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 the changes in overall morphology and in stem cell differentiation and distribution that occur as a free-swimming, solid hydrozoan planula larva is transformed into a sessile, hollow adult polyp. Eight stages of development are described: young 10-hour planula, mature 48-hour planula, attaching planula, disc, pawn, crown, immature polyp, and primary adult polyp. The larval interstitial stem cell population (interstitial cells, nematocytes, ganglion cells) undergoes dramatic changes during metamorphosis: (1) distribution patterns change, (2) certain larval derivatives disappear, (3) new types of derivatives differentiate, and (4) migration patterns become more complex. This study is the first to examine how a stem cell system develops in an organism that goes from embryo to larva to adult.