| AMELOBLASTIN ( AMELIN, SHEATHLIN, 13-17KDa PROLINE RICH NON-AMELOGENINS) |
| Ameloblastin is probably the second most abundant enamel matrix protein after amelogenin (although it still only accounts for 5-10% of the total protein). The following histological section through developing bovine enamel was stained with antibodies to ameloblastin. Ameloblastin is localised around the prism peripheries. However, in the newly secreted surface layer it is more uniformly distributed and seems to be within the prism bodies. Many of the prisms near the centre are in perfect cross section whilst those top right and bottom left are cut more oblique in section - hence the prism outlines vary between almost circular to elongated elipsoids. |
| Ameloblastin tissue biochemistry Ameloblasts sitting on the enamel surface (not present in this slide) secrete "parent" ameloblastin (molecular weight ~65KDa). It is very quickly processed to give a range of smaller polypeptides ranging from about 13-20KDa in size. It is these processing products that can be seen around the prism peripheries. As with amelogenin, ameloblastin is completely removed from maturation stage enamel. The exact function of ameloblastin is unknown. As a structural protein it may serve to keep the individual enamel crystals parcelled together to form the prisms. Recently it has been suggested that ameloblastin may have a role in cell differentiation (see below). |
| The slide above showed ameloblastin actually in the developing enamel matrix perhaps in a structural role. However, it may play a cellular role: Fig 2a below shows an histological section through a developing rat incisor treated with probes specific for ameloblastin mRNA (technique called in situ hybridisation). Cells expressing ameloblastin mRNA are stained grey-purple. The rat incisor is continually erupting left to right as we look at this picture. So the left side of the picture shows cells at an earlier point in development than cells on the right side. Ameloblastin is expressed by dentine forming odontoblasts (Od) before it is expressed by ameloblasts (Am). As odontoblasts differentiate before ameloblasts it has been suggested that ameloblastin may play a role in the sequential differentiation of these cell types. Fig 2b shows the same section stained for standard histology (haematoxylin and eosin). This slide clearly shows the classic histological picture of early tooth formation (Am = ameloblasts, Od = odontoblasts, bv = blood vessels). |
Ameloblastin expression by maturation stage ameloblasts
| (B) shows in situ hybridisation of ameloblastin (amelin) mRNA (dark staining in the ameloblast cell layer). It is expressed throughout enamel development - all through secretory stage, through transition and through maturation (the yellow line marks the end of secretion). However, ameloblastin cannot be detected in the enamel matrix during maturation (unpublished observation). The ameloblasts seem to keep expressing ameloblastin but do not secrete it during maturation. The ameloblastin present from the secretion stage is degraded by the serine proteases mentioned previously and is removed from the matrix prior to maturation. This again suggestes that ameloblastin may have a role inside cells as well as in the enamel matrix. (A) reminds us that in contrast to ameloblastin, amelogenin is only expressed during the secretory phase of enamel formation. |
