Stem-cell nomenclature is in a muddle! So-called stem cells may be self-renewing or emergent, oligopotent (uni- and multipotent) or pluri- and totipotent, cells with perpetual embryonic features or cells that have changed irreversibly. Ambiguity probably seeped into stem cells from common usage, flukes in biology's history beginning with Weismann's divide between germ and soma and Haeckel's biogenic law and ending with contemporary issues over the therapeutic efficacy of adult versus embryonic cells. Confusion centers on tissue dynamics, whether stem cells are properly members of emerging or steady-state populations. Clarity might yet be achieved by codifying differences between cells in emergent populations, including embryonic stem and embryonic germ (ES and EG) cells in tissue culture as opposed to self-renewing (SR) cells in steady-state populations.

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Histology's nomenclature has grown and changed since tissues were first conceived and identified grossly, sectioned, and observed microscopically. But this nomenclature has been dominated by static views of adult tissues and has not incorporated insights acquired through modern techniques for preparing and examining tissues and contemporary theories of tissue dynamics (e.g., stem cells). Hopefully, incorporating dynamics into tissue nomenclature will illuminate tissues' relationships to each other and their evolution, and alter concepts of tissues' mechanisms of development, maintenance, and pathology.

The dimensions of an epithelium such as gastrodermis of Hydra viridis may be governed by several different cellular activities. Changes in the total cell number of this epithelium and in the movements of markers placed within it by grafting show what roles cell division and cell loss play in determining the epithelium's length.A hydra's rate of lengthening is ten fold greater during its development as a bud than after it has detached from its parent. Changes in the rates of movement of gastrodermal cells onto developing buds, and of sloughing a t tentacles or foot are, however, not correlated with this change in the rate of lengthening. Buds do not begin forming until at least two days after the rate of lengthening in the parent decreases. The rate at which the bud's gastrodermal cells move out onto developing tentacles is as great as the sum of the rates of movement of parental gastrodermal cells onto mature tentacles and toward the foot.The time at which the rate of lengthening of the animal changes is also not correlated with changes in the rate of growth of the gastrodermis. The combined rate at which a bud's gastrodermis accumulates cells from the parent's gastrodermis and grows within itself (intrinsic growth), is the same as the rate of gastrodermal growth in freshly detached animals which is entirely intrinsic. Furthermore, these rates are the same as that of the gastrodermis of budding parents, if the amount of growth replacing cells moving onto developing buds (cryptic growth) is added to the amount providing additional cells to the parent's gastrodermis. It seems that the gastrodermis of Hydra consists of four separate self-generating cell populations: The populations at the ends support the tentacles or foot. Between these are two more which meet at the lower edge of the budding region and support the development of buds. These more central populations can expand when the animal lengthens.

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