ZebraFish

Background Information

Model organisms, particularly vertebrates, have been researched extensively in science to reach a greater understanding of human embryo development without facing the ethical issues such studies would arouse if performed on human embryos. They are used, not only in the study of development of a healthy embryo but also for the study of diseases and in particular the role genetics plays in the presence and development of such diseases.

Why zebrafish?

Among these models, the zebrafish is one of the most useful due to particular unique characteristics which allow it to be more easily studied than others. These advantages include the high proliferation rate of the zebrafish (something which is important for the study of genetics) and in addition to this, scientists are able to maintain them in breeding condition all year round. The fertilisation of the zebrafish is an external process; this means that the gametes can be fertilized individually by scientists, allowing for the manipulation of genetic material for the purpose of study of individual genes and the roles they play. Also, the unique transparency of the zebrafish embryo allows scientists to closely observe the development of the phenotypes without disturbing the process of development, allowing for the observation of the development of individual embryos from beginning to end.

"‘You can see different cell types, watch individual cells develop, do transplantation experiments’, Eisen enthuses, ‘and development is quick but not too quick’. Being able to watch individual neurons developing in real time opened up whole new avenues of research for Eisen and other neurobiologists." (Bradbury, 2004)

This is different to the study of other embryos in that their development must cease in order to dissect the embryo for study. This hinders the study of the embryo greatly, forcing scientists to effectively ‘fill in the gaps’ of information they could not observe due to the disruption of embryo development.

History of Model Use

In the Begining... George Streisinger

The use of the zebrafish as a model organism began in the late 1960s when a scientist at the University of Oregon, George Streisinger, chose to study it based not only on the many advantages stated above but also on his love for tropical fish.

Streisinger wanted to analyse mutants among the zebrafish in order to study embryological development and stated that his aim was to “study features of the organisation and embryological development of the vertebrate nervous system through the use of mutant strains” (Grunwald and Eisen, 2002). In order to do this, he decided that first the phenotypes showing rare recessive mutations must be used to reproduce heterozygous offspring. It wasn’t until 1976 that Streisinger and fellow scientist, Charline Walker managed to produce wholly or partially homozygous offspring using sperm to activate an egg but in such a way that the sperm (genetically impotent due to UV damage) did not contribute to the genetic make-up of the offspring. In order for the eggs to remain diploid without gaining an extra set of chromosomes, the second meiotic division was inhibited, giving the offspring the entire set of chromosomes needed. This allowed for much ease in mapping out genes and correlating them with phenotypes.

Charles Kimmel

Streisinger was not the only scientist working on the embryologic development of the zebrafish. Neurologist Charles Kimmel (also at Oregon) used the zebrafish to model the development of the nervous system and vertebrae and by the mid- 1970s, Kimmel uncovered the segmental structure of its brain.

Hesitations

Problems soon arose in the form of doubt when it came to using a zebrafish to understand human genetics. “There was little theoretical appreciation of the degree to which vastly diverged species would share the regulatory pathways that govern cell behaviour and embryonic development” (Grunwald and Eisen, 2002). At the time, there was no gene cloning and little understanding about genes, making the whole zebrafish project almost a gamble to follow through with. This put much strain on the funding given to Streisinger and the credibility of his work. This however did not affect Neurologists such as Kimmel as they were not much worried about the relevance of their work. According to them, as long as they were studying nerve cells, it was all relevant.

Taking the Plunge

The Institute of Molecular Biology at the University of Oregon seemed to nurture the gamble-like quality in Streisinger’s work and provided him with the right environment as well as facilities to develop his model. The attitude of the institute was one which encouraged personal endeavours and above all else, it was important to be “committed to answering a question” (Grunwald and Eisen, 2002).

Once a method to produce heterozygous diploids was established, non- mutant strains of zebrafish were created and then specific mutations induced to study the phenotype and genotype relations. A 1981 issue of Nature journal published the first article pertaining to the zebrafish research which outlined the methods used by Streisinger to produce the homozygous clones of the zebrafish. The cover of the historical issue of the Nature journal [1]

Around this time, Kimmel had “described more identifiable neurons in the zebrafish than had been recognised in any other vertebrae” (Grunwald and Eisen, 2002). They also traced the growth of the axons of spinal cord motor neurons. The common ground on which Streisinger and Kimmel based their work lead to a collaborative effort to record “patterning and differentiation of the nervous system” (Grunwald and Eisen, 2002). In 1982, Kimmel began a 10 year program to “illuminate the developmental steps that led to the origin and organisation of distinct tissue types in the zebrafish embryo” (Grunwald and Eisen, 2002). Although Streisinger died in 1984, the project went on to describe a crucial stage in embryonic development which occurred just before gastrulation. This study was groundbreaking as the step described was the one in which the entire body plan emerged for the embryo, giving future researchers a conceptual framework (or map) from which to explore and monitor the development of cells. It also “placed the zebrafish in the context of vertebral biology” (Grunwald and Eisen, 2002). That is, when corroborated with the work of embryologists of other species, it provided common principles of development for all vertebraes, leading Kimmel to claim excitedly “the fish is a frog... is a chicken... is a mouse” (Grunwald and Eisen, 2002). This illustrates the dawning of the idea that genetic and biological organisation is the same among a great range of animals, and that by studying any one of these vertebrates, it was possible to learn more about humans.

Stages and Timeline of Embryonic Development

[2] Movie- zebrafish egg developing over a 24 hour period

Genetics of ZebraFish and Embryology

Current Embryology Research

ANAT2341 group projects

2009 Group Project 3

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