Cochlear apical morphology in toothed whales: Using the pairing hair cell-Deiters' cell as a marker to detect lesions.

The apex or apical region of the cochlear spiral within the inner ear encodes for low-frequency sounds. The disposition of sensory hair cells on the organ of Corti is largely variable in the apical region of mammals, and it does not necessarily follow the typical three-row pattern of outer hair cells (OHCs). As most underwater noise sources contain low-frequency components, we expect to find most lesions in the apical region of the cochlea of toothed whales, in cases of permanent noise-induced hearing loss. To further understand how man-made noise might affect cetacean hearing, there is a need to describe normal morphological features of the apex and document interspecific anatomic variations in cetaceans. However, distinguishing between apical normal variability and hair cell death is challenging. We describe anatomical features of the organ of Corti of the apex in 23 ears from five species of toothed whales (harbor porpoise Phocoena phocoena, spinner dolphin Stenella longirostris, pantropical spotted dolphin Stenella attenuata, pygmy sperm whale Kogia breviceps, and beluga whale Delphinapterus leucas) by scanning electron microscopy and immunofluorescence. Our results showed an initial region where the lowest frequencies are encoded with two or three rows of OHCs, followed by the typical configuration of three OHC rows and three rows of supporting Deiters' cells. Whenever two rows of OHCs were detected, there were usually only two corresponding rows of supporting Deiters' cells, suggesting that the number of rows of Deiters' cells is a good indicator to distinguish between normal and pathological features.

Echolocating Whales and Bats Express the Motor Protein Prestin in the Inner Ear: A Potential Marker for Hearing Loss.

Prestin is an integral membrane motor protein located in outer hair cells of the mammalian cochlea. It is responsible for electromotility and required for cochlear amplification. Although prestin works in a cycle-by-cycle mode up to frequencies of at least 79 kHz, it is not known whether or not prestin is required for the extreme high frequencies used by echolocating species. Cetaceans are known to possess a prestin coding gene. However, the expression and distribution pattern of the protein in the cetacean cochlea has not been determined, and the contribution of prestin to echolocation has not yet been resolved. Here we report the expression of the protein prestin in five species of echolocating whales and two species of echolocating bats. Positive labeling in the basolateral membrane of outer hair cells, using three anti-prestin antibodies, was found all along the cochlear spiral in echolocating species. These findings provide morphological evidence that prestin can have a role in cochlear amplification in the basolateral membrane up to 120-180 kHz. In addition, labeling of the cochlea with a combination of anti-prestin, anti-neurofilament, anti-myosin VI and/or phalloidin and DAPI will be useful for detecting potential recent cases of noise-induced hearing loss in stranded cetaceans. This study improves our understanding of the mechanisms involved in sound transduction in echolocating mammals, as well as describing an optimized methodology for detecting cases of hearing loss in stranded marine mammals.

Characterizing respiratory capacity in belugas (Delphinapterus leucas).

We measured respiratory flow, breath duration, and calculated tidal volume (VT) in nine belugas (Delphinapterus leucas, mean measured body mass: 628 ± 151 kg, n = 5) housed in managed care facilities. Both spontaneous (resting at station) and trained maximal respirations (chuffs) were measured. The mean (±s.d.) inspiratory VT for spontaneous breaths (16.7 ± 4.7 l, range: 7.5-18.7 l) was larger than those predicted based on respiratory scaling equations from terrestrial mammals and was 32 ± 10% of estimated total lung capacity (TLCest) based on an equation from static measurements made on a range of cetaceans and pinniped lungs, and 52 ± 18% of estimated vital capacities (VC, mean: 27.7 ± 8.9 l, range: 16.7-40.3 l) based on respiratory measurements obtained during trained maximal respirations. Expiratory flow (V˙exp, spontaneous: 26.1 ± 5.5 l s-1, chuff: 66.8 ± 22.5 l s-1) was significantly higher as compared with inspiratory flow (V˙insp, spontaneous: 22.3 ± 4.6 l s-1, chuff: 30.1 ± 8.4 l s-1), and the maximal expiratory flow recorded was 212 l s-1. The breath duration was shorter for forced breaths (Expiration: 518 ± 101 ms; Inspiration: 905 ± 170 ms; Total: 1423 ± 227 ms) as compared with spontaneous breaths (Expiration: 995 ± 176 ms; Inspiration: 1098 ± 219 ms; Total: 2093 ± 302 ms). These data provide baseline estimates of the respiratory capacity of belugas.