Lymph nodes

With the exception of the cisterna chyli, the lymph sacs become subdivided by thin connective septa. They are transformed into lymph nodes and can be identified from week 9. The outer part of the sacs becomes the subcapsular sinus (marginal sinus) of the lymph node. The developing lymph nodes become invaded by lymphoid cells. Differentiation of both the medullary and cortical parts can be observed in the 2nd trimester. Definitive lymph follicles with germinal centers form from the beginning of the first year after birth.
Primary and secondary lymph nodes can be distinguished. The primary lymph nodes develop from the lymph sacs, with the first ones arising from the jugular sac as deep cervical lymph nodes. They form three chains: along the jugular vein, accompanying the accessory nerve, and as supraclavicular lymph nodes. Shortly thereafter, axillary, parasternal, mediastinal and scapular lymph nodes form around the evaginations of the jugular lymph sac. The retroperitoneal lymph sac gives rise to the upper group of lumbar lymph nodes. The lower group originates from the lumbar lymph sacs. The inguinal lymph nodes develop from the inguinal plexus, which represents a continuation of the lymph sacs in a caudal direction. The secondary lymph nodes arise along the peripheral lymphatic vessels as popliteal, cubital, mesenteric and omental lymph nodes.