Which Came First: the Chicken or the Zebrafish? Exploring Embryonic Development Through Model Organisms

BY ROHAN BADLANI

Embryonic development has a number of different applications. From medicine to evolutionary biology to genetics, the study of this process is an important part of the world of science. In medicine, the development of the embryo can be monitored to understand the onset of congenital disorders such as heart malfunctions or musculoskeletal defects. Viewing the study through the lens of evolution, it is apparent that the similarities in embryonic structure and function across a variety of different species can be used as a means of proving common ancestry. Within the framework of genetics, it allows an individual to explore the activation of genes from an early stage in life.

One of the most fascinating aspects of developmental biology lies in the manner in which cells find their destined locations within the early growth phases in an organism. It is this subject that has inspired Dr. Isaac Skromne, assistant professor at the University of Miami’s Biology Department. Skromne has been exploring this aspect of growth development of both zebrafish and chickens as a part of his ongoing research. “Imagine a Cartesian plane. Different cells have to be in different positions, just like points on the plane,” Skromne explained.

The major phenomena investigated in the laboratory are segmentation and patterning. While segmenting involves the actual subdivision of tissues within an embryo, patterning involves the assigning of a specific identity to each group of cells. The study of both of these processes ultimately results in a greater understanding of cell differentiation as a whole within the embryonic stages of life. Skromne’s laboratory, located in the Cox Science Center at the University of Miami, investigates segmentation and patterning via the observation of specially chosen organisms.

The use of zebrafish and chickens as experimental models allows Skromne to explore complementary systems in the world of developmental biology. Zebrafish offer the advantage of a rapidly reproducing organism that can be crossed to ensure that certain genes are passed on to the next generation — researchers can then accordingly observe the development that takes place as a result of the expression of certain genes. These zebrafish are housed in a special aquarium facility located in the basement of the Cox Science Center. Through crossbreeding, the laboratory produces strains of zebrafish that express different pigmentations. The importance of such a manipulation is reflected in the use of the transparent pigmentation to observe the inner anatomy of the fish and its various features, such as the flow of blood.

Chickens have been used as model organisms since the age of Aristotle. Unlike the case of zebrafish, however, the Skromne laboratory does not possess the means to maintain populations on site. “We can’t have a chicken coop in the University of Miami,” Skromne joked. However, the large size of chicken embryos (fertilized eggs similar in size to those found in the grocery store) allows the researcher to actually create a window in the egg and surgically remove and replace tissue sections in order to observe their effects on development. This, in turn, would show whether or not the cells in a particular region have differentiated.

The fruits of the work of Skromne’s laboratory can be best seen through exposure to some of their contributions to scientific literature. In a paper published by the laboratory in November 2014 titled “Retinoic acid regulates size, pattern, and alignment of tissues at the head trunk transition,” Keun Lee and Skromne recorded their investigation of the coordination of neural and mesodermal tissues in zebrafish. Changes in these tissues can compromise their integration and function. It was shown that the process of this integration is heavily dependent on two functions of the signaling factor retinoic acid. These two functions involve specifying the size and the position relative to the mesodermal structures of the hindbrain, respectively.

These two functions of retinoic acid are independent yet coordinated with its own function in the patterning of the hindbrain. The investigation demonstrated retinoic acid alignment of neural and mesodermal tissues through the observation of neural and mesodermal landmarks. The findings indicated that the functions of retinoic acid were implemented prior to the specification of hindbrain and spinal cord territories, as well as before the activation of hox gene transcription.

Through the use of cell transplantation, it was shown that the signaling molecule regulated the patterning of the hindbrain and size specification in different ways. While retinoic acid had a direct effect on the hindbrain patterning, the regulation of the size specification was partially dependent on Wnt signalling pathways, a group of signal transduction pathways consisting of proteins that involves the transmission of signals into a cell from the extracellular environment.

The study of the development of embryos offers exciting opportunities to aspiring undergraduates with the patience and drive to excel. “Don’t expect immediate results,” Skromne advised. “There is a steep learning curve, so you should start early in freshman or sophomore year.” Although he warned of the difficulty of such research, Skromne also emphasized that the faculty at the University of Miami are interested in training undergraduate students and helping them become successful. However, he stressed that students must look for the various available positions. “The opportunities are out there, but they are not going to look for you. You have to knock on doors.”

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