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Nematocyte differentiation in hydra: commitment to nematocyte type occurs at the beginning of the pathway.

In hydra the four types of nematocytes arise by differentiation from the multipotent stem cells among the interstitial cells. It has been unclear where along the nematocyte pathway commitment to type occurs. Some evidence suggests that this commitment occurs at the beginning of the pathway, while other data suggest that it occurs at the terminal cell cycle midway through the pathway. Upon reduction of cell population sizes of the interstitial cell lineage by treatment with hydroxyurea, interstitial cells entering nematocyte pathways frequently undergo an amplification division. A nearest-neighbor analysis of pairs of nematoblast nests in such depleted animals has shown that the fraction of the nearest-neighbor pairs that are matched pairs, in which both nests are of the same type, is higher than predicted. A very high fraction of the matched pairs were identical pairs in which the number of cells in each nest was the same. Also, in a large majority of the identical pairs the nests were shown to be in the same stage of development. The simplest interpretation of these results is that the two daughters of the amplification division giving rise to the matched pair were committed to nematocyte type before the division occurred. In another experiment we show that the length of the G2 phase of the next-to-terminal cell cycle differs between desmonemes and stenoteles. This indicates that differences in the differentiation pathways of these two types exist before the terminal cell cycle. This result also supports the idea that commitment to type occurs at the beginning of the pathway. A means of reconciling the view that commitment occurs early with the view that it occurs late in teh differentiation pathway is discussed.