Abstract
The postnatal growth and involution of the blood-forming tissues of the albino rats at different ages were studied chiefly by means of the quantitative analysis. One hundred and twenty rats of a subline of the WISTAR strain were used in this study. The animal material consisted of 12 different age groups; 1, 2, 3 and 4 weeks and 2, 3, 4, 5, 6, 8, 12 and 18 months after birth, in groups of 10, 5 males and 5 females, on each occasion. These animals were bred in our own laboratories under optimal environmental conditions. Breeding animals were inspected twice weekly, pregnancies and birth were recorded, the young were weaned and the sexes were separated at 28 days of age. After stunning the thymus, mesenteric lymph nodes, spleen and bone marrow were removed from each animal, the estimation of the total number of nucleated cells in these organs being made according to the method described by AWAYA in 1962. At the same time, histological studies of section were also made of these organs.
The alterations of the absolute weight and number of nucleated cells in the individual organ in response to aging, i. e. age changes of the absolute growth and involution, are shown in Tables 1 and 2 and Figs. 2, 3, 4 and 5. There occurred a progressive absolute growth of these organs until the end of the 3rd or 4th month (6th month in spleen). This was followed by an absolute involution which continued throughout the succeeding ages studied. On the other hand, if the organ weight and the total number of nucleated cells were calculated relative to body weight, the pattern of the growth and involution curve was characteristic of the individual organ, the thymus in particular (Fig. 6 and 7). At the ages of the 2nd or 3rd week (in females and males, respectively), there occurred a maximal relative growth of the thymus, which was followed by a progressive age involution. This was distinctly different from growth curves of the mesenteric lymph nodes and spleen. The mesenteric lymph nodes initiated their development in this period. The growth was maintained until the end of the 3rd and 4th months in the spleen and mesenteric lymph nodes, respectively. This showed that the thymus underwent a very rapid physiological involution while the mesenteric lymph nodes and spleen were still growing.
Recent work from several laboratories has established that the thymus plays an essential role in maturation of immunologic function and in the development of the anatomic integerity of the lymphoid tissues such as lymph nodes and spleen. Animals thymectomized at birth have a marked diminution of lymphocytes in their blood and tissues and severe defects in immune responses (ARCHER and PIERCE 1961, MILLER 1961, 1962, MARTINEZ et al. 1962, JANKOVIC et al. 1962, ARNASON et al. 1962, 1962, DAMESHEK 1962, BURNET 1962, GOOD et al. 1962, DALMASSO et al. 1963). These animals often develop a wasting syndrome, with failure of body growth and early mortality (GOOD et al. 1962, WAKSMAN et al. 1962, PARROTT and EAST 1962, 1964, SHERMAN et al. 1962, 1963). The results presented here support the above recent views on thymus function.
On the basis of per unit of body weight, the weights and the total numbers of nucleated cells in blood-forming tissues were found to be greater in females than in males throughout the period studied with the exception of bone marrow, although there were no sex differences in absolute values (Figs. 6 and 7). This indicates that the growth and involution of lymphatic tissue is influenced by sex factors.
Histologically, the thymus was made up of lobules, each of which consisted predominantly of a cortical area merging into a very small portion of medulla at the age of 1 or 2 weeks (Fig. 8), presenting normal adult architecture with a relative enlargement of a medullary portion at the age of 3 weeks (Fig. 9) when other lymphatic tissues were very incompletely developed. Since the thymus, from the quantitative views presented here
