Microanalytic and light microscopic studies on the developing otic capsule.

The structure of the middle layer of the bone of the otic capsule is histologically unique: it is of endochondral origin, has no Haversian canals and the ossification develops from several centres which fuse to form the hardest bone in the body. Our study has been concentrated on the development of the otic capsule in the CBA/CBA mouse, followed from the 13th gestational day to early postnatal age. In the 14th gestational day inner ear, a condensation of mesenchyme is detected around the membranous labyrinth. A cartilaginous capsule is present on the 15th-16th gestational day. Prior to birth, ossification centres occur close to the stapedial footplate. Serial cryosectioning of the newborn inner ear reveals very few regions containing high levels of calcium (microprobe analysis) although by light microscopy, several ossification centres can be identified.