Cell differentiation in the head of Hydra

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Abstract The differentiation pathways of cells in the head of hydra were investigated using cell-cycle analysis and tracing of cell movements. In the tentacles of Hydra oligactis, ectodermal epithelial cells (battery cells) and endodermal epithelial cells were terminally cell-cycle-arrested in G2. In contrast, the hypostome contained proliferating populations of ectodermal and endodermal epithelial cells. Proliferating cells were restricted to the outer ring area of the hypostome. In the ectoderm, tissue movement occurred toward the tentacles and also toward the mouth. During tissue movements toward the mouth, ectodermal epithelial cells of the hypostome differentiated into support cells for sensory nerve cells, which are located at the center of the hypostome. Tissue movements toward the tentacles correlated with the differentiation of the tentacles to battery cells. Both differentiation products were cell-cycle-arrested in the G2 phase. In the endoderm of the hypostome, the proliferation rate of endodermal epithelial cells was very low. A continuous flow of endodermal tissue occurred from the gastric column into the hypostome and tentacles. In contrast to the tentacles, the cell-cycle arrest in the endodermal epithelial cells of the hypostome occurred in G1. The tissue ring around the hypostome, where the tentacles insert, termed “tentacle bases”, represented the area of transition from gastric to head-specific tissue. Processes taking place in this area were: differentiation of ectodermal epithelial stem cells in the gastric column and hypostome into battery cells of the tentacles; differentiation of endodermal epithelial stem cells in the gastric column into endodermal epithelial cells of the tentacles and the hypostome; and differentiation of interstitial cells into hypostomal mucus cells. The origin of hypostomal mucus cells was studied in the mutant sf-1 of Hydra magnipapillata. These cells, located in the endoderm, were shown to differentiate from interstitial stem cells of the gastric column. The bulk of the hypostomal mucus cells were present in the G1 phase of the cell cycle, but some proliferation also occurred. Almost no interstitial cells were present in the hypostome.