How-To-Do-It The Sea Urchin Embryo: A Remarkable Classroom Tool

The experimental material of choice for the investigation of many develop-mental mechanisms is the sea urchin embryo (Giudice 1986; Davidson, et al. 1982; Oppenheimer & Lefevre 1989). It is the organism of choice be-cause of specific qualities that also make it ideal for classroom use. Unlike chick embryos, which are covered by shells and available in small numbers, and unlike frogs, whose males are often sacrificed to obtain sperm and females are hormonally induced to ovulate, sea urchin embryos are avail-able by the billions and clearly display embryonic development. No shells are present to block viewing and all experiments are done in the simplest of media-natural or artificial sea water. And, teachers can easily obtain sea urchins. Sea urchin embryos have been used for decades in the classroom and re-search laboratory and are the finest tools available for introducing stu-dents to the wonders of embryonic de-velopment and the world of research science. This article will illustrate how the sea urchin has been used to un-cover key developmental mechanisms and how it can be used in the classroom to excite the students ' curi-osity and facilitate their introduction to well defined research experiences. The Sea Urchin in Research Many important discoveries in the areas of fertilization and early devel-opment resulted from experiments with sea urchin embryos. One such exciting discovery is the story of egg activation. How does a tiny sea urchin sperm that is only 0.0002 percent of the egg surface trigger the multitude of changes that occur in the fertilized egg? The sea urchin has been most in-strumental in answering this ques-tion. Within three seconds after the sperm binds to the sea urchin egg, a membrane potential change occurs (Whitaker & Steinhardt 1985). By 30 seconds, calcium ions begin to be re-leased from the endoplasmic retic-ulum to a free state in the cytoplasm, followed by the cortical reaction, in which cortical granules that line the inner surface of the egg plasma mem-brane begin to fuse with the mem-brane, releasing their contents. This leads to the formation of the fertiliza-tion membrane that blocks the entry of additional sperm (Whitaker &amp