Supplementary Studies on Postnatal Histogenesis of Thymolymphatic Tissues of Albino Rats. I. Postnatal Histogenesis of Thymus and Maturation of Peripheral Lymphoid Tissues.

The purpose of the present paper is to report a continuation of the work on postnatal development of the thymolymphatic organs of albino rats made by Awaya and Oda (1965). The male 55 rats which were employed in the previous study served as material. They were divided into 11 age groups of 1, 2, 3 and 4 weeks and 2, 3, 4, 5, 6, 8 and 12 months of age, each groups containing 5 male rats. The thymus, mesenteric lymph nodes and spleen were removed from these animals. The total number of nucleated cells in these organs was estimated by the previous method, and differential counts were made on the Wright stained imprint. From the count thus obtained the absolute and relative numbers of lymphocytes in each of these organs were　estimated. The relative values were expressed as values per 100 gram body weight. At the same time, the development of lymphoid tissue was reinvestigated by a scrutiny of histology in the sections of these organs. The alteration of the absolute number of lymphocytes in the individual organ in response to aging, i. e. age changes of the absolute growth and involution are shown in Fig. 1. There occurred a progressive absolute growth of these organs until the end of the 4th month (the 6th month in the spleen). This was followed by an absolute involution which continued throughout the succeeding ages studied. If the relative number of lymphocytes-relative growth in the individual organs was observed, a more characteristic growth curve was obtained (Fig. 2). There occurred a maximal growth of the thymus at 3 weeks of age, which was followed by a progressive age involution. On the other hand, the mesenteric lymph nodes and spleen initiated their development in this period, and their growth was maintained until the end of the 3rd or 4th months.