Histological and ultrastructural studies of the basal disk of Hydra

Summary

In addition to glandulomuscular cells, three other cell types are found in the basal disk of Hydra. These are interstitial cells, cnidoblasts and nerve cells. Although only a few of the two former cell types are recognized in this region, the observations from this study refute previous statements to the contrary. Nerve cells are concentrated in the basal disk where they form a type of “network” system, due to the presence of bipolar, tripolar and multipolar cells. In some specimens, they assume a circular arrangement around the base of the polyp. Using morphological criteria for identifying the three types of epidermal nerve cells (neurosensory, neurosecretory and ganglionic) in other body regions, only neurosensory and neurosecretory cells are observed in the basal disk. These are indistinguishable ultrastructurally from their respective counterparts in other regions. It is possible that ganglionic cells are also present in the basal disk, but there may be few such cells.

It is suggested that the three cell types originate from the budding region and these precursor cells are then forced proximally. Interstitial cells, escaping their differentiative function, do not develop apparently into other cell types of the basal disk. Cnidoblasts contain normal nematocysts but their functional ability is uncertain. Neurosensory and neurosecretory cells arise directly and independently from interstitial cells in the budding region, as evidenced by the appearance of immature nerve cells in the peduncle and their absence in the basal disk. Although viable cells may be discarded from the basal disk, it is believed that most cells die in situ and are then eliminated. The possible role of nerve cells is discussed briefly.