Tritiated thymidine autoradiographic study on postnatal development of epididymal adipose tissue in the normal mouse.

Cell proliferation was studied in the developing epididymal adipose tissue of mice by means of 3H-thymidine autoradiography. 4 mice in each age group were killed 30 min after a single injection of 3H-thymidine given at the 6th, 10th, 16th, 21st, 28th, 35th, 42nd, 49th or 56th postnatal day. The labeling index of the adipose tissue was high for several postnatal days but it was highest on the 6th day. The labeling index decreased thereafter, and cell proliferation appeared to almost cease after 56 days. Another 16 mice were given a single injection of 3H-thymidine at 6 days old; pairs of animals were killed at 30 min, 2, 4, 6, 8, 10, 14 and 18 days later. From the 2nd to 6th days of pulse labeling, the number of labeled fat cells increased significantly, whereas the number of labeled poorly differentiated mesenchymal cells decreased reciprocally. The labeling index of fat cells remained unchanged from the 6th to the 18th day. These results suggest that there is a population of fat cell-precursor cells in the mesenchymal tissue of the new born mouse, and that it takes 2-6 days for the precursor cells to become mature fat cells in the epididymal fatty tissue.