Large numbers of DC develop in the presence of IL-7. Thymic lobes were organ-cultured under control conditions (original magnifications: A, 130×; B, 650×) or in the presence of IL-7 (original magnifications: C, 130×; D, 650×) for 12 days. Note the peripheral band of pale cells in IL-7–treated lobes (C). These cells mostly correspond to DC (asterisks; D). In contrast, thymocytes (arrows) predominate in control lobes (B).