Ontogeny of human fetal lymph nodes.

Developing lymph nodes from 30 human embryos and fetuses with crown-rump lengths (CRL) of 18 mm (5.6 wk) to 245 mm (26 wk) were examined by light microscopy. The nodes were embedded in araldite, and the sections examined were approximately 1 mu in thickness. The development of nodes was divided into three stages: 1. the lymphatic plexus and connective tissue invagination (30 mm to 67 mm CRL); 2. the early fetal lymph node (43 mm to ,5 mm CRL); and 3. the late fetal lymph node (CRL greater than 75 mm). The lymphatic plexus was formed by connective tissue invaginations and bridges which divided a lymph sac into a meshwork of channels and spaces. Connective tissue invaginations were endothelially-lined and were surrounded by lymphatic space. Reticular cells, macrophages, and blood vessels were found in these invaginations. Early fetal lymph nodes were formed from invaginations when the cellular density and lymphocyte content increased. The lymphatic space surrounding the early node was the developing subcapsular sinus. With further development the early node became packed with lymphocytes, increasing the cellular density and size of the node. The connective tissue surrounding the subcapsular sinus condensed to form the capsule. Afferent lymphatic vessels pierced the capsule. Capillaries, veins, postcapillary venules, and occasional arteries were found in early and late nodes.