Colloid milium: a final degeneration product of actinic elastoid.

Serum amyloid P component (SAP) is present not only in all types of cutaneous amyloidosis, but also in the normal dermal elastic fibers as well as abnormal elastic fibers, such as elastotic degeneration of elastic fibers in actinic keratosis. It was previously postulated that the colloid substance in colloid milium is a final degeneration stage of actinic elastosis. In a lesion of colloid milium SAP was intensely positive with an indirect immunofluorescence method using rabbit antiserum to human SAP. Electron microscopy of the colloid lesion revealed components of normal elastic fibers as well as those of actinic elastoid as seen in actinic elastosis. It is concluded that the colloid substance derives, at least in major part, from elastic fibers through actinic degeneration.