DL4 is expressed abundantly in subcapsular thymic epithelial cells. (A) Thymi from 3- to 6-month-old DL4^+/LacZ mice were whole-mount X-Gal–stained, paraffin-embedded, sectioned, and then counterstained with neutral red (i-ii, original magnifications, 100× and 800×). (B) Thymi from 3- to 6-month-old DL4^+/LacZ mice were whole-mount X-Gal–stained, sucrose-infused, and embedded in Tissue-Tek optimal cutting temperature compound. Cryosections were stained with anti-cytokeratin 8 (original magnification, 600×). (C) Thymi from 3- to 6-month-old DL4^+/LacZ mice stained in whole-mount preparations with Bluo-Gal that was visualized by scanning electron microscopy (original magnification, 10 000×). Images are representative of those obtained from at least 3 mice of each genotype analyzed in a minimum of 3 different experiments.