Appearance and distribution of two Ca²⁺-binding proteins during development of the cochlea in the musk shrew

In the developing cochlea of the musk shrew, Suncus murinus, the localization of two Ca²⁺-binding protein, calbindin and calmodulin, which are thought to play different roles in the nervous system, was examined during gestational and postpartum periods. Calbindin is thought to play a Ca²⁺ buffering role, while calmodulin activates other proteins. Cochleae from the musk shrews sacrificed from gestational day (GD) 15 to postnatal day (PP) 9 and as adults, were immunohistochemically analyzed. The localization and order of appearance of calmodulin in sensorineural elements were similar to those of calbindin, except for timing of appearance. Calmodulin-staining was recognized first in the spiral ganglion neurons on GD21, followed by the inner hair cells (IHCs) on GD23 and outer hair cells (OHCs) on GD26, while calbindin immunoreactivity in the spiral ganglion neurons on GD19, the IHCs on GD21 and the OHCs on GD23. In hair cells, during development, immunostaining of calbindin and calmodulin was initially seen in the cytoplasm, followed by the cuticular plate. Cytoplasmic staining then decreased in mature hair cells. Non-sensorineural components also showed positivity for both calbindin and calmodulin. The lateral wall of the cochlear duct was positive for calbindin, while the stria vascularis was positive for calmodulin. Immunoreactivity for calbindin was present earlier than that of calmodulin in sensorineural elements, suggesting that in the developing cochlea, calbindin and calmodulin have different functions and that Ca²⁺ buffering capacity, which is regulated by Ca²⁺ buffer proteins, such as calbindin, may be required before trigger proteins, such as calmodulin, function.