The enveloping of intercellular collagenous fibrils by reticular cell processes in postnatal development of rat lymph nodes.

In the lymph nodes of adult rats reticular fibers are known to be covered by the processes of reticular cells. This study aims to visualize the sequence of the envelopment of reticular fibers by reticular cells during development. Rat popliteal lymph nodes of one to twenty-three days after birth were examined by electron microscopy. At the earliest stages, collagenous protofibrils were found in the intercellular space between studded mesenchymal cells. The protofibrils clustered around the plasma membrane of immature reticular cells and then became arranged into microfibrils of 30-40 nm in diameter. Bundles of the fibrils which might be called reticular fibers were surrounded by processes of more than one reticular cell. Then the reticular fiber came to be enclosed by the cytoplasmic process of a single reticular cell. Finally at 16-23 days after birth, the reticular fiber was completely ensheathed by the thick cytoplasmic process of a single reticular cell closed with a junctional complex. Throughout these periods, basal lamina-like materials existed between the reticular fiber and cytoplasmic process. Clumps of fibrils were rarely in contact with leukocytes, including lymphocytes. Immature elastic fibers appeared among collagenous fibrils of the reticular fiber when the fiber came to be enclosed by processes of some reticular cells. It was shown that the enclosure of the reticular fiber by the reticular cell did not result from physical pressure due to the increase of the number of lymphocytes, but the reticular cell actively enclosed the reticular fiber.(ABSTRACT TRUNCATED AT 250 WORDS)