[Electron microscopic and immunofluorescent microscopic studies on the structure of the bony labyrinth]

The bony labyrinth of the guinea pig is composed of the inner endosteal, the intermediate endochondral and the outer periosteal layer. In addition, the endosteal layer is partially covered by an endosteal cell layer. At the boundary to the spatium perilymphaticum the endosteal layer is covered by an osmiophilic line. Osmiophilic lines also appear as a coating of bone cavities and canaliculi as well as on the periphery of the interglobular spaces in the endochondral layer. Osmiophilic lines were also found at the boundary between the three layers. Within the endochondral and especially the endosteal layer, the collagen fibrils are relatively loose and cover spindle-shaped areas which possess protein-carbohydrate complexes. In the periosteal layer the collagen is arranged in lamellae. Here, it was found that the protein-carbohydrate complexes are not quite as numerous. The high concentration of protein-carbohydrate complexes in all of the three layers is possibly responsible for the exceptional hardness of the otic capsule. Collagen type I is demonstrable in all three of the above layers. Collagen type III was also found to be present in the endosteal and the periosteal layer. The interglobular spaces of the endochondral layer contain only collagen type II and a cartilage-specific proteoglycan. The occurrence of the interglobular spaces, the strand-like formation of the collagen fibres, the areas with the protein-carbohydrate complexes, and the existence of collagen type III support the assumption that the bony labyrinth of the adult guinea pig remains at a low level of development. However, the uniform mineralization, the abundance of osmiophilic lines, and the small amount of the cytoplasm in the osteocytes lead to the conclusion that in the bony labyrinth very little change or transformation actually occurs.