Discussion Retinoic acid is expressed in a gradient along the anterior-posterior axis of the zebrafish. Because of the localization of a molecule which degrades retinoic acid in the anterior of the embryo, cytochrome P450, very little to none expressed in the extreme anterior of the embryo. The concentration of retinoic acid peaks in the mid-region of the embryo and tapers toward to posterior of the embryo due to degradation by another localized molecule which breaks down retinoic acid, retinaldehyde dehydrogenase 2 (Cebra-Thomas, Lecture). Different.Hox genes are expressed along the anterior-posterior axis according to the concentration of retinoic acid present in that region. Truncation similar to that noted in mice was readily observed in the zebrafish embryos treated with the highest concentration of retinoic acid (10-8 M) because this exogenous retinoic acid altered the natural gradient, causing ectopic expression of Hox genes. Regions that would normally have a lower concentration of retinoic acid now had a higher concentration and hence expressed genes similar to those expressed in the hindbrain and trunk region of the embryo, where the largest concentration of retinoic acid is usually found. The magnitude of the effects of treatment with exogenous retinoic acid appeared to be concentration dependent. Embryos treated with lower concentrations, 10-10 M and 10-11 M, developed relatively normally. The embryos treated with the 10-8 M also had larger yolk sacs, which may be indicative of their stunted growth and a less that normal level of metabolization of the yolk sac nutrients. While there was variation in the amount of visible melanocytes along the anterior–posterior axis of the embryos treated with lower concentrations of retinoic acid, this variation could not be correlated to the amount of exogenous retinoic acid to which the embryos were subjected. The embryos treated with a 10-11 M concentration of retinoic acid exhibited very little pigment along the axis, while those treated with higher concentrations (10-10 M and 10-8 M) exhibited an amount comparable to the control, which was treated with no exogenous retinoic acid. This observation may be due to a slight age difference in the embryos or other natural factors. In order to gain a better understanding of the effects of retinoic acid, the embryos from this study should be examined on a molecular and physiological level. Specific to the anterior anomalies known to result from overexposure to retinoic acid, other studies have been conducted to examine the effects of retinoic acid on the development of the zebrafish retina. In zebrafish, as in other vertebrates, retinoic acid is synthesized from two different enzymes, one ventral and one dorsal. In early eye development, only the ventral enzyme is present. When this enzyme is inhibited in vitro, the eyes lack a ventral retina, but can be rescued by treating them with retinoic acid. This result suggests that retinoic acid is necessary for the proper development of the ventral retina (Marsh-Armstrong, et al, 1994). Overexposure of the retina to retinoic acid induces a duplication of the retinas during development (Hyatt, et al, 1992). It also results in a precocious development of rod photoreceptors and inhibition of cone photoreceptors maturation, suggesting that the retinoic acid signaling pathway is involved in the differentiation and maturation of rod and cone photoreceptors (Hyatt, et al, 1996).   Literature Cited Cebra-Thomas, J. Spring 2004. Embryology Laboratory CD. Swarthmore College. Cebra-Thomas, J. April 8, 2004. Lecture 19: Neural Crest and A-P Identity. Swarthmore College. Gilbert, Scott. 2003. Developmental Biology. Sinauer Associates, Inc.: Sunderland, MA. See also: www.devbio.com. Hyatt, G. A., E. A. Schmitt, N. R. Marsh-Armstrong, J. E. Dowling. September 1992. Retinoic acid-induced duplication of zebrafish retina. Neurobiology, Vol. 89. pp. 8293-8297. Hyatt, G. A., E. A. Schmitt, J. F. Fadool, J. E. Dowling. September 1992. Retinoic acid receptors in vivo. Neurobiology, Vol. 93. pp. 13298-13303. Ligas, Adam. 2000. Testing the effects of retinoic acid on tail formation of developing zebrafish embryos. accessed March 13, 2004. http://www.swarthmore.edu/NatSci/sgilber1/DB_lab/Fish/ZF_RA.html. Marsh-Armstrong, M., P. McCaffery, W. Gilbert, J. E. Dowling, U. C. Drager. July 1994. Retinoic acid is necessary for development of the ventral retina in zebrafish. Neurobiology, Vol. 91. pp. 7286-7290.