Changes in adipose tissue of the rat due early undernutrition followed by rehabilitation. 3. Changes in cell replication studied with tritiated thymidine.

1. Well-nourished rats were injected with tritiated thymidine at 15, 22, 28 or 84 d of age. At 1, 6, 11 and 16 d after injection animals from each group were killed, samples of adipose tissue were removed from two subcutaneous sites (abdominal and scapular) and separated, using collagenase (EC 3.4.24.3), into 'fat cell' and 'stromal cell' fractions. The specific (radio)activity of DNA isolated from each fraction was measured. The specific activity of DNA isolated from two 'deep body' sites (perirenal and epididymal) was measured only in the animals injected at 84 d of age. 2. Animals undernourished from birth up to 84 d of age were injected with tritiated thymidine at 22, 28 or 84 d of age. Animals were killed 1 and 11 d after injection, adipose tissue removed, and the specific activity of DNA measured. Other undernourished animals were rehabilitated from 84 to 107 d and injected at 91 d of age with tritiated thymidine. The animals were killed 1, 6, 11 and 16 d after injection, adipose tissue was removed from the subcutaneous and deep body sites and the specific activity of DNA determined as before. 3. In well-nourished animals fat cell replication had largely ceased by 12 weeks of age in the subcutaneous depots. There were differences between the various sites of adipose tissue regarding the period of hyperplastic growth, its timing or rate of replication or both. 4. In undernourished animals replication was slow in the subcutaneous depots compared with well-nourished animals of the same age. Rehabilitation from undernutrition stimulated replication which resulted in higher rates in all four depots examined compared with those in well-nourished animals. 5. The findings are discussed in relation to the concept of a finite period of hyperplasia for adipose tissue.