Vegetative Reproduction by Budding in Hydra: A Perspective on Tumors

VEGETATIVE REPRODUCTION BY BUDDING INHYDRA: A PERSPECTIVE ON TUMORS STANLEY SHOSTAK* I.Introduction Analogies and models, useful adjuncts to scientific thought, are a shorthand, a way of substituting the accessible and simple for the inscrutable and complex. The analogy must not be fanciful, however, for its usefulness depends on its specificity, and its consequences must be testable. The microbial cell has proved an important model for tumor cells [1], for example, and, at a higher level of organization, vegetatively reproducing invertebrate organisms [2-4] suggest models for the growth and metastasis of tumors. Specific information and concepts from microbial genetics and physiology spawned the somatic mutation theory of cancer and clarified many aspects of tumor cells' metabolism. Recently, studies on the dynamics of cell populations during budding in the fresh water coelenterate, Hydra, have also advanced to a high order of specificity. The object of this report is to discuss the results and interpretations of recent work on Hydra and explore their implications to tumors. II.Hydra: The Steady State Animal Normal representatives of the fresh water coelenterate, Hydra, can be of different sizes. Nonbudding or slowly budding hydras (i.e., animals that only bud occasionally) are conspicuously smaller than rapidly budding animals with two or more buds [5-7], and changes from one degree of budding to another involve changes in the parent's size. Given a constant feeding schedule, laboratory animals remain at about the same size. A heavier feeding schedule can stimulate growth, and a lighter feeding schedule can lead to shrinkage. Feeding schedules generally adopted in laboratories promote the rapid growth of the small freshly detached buds into relatively large animals with two or more buds on them at every time. Such animals are said to be in the steady state and ?Department ofLife Sciences, University of Pittsburgh, Pittsburgh, Pennsylvania 15260. Perspectives in Biobgy and Medicine ¦ Summer 1977 | 545 are spoken of as standard hydras (fig. 1). For example, a standard Hydra viridis fed 7 days a week in the laboratory is about 2.5 mm long (without tentacles) and has an average of 2.3 attached buds which take about 2.5 days for their development [5, 8]. This hydra releases a bud and initiates a new bud on an average of once every 27 hours. Similar statistics forH. attenuata [6, 9] differ only slightly. The animal's body size is generally assumed to be a function ofthe size of its structural cell population, the epitheliomuscular cells of its outer cell layer and the digestive cells of its inner cell layer. How is the size of the structural cell population regulated? A. CELL POPULATION DYNAMICS OF HYDRA AND TUMORS The size of a cell population can be regulated by either or both of two o .!2 TJ FOOT TENTACLES ")¦ HEAD HYPOSTOME . GASTRIC REGION BUDDING REGION PEDUNCLE BASAL ADHESIVE DISK) Fig. 1.—Hydra. This line drawing is of a representative "standard" Hydra viridis. The animal's head consists of six tentacles and the elevated hypostome. Proceeding in the disto-proximal direction are the large gastric region, the budding region, the peduncle, and foot. The budding region is identical to the gastric region except for the presence of buds whose walls developed in continuity with the wall of the parent and whose gastral cavities, likewise, arose in continuity with that ofthe parent. The peduncle is distinguished from the more distal regions by its greater translucence, a function ofthe presence oflarge vacuoles in the cells ofthe inner body layer. The foot is the differentiated proximal end of the peduncle and is sometimes called the basal adhesive disk although it also has the capacity for secreting a gas bubble which allows the animal to escape from contact with its substratum. This animal has three buds labeled a-c in order of their age and stage of development. The bud indicated by a has just appeared and will develop for 1?? days before it detaches. The b bud is in the stage of tentacle and peduncle formation and differentiation. The cleft which will ultimately cleave this bud from the parent has just begun on its under side. The c bud is completely formed and is about to detach from the...