About the Cover
Cover
The size of neuronal populations in developing metazoan nervous systems is thought to be influenced by two processes: overproduction of nerve cells, which ensures that all necessary and appropriate connections are made; and programmed cell death, which culls the excess neurons that have not found a target. Programmed cell death during neural development has been described in many metazoans-both vertebrates and invertebrates. The Cnidaria are among the most primitive metazoans with a nervous system; and, indeed, the molecular mechanism for apoptosis, a type of programmed cell death, is uniformly expressed among the cells of Hydra vulgaris1.
Gordon B. Brumwell and Vicki J. Martin have raised embryos of Hydra vulgaris from fertilized eggs and have counted defined neuronal populations throughout development. They report in this issue of The Biological Bulletin (p. 70) that from the time that neurons of a particular type appear in the embryo, until development is complete, the population of these cells increases. From embryo to adult, no losses in any of the neuronal populations were detected. These data open the possibility, therefore, that no large-scale programmed cell death occurs in hydra during the development of the nervous system. Moreover, this study also suggests that culling of excess neurons-as a critical process in nervous system development-may have arisen in evolution after the appearance of the Cnidaria.
The neuronal populations described by Brumwell and Martin were first identified on the basis of their immunoreactivity to two antibodies, and then divided into subpopulations on morphological criteria. One of the immunoreactivities-to an antiserum raised to RFamide (Arg-Phe-NH2)-occurs early in development; and, in an embryo 5 days after hatching, defines a pair of well-developed networks in the base and oral region. Such a 5-day embryo (about 0.9 mm from base to mouth) is shown on the cover, in the inset (blue). The larger image on the cover shows a nerve net in the base of an adult stained with both antibodies (the scale, in micrometers, is about 10 times that of the inset). Immunoreactivity to anti-RFamide (red) dominates the net, but a few neurons (yellow) are also reactive to the second antibody-a monoclonal to JD1, an uncharacterized antigen (green).
The cover images were provided by Gordon Brumwell and Vicki Martin. The inset image, from Fig. 2b, p. 74, was modified with Adobe PhotoShop by Beth Liles, MBL, who designed the cover.
1Cikala, M., B. Wilm, E. Hobmeyer, A. Bottger, and C. N. David. 1999. Identification of caspases and apoptosis in the simple metazoan Hydra. Curr. Biol. 9: 959-962.
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Copyright © 2003 by the Marine Biological Laboratory.
