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		Development of the Heart Morphogenetic Field in the Axolotl Embryo   Marielena Vélez, Swarthmore College, 2004 Miha Krsmanovic, F&M College 2001    Introduction There are two modes of development common to most species in the animal kingdom.Virtually all embryos undergo both mosaic and regulative development at some point during their growth. Regulative development generally occurs in early gastrulation when cells are induced to form different structures according to the cell-cell signaling interactions in a specific area of the embryo that lead to the conditional specification of a cell's fate. A cell undergoing regulative development can be transplanted to another part of the embryo and form whatever structure belongs in that area instead of the structure that it would have originally formed because it is competent to receive the different signals from the new cells around it. The second mode of development is mosaic development. Mosaic development results from the autonomous specification of a cell's fate. These cells, instead of depending on cell-cell interactions, are determined by cytoplasmic factors contained within the cell itself. These cells will form a given structure even if they are moved to a new location and are exposed to cell-cell interactions and signals that differ from their original position. Most organisms contain tissues which may undergo one or both of these developmental mechanisms at a given time (Gilbert, 2003). The purpose of this experiment is to determine whether the heart morphogenetic field of the axolotl embryo undergoes regulative or autonomous development. The developmental pattern of the amphibian heart will be studied by bisecting the heart primordial field and observing the subsequent development of the heart, if present. If the heart were to develop using an autonomous mechanism, the cells would develop normally even if the two sides were not in physical contact with one another. This would lead to half a heart growing on either side of the incision. However, if instead the heart developed using a regulative mechanism, then the cells on the two sides would compensate for the perceived absence of the other cells. This would lead to the development of two distinct, and perhaps functional, hearts (Hamburger, 1960). The heart morphogenetic field was split using two different techniques. First the field was split using aluminum foil. This bisection ensures that an impenetrable barrier exists between the two primordia and cuts off all signaling between cells on opposite sides of the foil. However, given that this material is completely unnatural to the embryo, this implantation technique could be excessively stressful to the embryos. Thus a second technique was modeled after the Hamburger experiment for the formation of two hearts, and involved the bisection of the heart morphogenetic field of using a transplant of non-heart tissue from older embryos. These tissues would serve as a barrier to the intracellular signals and already be specified as some other tissue. Therefore the grafts would be impervious to the heart signals. Since these transplants would be of axolotl tissue as opposed to some foreign source, such bisection would be less stressful and would presumably heal faster than bisection using foil.