Sweat duct milia--immunohistological analysis of structure and three-dimensional reconstruction

The fine structure of sweat duct milia and the pathomechanism in their aetiology are still unknown. To examine the relationship and connection of milia to the sweat ducts as well as to the overlying epidermis, nine sweat duct milia, six incomplete and three complete, were studied by three-dimensional reconstruction (3DR) analysis based on photomicrographs obtained after histological and immunohistochemical staining with antibodies against carcinoembryonic antigen (CEA), cancer antigen (CA 50) and human cytokeratin 19 (CK 19). In both incomplete and complete milia, an eccrine duct expressing the antigens penetrated into the cyst wall at the centre of its base, formed a circular path within the wall, and opened into the inner cavity. The eccrine duct was mature in eight milia and immature in one. In the cyst wall, CA 50 and CK 19 were detected throughout the entire cyst except for the most apical portion of incomplete milia, where the cyst wall fused with the overlying epidermis which did not express any of the antigens. CEA was distributed mainly in the basal half of the milia. The finding that the path of the eccrine duct within the cyst wall is circular conflicts with the currently accepted concept of simple penetration of the eccrine duct into the wall, suggesting an acrosyringeal origin of the milia. An incomplete milium is the result of fusion between cells derived from an eccrine duct and those derived from the surrounding epidermis, while the formation of a complete milium does not involve this fusion.