The rheology and molecular organization of epithelial mucus 1

Abstract

In most instances where sliding transport has to occur over an epithelial tissue surface, the surface is covered by a system of cells capable of secreting a thick flowing fluid, mucus. The sliding motion can involve the transport of the whole organism, the transfer of food, the facilitation of sperm or ovum movement, the clearance of respiratory air ways, etc. There is considerable evidence that the major biopolymeric component involved is a glycoprotein, 80% of whose weight is carbohydrate and 20% is protein. The carbohydrate is attached in the form of short oligomeric side chains to about 4/5 of the protein backbone leaving 1/5 of its length bare. The bare part of the protein involves disulphide bridges which mayor may not be intramolecular. These glycoproteins are crosslinked into a weak gel i.e. a system which is viscoelastic and is endowed with long, if not infinite, mechanical relaxation times.

Where, as is often the case, mucus is in mechanical contact with an array of metachronously beating cilia it acts as coupler in facilitating transport by ciliary propulsion. In relation to the period of the ciliary beat its characteristic relaxation time is sufficiently long so that the response of the mucus layer lying on top of the bed of beating cilia is that of a semi-solid sheet having an essentially rigid interaction with the moving wave. The requirements are strongly linked to ciliary beat characteristics. While solid-like properties are essential they must fall into a certain class. The characteristics of an incipient gel are most favourable in establishing transport.

Mucus also possesses properties whose effects and purposes are not yet understood. Mucus, for example is unstable and its gel-like structure declines with time. This occurs rather slowly at body, but more rapidly at higher temperature. Hence the crosslinks of mucus are labile and of a character not yet established. The glycoprotein molecules building the networks are but for the cross links molecularly disperse. This is indicated by measurements of the hydraulic permeability of mucus which is far too low for any other arrangement of the glycoprotein strands to be considered.