Ultrastructural radioautographic analysis of neurogenesis in the hypothalamus of the adult frog, Rana temporaria, with special reference to physiological regeneration of the preoptic nucleus. II. Types of neuronal cells produced.

Light- and electron-microscopic radioautography was used to identify the newly formed neuronal cells in the hypothalamic preoptic area of the frog. Adult Rana temporaria that had been caught in May/June received repeated 3H-thymidine injections and were sacrificed 30 days later. Heavily labeled cells were found in 1-micron plastic coronal sections of the preoptic area and then analysed in electron-microscopic radioautographs of neighbouring thin sections. The cells were identified as newly generated by the presence of 3H-thymidine label over the nucleus. All frogs showed considerable numbers of new peptidergic neurosecretory cells, small conventional neurons, and glia in the preoptic area. Some new ependymally located cells contacting the cerebrospinal fluid displaying ultrastructural characteristics of monoaminergic cells were also revealed. We conclude that prominent ventricular neurogenesis normally exists in the intact adult frog hypothalamus. The birth of small hypothalamic neurons seems to represent a case of sustained growth leading to a net increase in neuron numbers without loss. Conversely, the birth of large peptidergic neurosecretory cells, in which the increased secretory activity often leads to natural death of some cells, is considered as a neuronal replacement phenomenon, referred to as physiological regeneration of the magnocellular preoptic nucleus. The possible significance of this phenomenon in adult Anamnia is discussed.