Experimental Embryology In Leeches: Cellular and Molecular Approaches

Abstract

Among the annelids, glossiphoniid leeches, and in particular Helobdella triserialis, have become a favorite preparation for modern experimental embryology, and are currently used both for cellular and molecular genetic studies of development. The principal adventages of Helobdella as a model organism accrue from the large cells and well defined lineages, important experimental advantages lacking in animal species such as Drosophila. The eggs of glossiphoniid leeches are relatively large and the yolk of the egg is the major nutrient source for the developing embryo, enabling embryos to be cultured separately from the parent in a simple salt solution. Development from egg to adult is direct with no intervening trochophore or metamorphosis seen in polychaet annelids. The early embryos comprise large cells that are individually identifiable and accessible to experimental manipulation. Their suitability as experimental material was recognised by Charles Whitman in the 1890’s (Whitman, 1878). On the basis of his observations in the light microscope on the early cleavage of a glossiphoniid leech egg, Whitman first stated the idea that each identified cell in the early embryo is developmentally distinct and that each identified blastomere and the clone of its descendant cells plays a specific predestined role in later development. In the 1970’s, new techniques for lineage analysis by intracellular injection of tracers were developed using leeches. This has enabled the construction of detailed lineages for identified cells of the early embryo. Subsequent experimental work has been concerned with: the role of cytoplasmic determinants, the relative roles of cell ancestry and cell interactions in determining developmental fate, and the control of cell cycle in embryonic cells. Recently, Helobdella has also been used for molecular genetic studies of development. Highly conserved homeoboxes from Drosophila genes have been used to identify genes in Helobdella that may be important for regulating development.