CELL POPULATION KINETICS OF THE SCLEROBLAST DURING ETHISTERONE-INDUCED ANAL-FIN PROCESS FORMATION IN ADULT FEMALES OF THE MEDAKA, ORYZIAS LATIPES

Proliferation and migration of the scleroblasts and their precursor cells during ethisterone-induced anal-fin process formation in adult females of the medaka, Oryzias latipes , were investigated using a radioautographic technique with ³H-thymidine. In the posterior margin of the joint plate, rapid increases in labeling indexes and scleroblast population began simultaneously in the central portion on the third day of ethisterone treatment, and the population became approximately ten times that in the control fish on the fifth day. However, nomarked increase in scleroblast population occurred in the peripheral portion, although the scleroblasts showed nearly the same labeling index as those in the central portion throughout the experiment. These observations indicate that the scleroblasts and their precursor cells migrated from the peripheral to the central portion along the proxi-distal axis of the joint plate. Some cells in the inner and hind spaces also migrated to the posterior margins and subsequently developed into scleroblasts. It may be concluded, therefore, that the scleroblasts are derived from precursor cells situated not only along the posterior margins but also in the adjacent spaces. These precursor cells proliferate and/or migrate to the central portion of the posterior margins and aggregate to form the scleroblast mass.