Three-dimensional analysis of inner ear development in human embryos.

The development of the inner ear is difficult to understand morphologically, because it proceeds in a complicated manner. Chronological 3-D reconstructed models of the inner ear primordium in human embryos (Carnegie stage 16-22) were created from the histological serial sections in the Kyoto Collection of Human Embryos using 3-D-reconstruction software on a personal computer. The endolymphatic duct begins to extend at stage 18 and continues to extend. The formation of the anterior and posterior semicircular ducts begins at stage 17. The upper lateral region of the otic pouch starts to sink inward at stage 17 and then the epithelia of both sides face and fuse with each other. The fusion disappears and the mesenchyme appears in the primordium, which looks like a hole in the otic pouch at stage 18. The mesenchyme begins to enlarge in the otic pouch at late stage 18, and continues to enlarge until the formation of the loop of semicircular ducts at stage 19. The lateral semicircular duct is formed similarly at stages 18 and 19. In the mesenchyme of the lateral semicircular duct, we found apoptotic death near the epithelium of the otic pouch at late stage 19. The cochlear duct already begins to extend at stage 16. First it extends to the opposite direction of the future cochlear rotation at stage 16 and 17, and then turns to the future rotating direction at stage 18. The cochlear duct initiates rotation at late stage 19. The cochlear duct continues to rotate and forms approximately one winding at stage 22.