ULTRASTRUCTURAL AUTORADIOGRAPHIC ANALYSIS OF NEUROGENESIS IN THE HYPOTHALAMUS OF THE ADULT FROG, RANA-TEMPORARIA, WITH SPECIAL REFERENCE TO PHYSIOLOGICAL REGENERATION OF THE PREOPTIC NUCLEUS .1. VENTRICULAR ZONE CELL-PROLIFERATION

The localization and fine structure of proliferating cells in the hypothalamic preoptic area were studied by light- and electron-microscopic radioautography 1-2 h following single application of H-3-thymidine to adult Rana temporaria taken from their natural habitat in the spring and autumn. H-3-thymidine uptake by proliferating cells was much more pronounced in frogs caught in May/June, i.e., a month after the breeding period (labeled cells represent about 10% of the total ventricular zone cell population), compared to animals caught in mid-September, when it was very low. In both H-3-thymidine treatment groups the vast majority of labeled cells are found exclusively within the preoptic recess ventricular zone. With regard to ultrastructure, it contained proliferating cells of at least 4 types, ranging from immature forms (bipolar stem cells) to more differentiated elements (tanycyte-like ependymoblasts, ''classical'' ependymoblasts). All of them showed label over their nuclei indicating that these cells are capable of DNA synthesis and mitosis. The possible role of the preoptic recess ventricular zone as a source of precursor cells for new peptidergic neurosecretory cells, conventional neurons and glial cells in the hypothalamic preoptic area of the adult frog is discussed.