Tridimensional study of the deep cortex of the rat lymph node. VI. The deep cortex units of the germ-free rat.

The deep cortex of the normal rat lymph node consists of semirounded lymphocytic structures, termed deep cortex "units," each being centered on the opening of an afferent lymphatic. The aim of the present work was to investigate the morphologic features of the units in germ-free animals, in an attempt to evaluate the influence on natural exogenous antigenic stimulation on the development of the units. For this, the lymph nodes from various anatomic locations from 8-week-old Sprague-Dawley germ-free rats were analysed tridimensionally. The observations revealed that, in comparison with the lymph nodes of normal rats, the units of the cervical and mesenteric lymph nodes of the germ-free animals were underdeveloped, while those of the brachial, inguinal, and popliteal lymph nodes were unchanged. Moreover, the germ-free state modified the units of the mesenteric lymph nodes in a manner not encountered in the remaining lymph nodes. Other morphologic features of the peripheral cortex of the lymph nodes of germ-free rats also differed from those of normal ones. The significance of these differences is discussed with respect to immune responses and the process of lymphocyte recirculation. They are of interest because they support previous proposals regarding some aspects of the functioning of the normal lymph node, accounting for the features of the structures and overall architecture of the organ.