About the Cover
Cover
Pulmonate snails have unusually large, identifiable neurons, so the central ganglia of these animals have been used for years to study the details of neural physiology, biochemistry, and more recently, molecular genetics. Moreover, because the fertilized eggs of pulmonate gastropods develop directly into juvenile snails, such embryos are particularly well-suited-and have been used-to describe the development of central neurons, neural circuits, and their functions. The full potential of these model systems has, however, never been realized.
The difficulty is captured in essence by the image on the cover-an embryo of the pond snail Helisoma trivolis. Freshwater pulmonates develop within a tough egg capsule, which is filled with a nourishing, protective, and regulatory albuminous fluid. This system, though marvelously adaptive to the habitat of these animals, hinders experiments in which embryos are perturbed and then monitored for a time, so that the effect of the treatment on development and function can be determined. Treatment requires only that the embryos be isolated from their capsules and their salubrious medium, but monitoring requires that the exposed embryo develop and grow relatively normally for an experimentally appropriate period-usually hours to days.
In this issue of The Biological Bulletin (p. 278), Jeffrey I. Goldberg and his students report that, of various culture media they tested, none could sustain normal embryonic development for more than a short period. But the good news is that, when decapsulated Helisoma embryos were transplanted into host capsules, they survived for 13 days (from the 2-cell stage to hatching). Moreover, the development and growth of such transplanted embryos were similar to those of undisturbed controls. This novel methodology was also given a practical test: a pair of serotonergic nerves was laser ablated, the operated embryos were transplanted into host capsules, and a role for the neurons in the regulation of a ciliary rotational behavior was demonstrated the following day. The success of the transplantation technique indicates that a wide range of classic and modern embryological studies can now be performed on important model species with encapsulated eggs.
The Helisoma embryo on the cover is in stage E30 (i.e., 30% of development is completed, about four days after egg-laying); the length is about 325 &mgr;m, and the height, 200 &mgr;m. To the right is the domed protoconch being secreted by the underlying shell gland; to the left, the foot points ventrally and the radular sac is visible (see the legend for Fig. 1, p. 281).
The image was provided by Shawn Parries at the University of Alberta, and the cover was designed by Beth Liles, Marine Biological Laboratory, Woods Hole, Massachusetts.
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Copyright © 2003 by the Marine Biological Laboratory.
