Development of the Lymphatic System

The mesoderm cells of the developing veins form the precursor lymphatic vessels and nodes called lymph sacs.

The paired jugular lymph sacs are the first to form at the junction of the internal jugular and subclavian veins. They provide capillary networks to the thorax, upper limbs, neck, and head.

Importantly, each jugular lymph sac retains one connection to the jugular vein, with the left sac forming the upper part of the thoracic duct.

Next, the unpaired retroperitoneal lymph sac forms from the primitive vena cava and mesonephric veins, extending capillaries to the abdomen and diaphragm.

Simultaneously, the cisterna chyli sac forms and gives rise to the inferior portion of the thoracic duct.

The posterior lymph sacs, forming from the iliac veins, generate a capillary network for the abdomen, pelvis, and lower limbs.

Eventually, most of the lymph sacs get transformed into lymph nodes by the differentiating mesenchymal cells.

Additionally, weeks five through eight are characterized by the development of the spleen from the mesoderm.