Detection of the Acoustic Stapedius Reflex in Infants Using Wideband Energy Reflectance and Admittance

This study examined the measurement of the contralateral acoustic stapedius reflex in six-week-old infants and adults using wideband shifts in admittance and energy reflectance (YR). The reflex activator was bandpass noise from 2,500 to 11,000 Hz presented at a maximum spectrum level of 51 dB SPL measured in the ear canal. Reflexes were detected by calculating a cross-correlation between one-twelfth-octave measurements of YR for the highest activator level and responses to lower levels. The reflex-induced shifts in YR for the infant ears were similar in pattern to adult responses but were noisy at frequencies below 1000 Hz. Infant reflexes were more successfully detected when the cross-correlation was calculated from 1000 to 8000 Hz, whereas adult reflexes were more successfully detected for a cross-correlation from 250 to 2000 Hz. This method may be useful in capturing the most robust frequency region for acoustic reflex detection across postnatal middle ear development.

Hoxa1 and Hoxb1 synergize in patterning the hindbrain, cranial nerves and second pharyngeal arch

The analysis of Hoxa1 and Hoxb1 null mutants suggested that these genes are involved in distinct aspects of hindbrain segmentation and specification. Here we investigate the possible functional synergy of the two genes. The generation of Hoxa1(3′RARE)/Hoxb1(3′RARE) compound mutants resulted in mild facial motor nerve defects reminiscent of those present in the Hoxb1 null mutants. Strong genetic interactions between Hoxa1 and Hoxb1 were uncovered by introducing the Hoxb1(3′RARE) and Hoxb1 null mutations into the Hoxa1 null genetic background. Hoxa1(null)/Hoxb1(3′RARE) and Hoxa1(null)/Hoxb1(null)double homozygous embryos showed additional patterning defects in the r4-r6 region but maintained a molecularly distinct r4-like territory. Neurofilament staining and retrograde labelling of motor neurons indicated that Hoxa1 and Hoxb1 synergise in patterning the VIIth through XIth cranial nerves. The second arch expression of neural crest cell markers was abolished or dramatically reduced, suggesting a defect in this cell population. Strikingly, the second arch of the double mutant embryos involuted by 10.5 dpc and this resulted in loss of all second arch-derived elements and complete disruption of external and middle ear development. Additional defects, most notably the lack of tympanic ring, were found in first arch-derived elements, suggesting that interactions between first and second arch take place during development. Taken together, our results unveil an extensive functional synergy between Hoxa1 and Hoxb1 that was not anticipated from the phenotypes of the simple null mutants.