© 1974 by The Society for Integrative and Comparative Biology
Cell Movements Associated with Terminal Growth in Colonial Hydroids
Department of Physiology and Cell Biology, University of Kansas Lawrence, Kansas 66045
A series of studies into the nature or leimiual growth movements in hydroids (chiefly the thecate Campanularia) is reviewed. Both stolon and pedicel tips of all hydroids observed grow by an endless repetition of "growth cycles." In thecate stolons, the geometry of the tip's excursions is relatively simple and successive cycles are predictable in their duration and per cycle growth. Conversely, athecate stolons are much more complex or variable in each of these respects. In Campanularia both cycle time and per cycle growth differ among genetic stocks.
Such environmental or environment-related factors as temperature, organic contaminants in the sea water, and the nutritive condition of the colony each affect cycle time and/or per cycle growth. Also, ingestion of a large meal temporarily inhibits the retractions that characterize most growth cycles.
Growth movements are a property of the stolon tip, since isolated tips less than 0.5 mm long move in basically normal fashion. However, two aspects of the cycle are, modified by more distant regions: the cycle is accelerated by a pacemaker lying 0.5 to 2.0 mm from the tip, and the extent of each cycle's retraction phase is determined by intrastolonic pressure generated by hydroplasmic flow. Certain neuroinhibitor drugs duplicate in intact stolons the cycle changes seen in surgical isolates.
Synchronized with each growth cycle is a pattern of epidermal thickening and thinning at the stolon tip, this layer being thinnest just after the crest and thickest about two-thirds of the way through the cycle. Cells behind the tip also move in relation to the growth cycle, generally approaching the tip during epidermal thickening and retreating during epidermal thinning.
Several types of metabolic inhibitors markedly affect the stolonic growth movements, indicating the need of oxidative metabolism and of protein synthesis using a short-lived mRNA for the tip's activities.
Campanularia pedicel growth cycles differ from stolon growth cycles in having a shorter and more variable duration, a lesser per cycle growth, and a shallower retraction phase, with changes in these features along the length of the pedicel correlated with changes in the type of growth occurring.
These studies and those of other investigators are related, their contributions to our understanding of elongation are discussed, and persisting problems are highlighted.