Pattern of Cell Production and Cell Migration in Mouse Thymus Studied by Autoradiography

Abstract

Autoradiographs prepared from histological sections of thymus were studied at different intervals after a single intraperitoneal injection of tritiated thymidine into male CBA mice. The initial uptake was located mainly in the cortex, especially in the periphery, to a lesser extent in the medulla. Immediate incorporation was seen in reticular cells and in large and medium‐sized lymphocytes, not in small lymphocytes. As time elapsed more cells became labelled, and a migration of labelled cells from the cortex to the medulla took place. Simultaneously an increasing number of small thymocytes acquired the label. The pattern of initial uptake and spread of label with time was similar for the thymus of mice aged 3 weeks, 7 weeks and one year. Autoradiographs of smears of whole thymus revealed that 13 per cent of the cells were labelled one hour after the administration of 3H‐thymidine. The labelling percentage increased with time and reached a maximum of 37 per cent, 3 days after the injection. After that time a fall in labelling percentage was observed with a final disappearance of labelled cells from the thymus about the 14th day. Calculations on the basis of mitotic counts with and without colchicine pre‐treatment gave an average mitotic rate in thymic cortex of 3.7 per cent per hour, which renders an average renewal time of 27 hours by cell division for thymic cortex ‐ indepent of age. It is concluded that a large cell production takes place in the thymus, especially in the outer cortex, the end result being the production of thymocytes, mainly small ones, which migrate to the medulla from where they disappear.

Thymus and Node Labelling by Intramediastinal Injection of Tritiated Tracers and Its Bearing on Present Concepts of Lymphocyte Cytokinetics

“…The observation that the labelling of the thymic lymphocytes is weaker than that of intestinal epithelial cells and of nodal plasmocytes, following an intravenous or a subcutaneous dose of 1 pC of "H-thymidine, is in line with that of other investigators (Kallenbach 1960, Borum 1968). On the basis of the above assumptions, the observation would mean that proliferation is much less intense in the thymus than in the two other cell populations.…”

“…In places, the cells in the peripheral cortex were more reactive than those in the deep cortex, but less so than those in the medulla. At times, cells with the same reaction intensity as that over cells in the medulla were seen in the layer of cortex adjacent to the cortico-medullary junction of a lobule, as reported by Borum (1968).…”

The Fate of Thymocytes: A Study of Kinetics of Thymocytes using High Alkaline Phosphatase Activity as an Endogenous Label

“…Because no significant differences were found between the frequencies of AP-positive lymphocytes in the thymic and other venous blood, the thymocyte release could probably not be of a very large magnitude. However, there are facts supporting the idea that medullary thymocytes alone or with cortical thymocytes leave the thymus (Borum 1968). There is evidence that the medullary thymocytes are immunologically more competent and may form the main population of long-living cortisone-resistant circulating lymphocytes (Warner 1964, Dougherty et al 1964, Abe & Ito 1970, Blomgren 1971.…”