Mussels attach to solid surfaces in the sea. Their adhesion must be rapid, strong, and tough, or else they will be dislodged and dashed to pieces by the next incoming wave. Given the dearth of synthetic adhesives for wet polar surfaces, much effort has been directed to characterizing and mimicking essential features of the adhesive chemistry practiced by mussels. Studies of these organisms have uncovered important adaptive strategies that help to circumvent the high dielectric and solvation properties of water that typically frustrate adhesion. In a chemical vein, the adhesive proteins of mussels are heavily decorated with Dopa, a catecholic functionality. Various synthetic polymers have been functionalized with catechols to provide diverse adhesive, sealant, coating, and anchoring properties, particularly for critical biomedical applications.

Mussel-Inspired Adhesives and Coatings

Homeotic transformations of murine vertebrae and concomitant alteration of Hox codes induced by retinoic acid

The fouling marine mussel Mytilus edulis attaches itself to various substrates by spinning byssal threads, the adhesive discs of which are rich in the amino acid 3,4-dihydroxyphenylalanine (dopa). An acid-soluble protein was extracted and purified from the phenol gland located in the byssus-secreting foot of the animal. This protein is highly basic and contains large amounts of lysine, dopa, and 3- and 4-hydroxyproline. The composition of this protein and its sticky tendencies in vitro strongly suggest that it contributes to byssal adhesion.

https://science.sciencemag.org/content/sci/212/4498/1038.full.pdf

Polyphenolic Substance of Mytilus edulis: Novel Adhesive Containing L-Dopa and Hydroxyproline.

Oral homeostasis is dependent upon saliva and its content of proteins. Reflex salivary flow occurs at a low 'resting' rate and for short periods of the day more intense taste or chewing stimuli evoke up to ten fold increases in salivation. The secretion of salivary fluid and proteins is controlled by autonomic nerves. All salivary glands are supplied by cholinergic parasympathetic nerves which release acetylcholine that binds to M3 and (to a lesser extent) M1 muscarinic receptors, evoking the secretion of saliva by acinar cells in the endpieces of the salivary gland ductal tree. Most salivary glands also receive a variable innervation from sympathetic nerves which released noradrenaline from which tends to evoke greater release of stored proteins, mostly from acinar cells but also ductal cells. There is some 'cross-talk' between the calcium and cyclic AMP intracellular pathways coupling autonomic stimulation to secretion and salivary protein secretion is augmented during combined stimulation. Other non-adrenergic, non-cholinergic neuropeptides released from autonomic nerves evoke salivary gland secretion and parasympathetically derived vasointestinal peptide, acting through endothelial cell derived nitric oxide, plays a role in the reflex vasodilatation that accompanies secretion. Neuronal type, calcium-activated, soluble nitric oxide within salivary cells appears to play a role in mediating salivary protein secretion in response to autonomimetics. Fluid secretion by salivary glands involves aquaporin 5 and the extent to which the expression of aquaporin 5 on apical acinar cell membranes is upregulated by cholinomimetics remains uncertain. Extended periods of autonomic denervation, liquid diet feeding (reduced reflex stimulation) or duct ligation cause salivary gland atrophy. The latter two are reversible, demonstrating that glands can regenerate provided that the autonomic innervation remains intact. The mechanisms by which nerves integrate with salivary cells during regeneration or during salivary gland development remain to be elucidated.

Regulation of salivary gland function by autonomic nerves.

Vertebrate cranial placodes I. Embryonic induction.

The rat submaxillary salivary gland has five distinct parenchymal zones.

1Acini consist of secretory and myoepithelial cells. An extensive network of canaliculi connect the many cells within an acinus to the main lumen. The fine structure of acinar secretory cells suggests that they are capable of great synthetic capacity; each cell having a large amount of ergastoplasm, many Golgi zones, and a great amount of secretory material. It is proposed that these cells are of the continually secreting type.
2Intercalated ducts consist of cuboidal cells and myoepithelium. This segment connects the acini to the main conduit system of the gland. The fine structure of the cuboidal cells indicates that they are essentially nonsecretory.
3The granular duct consists of three types of columnar cells; (a) dark narrow cells which contain many free ribosomes but no ergastoplasm or granules, (b) light granular cells which have varying amounts of ergastoplasm and granules, (c) dark granular cells which are full of granules while the other cell constituents including the nucleus, occupy a basal position. It is proposed that these three cells represent different secretory stages of the same cell type. This supports the interpretation that secretion in these cells is not continuous, but cyclic in nature.
4The striated duct forms a small portion of the total gland parenchyma and consists of tall columnar cells with extensive infolding of the basal plasma membrane, relatively little ergastoplasm and very few granules. It seems likely that ion and water metabolism is a specialized function of this segment.
5The excretory duct consists of three cell types: (a) tall columnar light cells, (b) dark columnar vesiculated cells and (c) small basal cells. The basal infoldings of these cells and the arrangement of many capillaries around these ducts suggests that this segment is primarily concerned with water transport.

The rat submaxillary salivary gland. A correlative study by light and electron microscopy.

The byssus attachment plaque and the tissues responsible for its formation were studied in M. califomianus by light microscopy and by transmission and scanning electron microscopy. It was shown that the plaque consists of at least three phases which ultrastructurally resemble three secretions considered to be collagen, mucoid material and polyphenol. The mucoid and polyphenol appear to mix as a colloidal suspension in which the latter is the continuous phase and forms the definitive bonding surface. Plaque collagen represents an extension of thread material into the cementing substance.



Stimulated secretion within the ducts and distal depression of the mussel's foot shows a continuum of increasing heterogeneity from the inner toward the outer regions. This reflects the distribution of exocrine cell apices wherein exocytosis of polyphenol granules predominate deeply, mucous granules superficially and collagen granules in between.



It is proposed that the morphology of the plaque conforms to theoretical physical-chemical requirements for adhesion under water.

Arnold Tamarin

Papers

The rat submaxillary salivary gland. A correlative study by light and electron microscopy.

The structure and formation of the byssus attachment plaque in Mytilus.

Myoepithelium of the rat submaxillary gland.

Submaxillary gland recovery from obstruction. I. Overall changes and electron microscopic alterations of granular duct cells

Submaxillary gland recovery from obstruction. II. Electron microscopic alterations of acinar cells.