Differentiation of inner ear fibrocytes according to their ion transport related activity

Fibrocytes in the lateral wall and limbus of the gerbil cochlea evidenced a capacity for ion transport activity by immunostaining for transport mediating enzymes including Na,K-ATPase, carbonic anhydrase (CA) and creatine kinase (CK). Fibrocytes of the spiral ligament unlike those in the suprastrial region and limbus decreased in abundance from base to apex. Spiral ligament fibrocytes at a given position along the cochlea varied in content of transport related enzymes, and on the basis of immunostaining, location and orientation, were classified into four types. Type I fibrocytes under the stria vascularis stained for CA isozymes II and III and CK isozyme BB. Type II fibrocytes under the outer sulcus and spiral prominence epithelium were found to contain only Na,K-ATPase. Type III fibrocytes lying adjacent to bone in the inferior region of the spiral ligament contained CA II and III and CK isozymes BB and MM. Type IV fibrocytes located more superficially in the inferior part of the spiral ligament stained variably for all the enzymes. Superficial fibrocytes in the suprastrial area disclosed Na,K-ATPase whereas the underlying fibrocytes stained for CA and CK. Limbal fibrocytes reacted with antisera to all the enzymes except CA III. Most fibrocytes in stromal plates beneath the vestibular system's neurosensory epithelium contained Na,K-ATPase and CA II but not CA III. These findings point to cooperativity in fluid and ion transport between epithelial cells and neighboring fibrocytes and demonstrate functional diversity of fibrocytes of the inner ear providing a basis for classifying those in the spiral ligament.