Fig. 1.

Histology of maxillary first molars from wild-type, kallikrein-related peptidase 4 (Klk4) null, matrix metalloproteinase 20 (Mmp20) null, and Klk4/Mmp20 (MK) double-null mice from postnatal days 5, 11, and 15. Day 5 (secretory stage): the enamel layers from all four genotypes are protein-rich and stain histologically. The Klk4 null molar is similar to that of the wild-type (WT) mice, while the Mmp20 null and MK double-null molars have irregular enamel layers that vary in thickness. Day 11 (maturation stage): a layer of retained enamel proteins is observed near the cervical margin in wild-type mice (where E is labeled), but the stained organic matrix thins and disappears near the cusp tip. The Klk4 mice molar shows a uniform thickness of enamel proteins. The Mmp20 null and MK double-null molars have a protein-rich enamel matrix that varies in thickness. Day 15 (near eruption): no protein layer is detected in the wild-type mice (the enamel space is clear, unstained). Proteins are retained throughout the enamel layer in Klk4 null and MK double-null mice. The Mmp20 null mouse molar is partially erupted, but a thick layer of protein is still visible in the enamel on the distal surface (left). Am, ameloblasts; D, dentin; E, enamel; Od, odontoblasts; P, pulp. Bars = 100 µm.