MODEL ORGANISMS

• Why we generally do not study development in humans?

  • Observation is difficult inside the uterus.
  • Morally and ethically, we do not want to perform experiments on human embryos.
  • We don't want to breed humans to look at effects of gene mutations on embryos.
• Model organisms

  • Researchers study development in model organisms to identify general principles
  • Developmental processes are so fundamental that there are striking similarities in the development of very varied organisms
  • Despite different adult forms, these organisms share similar developmental mechanisms.
• Model organisms

  • This is shown by the similarity of the basic embryo plan for vertebrate embryos.
  • The adult forms of these vertebrate animals look quite different and the eggs from which they arise are also different in size and design, but during development of the embryo, the overall design and body plan are very similar.
• Model organisms

  • When the primary research goal is to understand broad biological principles of development, the organism chosen for study is called a model organism
  • Researchers select model organisms that lend themselves to the study of a particular question
• For example, frogs were early models for observing development of an animal embryo because their large eggs are easy to observe and manipulate and fertilization and development occurs outside the mother's body.
• There are three considerations in choosing a model organism for research

  1. Biological considerations
  2. Practical considerations
  3. Historical considerations
• Biological considerations

  • Is this organism biologically suited to the type of study?
  • Obviously a frog cannot be used to study the particular features of mammalian development.
  • An important biological consideration for the study of development is accessibility of the egg and embryo for observation and manipulation.
• 2. Practical considerations

  • cost, space requirements, and the ease of handling and breeding the organism
• 3. Historical considerations

  if a model has been used in the past then its biology will be well understood and specific tools will have been developed.
• Development and morphological analysis

  • Advantages: large eggs, accessible embryos, short development time, and easy to keep in the lab
  • Model organism: frog and chicken
• Experimental embryology

  • advantages: large accessible embryos, robust embryos that tolerate manipulation, embryos grown in dish / culture
  • model organisms: frog or chick
• Developmental genetics

  • Advantages: ease of breeding in the lab, short generation interval (time from fertilization to sexual maturity), simple or small genome, and ease of observing embryos to see mutant effects
  • Model organisms: fruit fly, zebra fish, nematode worm
• Examples of invertebrate model organisms

  1. Fruit fly Drosophila melanogaster
  2. Nematode worm Caenorhabditis elegans
• Examples of vertebrate model organisms

  1. Frog Xenopus laevis
  2. Chicken Gallus sp
  3. Mouse Mus musculus
  4. Zebra fish Danio rerio
• Example of plant model organism

  1. Arabidopsis thaliana
• The fruit fly was first chosen as a model organism by geneticist Thomas Hunt Morgan and intensively studied by generations of geneticists after him. It is small, easily grown in the laboratory with a generation time of only two weeks, produces many offspring with 50 to 70 eggs per day and the embryos develop outside the mother's body.
• The fruit fly is also amenable to large scale mutational screens to identify genes involved in particular function such as forming the right body parts in the right place, so flies with mutated versions of all genes in the genome have been isolated. There are vast amounts of information on its genes and other aspects of its biology. The genome sequence and it contains 13,600 genes.
• Nematodes normally lives in the soil but easily grown in petri dishes. Only a millimeter long, it has a simple, transparent body with only a few cell types and grows from zygote to mature adult in only three and half days. with a sequenced genome.
• A fate map traces the development of an embryo
• Advantages of nematode worm

  • Easy to identify mutant animals: it is hermaphrodites
  • Easy to detect recessive mutations: self fertilization of heterozygotes will produce some homozygous recessive offspring with mutant phenotypes
  • Simple with only 959 somatic cells
  • This arise from the zygote virtually the same way for every individual
  • By following all cell divisions with the microscope, biologists have constructed the organism's complete cell lineage, a type of a fate map
• Advantages of frog as a model organism

  1. Ease of access and manipulation of the egg and embryo allowing experimental manipulation, assay inductive interaction
  2. Robust embryos can tolerate manipulation
  3. Parts of embryo can easily be cultured
  4. Rapid development 4 days to a free swimming tadpole
  5. A well established system 
• Disadvantages of frog as a model organism

  • No genetics
  • Complex and uncharacterized genome
  • Animals cannot be bred for multiple generation in the lab
• Advantages of chicken as model organism

  • Large and easily obtainable eggs
  • development can be observed by cutting a hole in a shell
  • the embryos can be removed and cultured allowing manipulation
  • methods develop for cell lineage analysis allows experimental manipulation and cell lineage studies
  • complex development similar to mammals
• Disadvantages of the chicken as a model organism

  • Not good for classical genetics
  • complex genome with many small chromosomes
  • but the genome is currently being sequenced
• Advantages of mouse as a model organism

  • It has a long history as a mammalian model of development
  • Much is known about its biology including its genes
  • Genome is well characterized and almost completely sequenced
  • Sophisticated and elaborate techniques exist to manipulate mouse genes: transgenic mice and mice in which particular genes are knocked out by mutation
• Disadvantages of mouse as a model organism

  • complex animals with genome large as ours
  • Embryos develop in the mother's uterus, hidden from view
  • embryo culture is difficult and limited
  • the generation interval is long (three months)
• Advantages of zebra fish as a model organism

  Recently chosen and developed because it has a unique combination of advantages

  • Easily observable eggs and embryos with advantages for carrying out genetic studies such as ease of breeding in the laboratory and small simple genome
  • Small fish 2 to 4 cm long
  • Easy to breed in laboratory in large numbers
  • Transparent embryos develop outside the mother's body, easy to observe
• For the zebra fish, the generation time is 2 to 4 months. The early stages of development proceed quickly. By 24 hours after fertilization, most tissues and early versions of the organs have formed. After two days, the fish hatches out of the egg case. Molecular techniques have been developed to manipulate genes. The genome was rapidly mapped and sequenced.
• Arabidopsis thaliana, a member of mustard family can grow and produce thousands of progeny after 8 to 10 weeks. It is also a hermaphrodite, each flower makes ova and sperm. For gene manipulation research, scientist can induce culture cells to take up foreign dna (genetic transformation). Its relatively small genome, about 100 million nucleotide pairs, has already been sequenced
• There are major differences between plant and animal development.

  • In animals, movements of cells and tissues are involved in transforming the embryo.
  • Ongoing development in adults is restricted to the differentiation of cells, such as blood cells, that must be continually replenished (2 million RBCs)
  • In plants morphogenesis and growth in overall size are not limited to embryonic and juvenile periods.
  • Apical meristems are responsible for the plant's continual growth and formation of new organs, such as leaves and roots.