Model Systems:
Most model systems were used to study the process of development due to the ease with which the embryos can be acquired, their presence outside of the animals body (eggs) their transparency (zebrafish eggs), the fact that they can withstand surgical manipulations (chick embryos). In this section we will look at the different model systems used to study the development of the neural tube. The image below shows an example of visualizing gene expression in a Drosophila embryo.
The main model systems we will be looking at are:
- Xenopus
- Drosophila
- Chick
- Zebrafish
- Mouse
(Image 9 - Drosophila embryo with visualised gene expression. Sourced from The SDB)
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Xenopus
Since this is the model organism which has been studied most, it was the main example given in the previous text. Xenopus is a widely used vertebrate model for several reasons. Firstly, mating can be induced easily by injecting hormones into adult animals, large clutches of eggs are produced at one time and they are fertilized and develop externally. Another useful advantage is the pigmenting found on Xenopus eggs - the animal pole has a dark pigment which makes visualizing certain aspects of development easy. The banner picture at the top of our website illustrates this beautifully. Unlike mammalian and other embryos, one of the body axis - the dorsal-ventral axis - is determined entirely by sperm entry point, and the rest of the body axis are set up in relation to that. The embryo undergoes cortical rotation - that is where cytoplasm in the animal pole is shifted towards the site of sperm entry, triggering changes in the cells that have been moved.
(Image 10 - A Xenopus embryo stained for Tubulin expression. Sourced from The SDB )
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Drosophila
The drosophila does not form a neural tube, it develops a nervous system rather differently from vertebrates. In fact the structure of invertebrates nervous system varies dramatically from vertebrates with 80% of their central nervous system being dedicated to sight.
(Image 11 - A diagram showing the opposite position of the germ layers in vertebrates and insects.)
In all arthropods, the nerve cord lies ventrally rather than dorsally as in vertebrates.
How is Drosophila used as a model system then? There are several basic processes that are common in most embryos, including certain signaling pathways and the basic plan of gastrulation, neural induction and organogenesis. These processes may appear to be very different in Drosophila but in fact there are a huge number of homologous genes between vertebrates and invertebrates. Neural induction in the Drosophila depends on a process called lateral inhibition through the Notch/Delta signaling pathway. After the ectodermal cells acquire their neural fates, they begin to leave the ectoderm through the process of delamination (another process common with vertebrate and human embryos) where the cells enlarge and move into the embryo. The first neuroblasts arrange themselves along the anterior/posterior axis and in 3 columns along the midline. These neuroblasts begin to divide, producing progeny - ganglion mother cells (GMC) - which give rise to the neurons and glia of the nervous system.
The Notch/Delta system is also present in vertebrates where it is also important in the development of the embryo. There are at least 4 Notch genes 3 of which are important in the development of the nervous system. It is also present in the Xenopus where it expands the neural plate at the expense of the ectoderm. The Notch/Delta pathway is one of the many reasons comparative biology is an important aspect of science, by discovering this pathway in the Drosophila it allowed scientists to search for homologs in other species.
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Chick
The chicken is an amniote - like a human, it’s embryo is surrounded by several layers which protect it from the outside environment, however unlike the human, these layers are hardened and allow the egg to be incubated outside of the female’s body. This not only makes the chicken a close system to man, but allows for easy manipulation of the embryo without having to perform surgery on the mother, at least after the egg has been laid. Chick embryos can also be cultured outside of the egg, and are (relatively) resilient to surgical procedures.
In the chick, the basic changes in an embryo that make the notochord and eventually the neural tube are essentially the same as the human, and indeed Xenopus. Gastrulation begins with the formation of the primitive streak (similar in some respects to the blastopore) which develops 16 hours after the egg is laid, extending from the posterior marginal zone, as the anterior part of the streak condenses forming and area known as Hensen’s node (the functional equivalent of the Spemann organizer in Xenopus) as the mesoderm and endoderm begin to move inwards the cells of Hensen’s node give rise to the notochord and somite as the cells begin to regress in a posterior direction and the notochord is formed immediately anterior to it. One dissimilarity between Xenopus and amniotes is the lack of cell growth and cell proliferation during gastrulation, which occurs in both the chick and mammals.
The neural tube then begins to form starting in an anterior-posterior direction, unlike in Xenopus, where the neural tube forms at the same time.
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Zebrafish
The fate of individual cells can be traced in a zebrafish due to the transparency of its embryo, it also has a short life cycle of 12 weeks. Gastrulation in the zebrafish follows a similar pattern as in the Xenopus with just one difference: Involution happens all around the outside of the blastoderm at the same time.
The Zebrafish undergoes secondary neurulation. The organizer tissue here is called "the shield", which acts in the same way as the Spemann organizer in the Xenopus and the Hensen's node cells in the chick.
(Image 12 - A photograph showing a Zebrafish embryo. Sourced from Wikipedia.)
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Mouse
The mouse develops from an embryo to an adult in 9 weeks, has a large litter and is easily genetically modified, making it ideal for the study of vertebrate development.
It does, however, have it’s drawbacks - the mouse gestates inside it’s mother until birth, so any neonatal investigations require surgery, and although the mouse has a quick life cycle for a mammal it is still much longer than Drosophila, and is much more expensive to keep.
The mouse also has primitive streak, similar to that of the chick, however there is no simple equivalent to the Spemann organizer or Hensen’s node - there seem to be two ‘organizer’ structures. One - the node - a bobble of cells at the anterior end of the primitive streak secretes chordin and noggin and is responsible for patterning the body of the embryo, whereas a group of cells in the anterior visceral endoderm help pattern the head.
(Image 13 - A Photograph showing a chick embryo. Sourced from The SDB.)