Biochemical and functional-structural aspects of human cervical mucus.

Human cervical mucus is composed of an insoluble polymer of glycoproteins (mucin) which forms a matrix in which the aqueous phase is dispersed. Both the mucin and the aqueous phase are considered to be of importance to the structural integrity and function of cervical mucus. The mucin is considered to be composed of coiled glycoprotein molecules. These molecules may then associate by the chelation of divalent copper, via the carboxylic acid groups of the terminal sialic acid moieties of the carbohydrate side chains, to form the mucin per se. The modulation of the mucin structure at midcycle may then be achieved by the reduction of the copper bound to the sialic acid, by the oxidation of L-ascorbic acid: the L-ascorbic acid becoming oxidized to dehydroascorbic acid. This will result in an increase in hydration of the mucin by the association of water molecules with the uncomplexed sialic acid molecules of the glycoproteins. Thus the biophysical changes of cervical mucus may occur without enzymatically induced biochemical reactions. The proposed biochemical structure of cervical mucus is in accordance with the honeycomb-like structure of cervical mucin viewed by scanning electron microscopy. Whether or not the mucin exists precisely in this form in vivo is debatable. However, the honeycomb-like structure provides a convenient model on which one may investigate the biochemical, physiological and pathological nature of cervical mucus.