The ontogenesis of pseudomonopolar cells in spiral ganglion of cat and rat.

The ontogenesis of type II ganglion cells (T II cell) in the spiral ganglion in the cat and in the rat was studied by light microscopy and by electron microscopy. In the cat, typical pseudomonopolar T II cells with light-coloured cytoplasm containing abundant neurofilaments are observed at birth in the basal part of the cochlea. No T II cells are observed in more immature areas such as the third turn. A constant number of T II cells is present and no typical sign of degenerative ganglion cells is observed throughout postnatal development. In the rat, T II cells can be recognized with confidence at the 6th day post partum. It is suggested that T II cells represent a population of normal cells in the spiral ganglion that can be seen late during ontogenesis.