(A) Radial finger primordia (circle) in the ante-rior GD13 limb bud cultivated in vitro in the SS medium for 3 days. Interdigital region (dashed line), epithelium (arrow), central zone of decay (asterisk), mesenchyme (cross); Azan. (B) PCNA expression in the GD13 front limb bud cultivated for 14 days in vitro. PCNA signal in the mesenchymal cell (arrow); PCNA signal in the chondrocytes contained in the lacuna (thin arrow); direction of formation of the future interphalangeal joint (hollow arrow). GD: Gestation day; SS: Serum supplemented; PCNA: Proliferating cell nuclear antigen.