Ultrastructure of the normal lymph node.

The "normal" lymph node has been studied by electron microscopy. The lymphoid tissue can be divided into three distinct zones. Zone 1 consists of loosely arranged cells surrounding the lymphatic sinuses and blood vessels. This is the only zone in which plasma cells are present. Zone 2 is surrounded by zone 1 and consists of compactly arranged cells in which lymphocytes predominate. Zone 3 (germinal center) appears only after antigenic stimulation. It is characterized by large, ribosome-rich cells and macrophages containing phagocytosed lymphocytes. These zones are arranged with their longest diameters pointing towards the hilus. Zone 1 is the longest and extends across the cortex, paracortex and medulla. Zone 2 spans across cortex and paracortex. Zone 3 usually is confined to the cortex. Our preliminary studies indicate that zone 1 is mainly bursal dependent, zone 2 is mainly thymic dependent and zone 3 is bursal dependent.