaneously, the cisterna chyli, located below the diaphragm's posterior abdominal wall, emerges. It gives rise to the inferior part of the thoracic duct.
The final lymph sacs, the paired posterior lymph sacs, develop from the iliac veins. They form capillary plexuses and lymphatic vessels in the abdominal wall, pelvic region, and lower limbs. These posterior lymph sacs eventually connect to the cisterna chyli and lose connections with nearby veins.
Except for the part forming the cisterna chyli, mesenchymal cells invade and transform all lymph sacs into groups of lymph nodes. The spleen develops from mesenchymal cells within the dorsal mesentery layers of the stomach, while the thymus originates as an outgrowth of the third pharyngeal pouch. This intricate process establishes the foundation for the lymphatic and immune systems in the developing embryo.
