Two new cases of colloid milium were studied by several methods to examine if collagen is the source of colloid. Woundhealing experiments in the lesion revealed that colloid is reproduced de novo with possible interaction with epidermal cells. Unfixed, native colloid did not contain collagen and salt extraction did not alter the ultrastructure of colloid. Tissue culture of colloid fibroblasts produced colloid-like substance. These results suggest that colloid is not a degeneration product of collagen. Disc electrophoresis of extracted colloid produced a distinct band, which is absent in the normal skin control. Amino acid analysis of colloid suggests that colloid may contain the recently described class of glycoprotein, called "structural glycoprotein" that is not related to collagen because it does not contain hydroxyproline and hydroxylysine and contains only a small amount of glucose. These biochemical data support the conclusion reached in electron microscopical studies.
Hashimoto K, Katzman RL, Kang AH, Kanzaki T. Electron Microscopical and Biochemical Analysis of Colloid Milium. Arch Dermatol. 1975;111(1):49-59. doi:10.1001/archderm.1975.01630130051005