STUDIES OF THE INCORPORATION OF PYRIMIDINES INTO DNA IN SINGLE LEUKAEMIC AND OTHER PROLIFERATING LEUCOCYTES

Circadian influence on the frequency of labeled mitoses method in the stratified squamous epithelium of the mouse esophagus and tongue

The frequency of labeled mitoses method (FLM) was used to study the cell kinetics in the stratified squamous epithelium of the mouse esophagus and tongue. FLMs were generated by injecting tritiated thymidine (TdR) at two different phases of the mouse circadian system: TdR was injected into one group of mice at 0900 and into a different, second group of mice at 2100. Three variables were monitored for each group; (1) the FLM, (2) the mitotic index and (3) the grain count over the labeled mitotic figures. In both the esophagus and the tongue there was a circadian rhythm in the mitotic index with the peak occurring during the first half of the diurnal phase and the trough occurring during the first half of the nocturnal phase. The FLM curves from each group revealed the following data.

The kinetics of cellular commitment during stimulation of lymphocytes by lectins

The kinetics of cellular commitment in the stimulation of lymphocytes by concanavalin A (Con A) has been analyzed by measurement of DNA synthesis, autoradiography, and histologic staining techniques. If the competitive inhibitor alpha-methyl-D-mannoside (alphaMM) is introduced into cultures of mouse spleen cells at various times after the addition of Con A, there is a gradual decrease in its capacity to inhibit the lectin-stimulated incorporation of [(3)H]thymidine. Addition of the saccharide 20 h after exposure of the cells to Con A had no effect on the level of the cellular response to the lectin. With increasing periods of contact with Con A, the percentage of blast cells and the percentage of [(3)H]thymidine-labeled blast cells increased in parallel with the total radioactive thymidine incorporated while the average number of autoradiographic grains per labeled blast cell remained relatively constant. These observations suggest that the rising level of [(3)H]thymidine incorporation results from an increase in the number of cells that respond to lectin stimulation and become refractory to inhibition with alphaMM. Once such cells become committed, they synthesize DNA at a rate independent of the length of exposure to the lectin. The combined results indicate that mouse splenic lymphocytes are heterogeneous in their capacities to respond to Con A and that different cells require different induction periods to be stimulated.

Cell population kinetics: a modified interpretation of the graph of labeled mitoses

Graphs of labeled mitoses, derived from autoradiographs of cell populations with (3)H-thymidine, show depressions in the curves at their midpoints. These depressions reflect interruption of DNA synthesis midway through S phase. Such interruptions revealed by the method of labeled mitoses should be considered when determining cell-cycle times.

Incorporation of sulfate by the mouse thymus: its relation to secretion by medullary epithelial cells and to thymic lymphopoiesis

The thymus was examined in suckling mice during normal development and the involution and regeneration produced by injection of cortisol, in experiments designed to test the hypothesis that medullary epithelial cells secrete a ymphopoietic hormone responsible for controlling the magnitude of thymic lymphopoiesis. Cellular events were observed by light and electron microscopy. Lymphopoiesis was assessed, after injection of thymidine-(3)H, by counting the proportion of lymphocytes labeled in radioautographs of thymus. Cortical lymphopoiesis was distributed heterogeneously, being concentrated in the subcapsular region, but medullary lymphopoiesis was statistically homogeneous in distribution and similar in magnitude to the average level of cortical lymphopoiesis in suckling mice. Therefore counts of the labeling index in the medulla were used to estimate the size of the proliferating population of lymphocytes. Epithelial secretory activity was estimated by measuring the incorporation of (36)sulfate by the thymus, using gel filtration chromatography to isolate soluble macromolecular (35)sulfate-presumed on radioautographic evidence to represent the mucoid epithelial secretory product. Incorporated (35)sulfate accumulated rapidly for 4 hr, reached a peak at 12 hr, and had fallen to half that level by 24 hr after a single injection-as would be expected of a secretory product. During normal postnatal development the size of the proliferating population of lymphocytes and the magnitude of (35)sulfate incorporation increased in parallel. During acute involution induced by cortisol both parameters diminished greatly but rose to high levels during subsequent regeneration. Accordingly, lymphopoiesis and sulfate incorporation -as defined and measured in these experiments-correlated linearly over a wide range of variation, providing circumstantial evidence to support the hypothesis that medullary epithelial cells secrete a sulfated mucoid lymphopoietic hormone. This conclusion is discussed in terms of the roles of thymus and adrenal cortex in development of the lymphoid system and maturation of immunological competence.