(a) Round or polygonal primitive epithelial cells (arrows), some of them in division (m), constitute the homogeneous thymic stroma devoid of lymphoid cells of a 13-day-old Sham fetal rat. Note the lack in these cells of prominent cytoplasmic organelles, except for a few, elongated mitochondria (arrow heads), and the enlarged intercellular spaces. Original magnification × 3,700. (b) Thymic primordium of a 13-day old Adx fetal rat. Numerous lymphoid progenitors (L) occur between a well constituted network of irregular thymic epithelial cells (TEC). Nucleolus (stars). Original magnification × 3,700. (c) Incipient cell-to-cell contact (arrows) between 2 primitive thymic epithelial cells of a 13-day- old Sham fetal rats. Mitochondria (Mt). Original magnification × 56,000. (d) Incipient cell junction (arrows) between 2 thymic epithelial cells of a 13-day-old Adx fetal thymus. Note the increased amount of cisternae of rough endoplasmic reticulum (RER) and mitochondria (Mt) as compared with the condition of Sham fetal rats of the same age (c). Original magnification × 35,700. (e) Electron-dense (DEC) and electron-lucent thymic epithelial cells (LEC), 13-day old Adx fetal thymus, rough endoplasmic reticulum (RER), lipid droplets (Li), mitochondria (arrows), Golgi complex (G), and incipient cell junctions (arrow heads). Original magnification × 6,000.