STAGES OF LARVAL DEVELOPMENT AND STEM-CELL POPULATION-CHANGES DURING METAMORPHOSIS OF A HYDROZOAN PLANULA

Scanning electron microscopy and light histology were used to reveal t he changes in overall morphology and in stem cell differentiation and distribution that occur as a free-swimming, solid hydrozoan planula la rva is transformed into a sessile, hollow adult polyp. Eight stages of development are described: young 10-hour planula, mature 48-hour plan ula, attaching planula, disc, pawn, crown, immature polyp, and primary adult polyp. The larval interstitial stem cell population (interstiti al cells, nematocytes, ganglion cells) undergoes dramatic changes duri ng metamorphosis: (1) distribution patterns change, (2) certain larval derivatives disappear, (3) new types of derivatives differentiate, an d (4) migration patterns become more complex. This study is the first to examine how a stem cell system develops in an organism that goes fr om embryo to larva to adult.