The byssus attachment plaque and the tissues responsible for its formation were studied in M. californianus 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.
The distal depression of the ventral pedal groove of Mytilus calffomianus was investigated by scanning and transmission electron microscopy. This part of the byssus forming system is responsible for the formation of the attachment plaque of the byssus thread.The longitudinal pedal ducts open into this area and the floor of the distal depression is covered by specialized cilia which terminate as biconcave flattened discs or "paddles." The disc is formed by a 360" curvature of the axoneme tip within the ciliary membrane. The diameter of the disc is about 1.33 p while that of the shaft portion is 0.24 p. There are about 11 cilia per square micron of surface area and the necks of the cilia are separated from each other by a web-like extension of apical cytoplasm extending from the epithelial cells.It is proposed that these specialized cilia function as microscopic spatulas for the application of the adhesive plaque material to substrate surfaces. The pattern of surface convection currents seen in vivo tends to support this hypothesis.Generally, cilia have a simple cylindrical shape, but there are descriptions of cilia having more complex morphology related to various specialized functional adaptations
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