Lymph node development in human fetuses with special reference to the initial lymphocyte aggregation and intranodal sinuses

A mouse lymph node develops inside of five lymph sacs in the trunk. However, there are uninvestigated, numerous peripheral nodes. We chose histological sections from 38 human fetuses at approximately 8-31 weeks of gestational age (36 to 260 mm of crown-rump length). Three sites of the extremity and seven sites of the trunk were examined. Lymphocyte aggregations developed within the jugular, para-aortic and iliac lymph sacs by 10 weeks. The aggregation lost lymph endothelia at the lumen side and, in the external side, it was attached to the surrounding mesenchymal tissue. Shortly thereafter, in place of the sac endothelia, abundant lymph vessels (LVs) surrounded the aggregation. In the posterior intercostal space, two to seven LVs fused to build a small sac-like structure and, within it, a lymphocyte aggregation developed by 12 weeks and maintained a contact to the surrounding mesenchymal tissue. The same manner was seen in the inguinal, popliteal and cubital regions. In the jejunal mesentery, LVs drastically increased in thickness at 9-11 weeks and, shortly thereafter, they fused to build a very long sac-like structure. However, in the pulmonary hilum at 9-10 weeks, LVs did not fuse together but simply thickened. Thus, keeping a wide mesenchymal connection, a lymphocyte aggregation protruded into a single LV. Being independent of the lymph sac, lymphocytes aggregated in the mesenchymal tissue adjacent to peripheral LVs, followed by its protrusion into a sac-like structure that was usually built by fusion of multiple LVs or a thickened LV.