LYMPHOPOIESIS AND LYMPH NODE HISTOGENESIS IN EMBRYONIC AND NEONATAL RABBIT

The thymus represents the first lymphocytic organ to exhibit lymphopoietic activity in the fetal rabbit with lymphoblastic transformation beginning by 17 days of gestation, and lymphocytes appearing by 18 days, one to two days before vascularization. Lymphocytes are first evident in lymph nodes at 18 days of fetal development, in the spleen on the twenty‐third day, and in the appendix just prior to birth. Medium‐sized lymphocytes and rare blast‐like cells represent the first lymphocytic cells found in the lymph node anlage. Both of these cell forms appear simultaneously and are distributed randomly. Lymphocytes remain relatively sparse increasing gradually in number until several days prior to birth when their numbers increase rapidly. Coincident with the augmentation in lymphocyte population, lymphocytes tend to cluster near the vascular channels, and are present within the lumina and walls of the smaller veins in the node. Morphological gradations between mesenchymal cells and medium‐sized lymphocytes provide suggestive evidence that the initial population of lymphocytes within the node arise by the direct transformation of the stromal cells into medium‐sized lymphocytes. Less frequently, deeply basophilic blast‐like cells may provide an intermediate stage in lymphocyte differentiation. These cells differ from typical lymphoblasts because of their smaller size and irregular cellular contour. It is postulated that the initial population of lymphocytes formed in the lymph node anlage is derived from the transformation of mesenchymal cells and subsequent homoplastic lymphocyte proliferation and that during later fetal life this population of lymphocytes is complemented by the colonization and proliferation of blood‐borne lymphocytes. Copyright © 1969 Wiley‐Liss, Inc.