An ultrastructural study of the primary follicle in the lymph node.

The present authors have studied the cellular composition of the primary follicle (PF) in the germ-free rat lymph nodes and the development of PF in the human lymph node anlages by using the light- and the electron microscopy. According to the ultrastructural study, the cellular composition of PF in the germ-free rat lymph nodes and classified into five cell types, 1) lymphocytic cells (small and medium-sized lymphocytes), 2) large lymphocytes (= large germinal center cells), 3) reticulum cells with desmosomes (RED), 4) desmodendric cells (DDC), and 5) histiocytes. The interstitial cells with desmosome are classified into DDC and RED based on the relationship to collagenous fibers and the morphological differences. There are amorphous electron-dense intercellular materials among the cytoplasmic processes of DDC, which are distinguished from histiocytes based on the morphological differences. PF emerges as a spherical aggregation of young DDC straight underneath the marginal sinus in the outer cortex of the human embryonal lymph node anlages on the 16th fetal week.