Tritiated thymidine ( H3-T) autoradiography has been applied to the duodenal progenitor population (crypts of Lieberkuhn) in four age groups of male Swiss albino mice to determine: (1) the size and distribution of the progenitor cell pool; and (2) to characterize the role of individual progenitor cells in previously reported age-related alterations in the proliferative activity of the population.
The progenitor cells which incorporated H3-T extended from the first cell below the crypto-villa1 junction to the last cell (Paneth) at the cryptal base in each age group. However, the majority of labeled progenitor cells were restricted to the central region of the crypt, while a smaller number was found in both the upper and basal portions. Thus, the crypt wall was divided into three zones according to the incidence of labeled cells; (1) zone of minimum cell proliferation (upper region), (2) zone of maximum cell proliferation (central region), and ( 3 ) zone of Paneth cells (basal region). The labeling incidence of the maximum zone was 3-5 times that of the minimum or Paneth regions.
Comparison of the labeling incidence at the different ages revealed an agerelated decrease in the overall frequency of labeled cells throughout the crypt, and particularly in the maximum zone. Estimations of the progenitor cycle duration from the ratio of S-phase duration to the DNA synthetic index revealed an increase in the progenitor cycle duration with age. It appears that the cycle duration increase, rather than a decrease in the size of the functional progenitor pool, may be the reason for the age-related change in the proliferative activity.
The labeling frequency of progenitor cells was least (about 1% ) at the cryptovillal junction and highest (about 50% ) in the central region of the crypt. It is concluded that factors in the microenvironment of the duodenal epithelial cell population may be regulating the proliferative activity of the progenitor cells.