Stereological study of postnatal development in the mouse utricular macula

Abstract

This study describes the morphometric changes taking place in the utricular macula of mice with ages in geometric progression from 1 to 512 days after birth. By using design-based stereological methods, the total volume and surface area of the sensory epithelium as well the total number of the hair cells and supporting cells were estimated. Finally, the numerical density, volume density, and mean volume of the individual cell types were determined. The major changes were found in the number of the individual cell types during the first couple of weeks, and a mature composition of cell types was not attained until 16 days after birth. There was no change in the total number of cells and no decline in the number of hair cells within the time period studied.

Over Half the Hair Cells in the Mouse Utricle First Appear After Birth, with Significant Numbers Originating from Early Postnatal Mitotic Production in Peripheral and Striolar Growth Zones

“…Those reports attributed the statistical increases to postnatal hair cell differentiation and the direct conversion of supporting cells into hair cells (Rusch et al 1998;Kirkegaard and Nyengaard 2005). However, we suspected that previously undetected populations of postnatally dividing progenitors might continue to produce progeny that would become new hair cells, thus contributing to the growth of the utricle's sensory epithelium.…”

“…Interpretation of those limited data appeared to lead to the conclusion that little or no mitotic production of hair cells occurs in mice after birth (Rusch et al 1998;Kirkegaard and Nyengaard 2005).…”

“…Consistent with this, early investigations in rodents reported that the production of hair cells and supporting cells peaks during the second half of embryogenesis and declines precipitously before birth (Ruben 1967; Sans and Chat 1982; Lee et al 2006). Yet, recent statistical density estimates have suggested that hair cell numbers in mouse utricles continue to increase in the weeks after birth (Rusch et al 1998;Kirkegaard and Nyengaard 2005).…”

The function and molecular identity of inward rectifier channels in vestibular hair cells of the mouse inner ear

“…The increase in Kir2.1 mRNA expression during the 1st postnatal wk also coincided with the continued maturation of the vestibular system in mice (Rüsch et al 1998). Furthermore, the total number of mature hair cells continues to increase until P16 (Kirkegaard and Nyengaard 2005;Li et al 2008), which may contribute to the higher Kir2.1 mRNA expression level measured at P25 compared with P7.…”

“…None of the other Kir2 family subunits displayed substantial expression at the developmental onset of I K1 or at any time point that we examined. Furthermore, Kirkegaard and Nyengaard (2005) found that hair cell numbers in mouse vestibular epithelia increase until P16. A larger number of hair cells expressing Kir2.1 are consistent with our data, which show an increase in Kir2.1 mRNA expression between P7 and P25.…”