Tridimensional study of the deep cortex of the rat lymph node. IV. Differential labelling of the deep cortex units with 3H-uridine.

Using tridimensional reconstruction, it was recently found that the deep cortex of rat lymph nodes comprises one to several basic "units." Each unit is a semi-rounded structure contiguous to the peripheral cortex and bulging into the medulla of a node. Other investigators reported that transfused lymphocytes, heavily labelled in vitro by 3H-uridine, became concentrated in an ill-defined region of nodes, referred to as the mid and deep cortex. This suggested to us that the in vivo labelling of nodes with 3H-uridine might allow to further characterize the units on a physiological basis. Therefore, rats were injected intravenously with a dose of 1--20 muCi of 3H-uridine/gm body weight and sacrificed 1 hour to 3 days later. The radioautographs of their nodes were exposed up to 535 days. The observations revealed that a large dose of 3H-uridine combined with a long exposure of the radioautographs yielded a differential labelling of the cell population of the units, characterized by a much more intense reaction than that of the surrounding structures. This demonstrated that the physiology of the lymphocyte population of the deep cortex units differs from the morphologically similar lymphocyte population of the extrafollicular zone of the peripheral cortex. The possible reason(s) for the differential labelling of the units is discussed.