Germ cells as interesting research objects

We human beings and other animals that reproduce sexually come in two sexes — male or female. When a sperm made by a male fertilizes an egg made by a female, this triggers the initiation of a new life. Sperm and eggs are mature reproductive cells known as gametes that have undergone a high degree of differentiation to enable the event called fertilization. These gametes and the cells that produce them are referred to collectively as germ cells. Germ cells are the original cells that give birth to new life, and are the only cells that transmit genetic information to the next generation.

Figure 1. Embryonic development of a loach. Fertilized eggs created through the coming together of a sperm with an egg start off as single cells, and undergo rapid cell division to produce a growing mass of cells. Then the cells start to work together as a team to form themselves into a single organism. They keep differentiating to form the various parts of the body of that organism. During this process of embryonic development, primordial germ cells, which become future germ cells, also differentiate.

In general, the somatic (body) cells of vertebrates have two genome sets, one each from their father and mother. During growth or regeneration, cells multiply by the process of cell division to create copies of themselves. This cell division is called mitosis. Germ cells also multiply by mitosis, but undergo a special kind of cell division known as meiosis to create gametes that contain only half the genome content of somatic cells. As such, when male and female gametes come together in fertilization, the resulting individual again has a genome comprised of two sets of genes, one each from the sperm and the egg.

The special characteristics of germ cells lies in the fact that they imbue gametes with genetic diversity through the process of meiosis, and give birth, through the coming together of those gametes, to the fertilized eggs that are the starting point of embryonic development — or in other words, of life itself. Using the germ cells of fish as my research objects, I conduct research mainly on meiosis and primordial germ cells (PGCs).

 

How on Earth can meiosis produce clones?

Figure 2. The loaches that I am studying, and the tanks in which they are kept. At top left is a loach that reproduces clonally. At bottom left is loaches with a pigmentation mutation. Clonal loaches cannot be visually distinguished from ordinary loaches.

Most people studying the basic principles and mechanisms of meiosis use yeasts or other model organisms as their research material, but I am using loaches. Most loach species undergo normal meiosis to give birth to genetically diverse progeny through sexual reproduction. However, loaches in certain wild populations lay eggs containing double the normal genome content that, after fertilization, undergo embryonic development without any genetic contribution from the sperm to produce clones that contain solely maternal genetic information. Loaches can be divided roughly into two populations which, although almost identical in appearance, differ in their genetic makeup. Clonal loaches are thought to have arisen from hybridization between these two populations. The fact that hybrids of the two populations bred in the laboratory through artificial insemination also produce abnormal gametes suggests that the genome composition of germ cells has a significant impact on the creation of gametes.

In the natural world, clones could be seen as heretics, but I am interested in exploring how the mechanisms behind the production of clonal gametes could be applied. Clonal reproduction techniques are important for the fact that they produce genetically homogeneous organisms, which means that they can be used to minimize variation and impart outstanding traits to all their offspring. If we could create cultured fish breeds capable of cloning themselves, it may be possible to produce large quantities of fish with superior traits.

 

Utility and application of primordial germ cells as genetic resources

As the progenitors of germ cells, PGCs are cells that appear during the process of embryonic development before the gonads are formed and migrate cell-autonomously to the location where the gonads will be formed. Then they can produce either eggs or sperm, according to their gonadal sex. Because they can produce the eggs that are the basis of embryogenesis, PGCs are extremely useful to store and utilize as genetic resources.

Figure 3. Transplantation experiments are conducted using stereo microscopes, because a lot of minute three dimensional movements are required in the experiments. Dexterity and concentration are necessary. Top right: Embryos in which PGCs have been identified with green fluorescence. Bottom right: PGCs that have been isolated emit a significant green fluorescence.

However, since individuals cannot be made directly from PGCs, they need to be first differentiated into gametes, and then generate individuals through fertilization. This requires that PGCs need to be transplanted to a surrogate host that provides an environment for the differentiation of the PGCs into gametes. Because PGCs are visually indistinguishable from other cells, we use a substance that causes only PGCs to emit a green fluorescence, enabling us to isolate them for transplantation.

This research can hopefully contribute not only to the preservation and regeneration of genetic resources, but also to the production of useful stock for aquaculture through its application to the efficient production of gametes of useful fish species (for example, by transplanting the PGCs of slowly maturing fish species to rapidly maturing species to shorten the maturation of the former).

 

My approach to research

The hydrosphere still contains many unsolved mysteries. If you look at textbooks, you may get the impression that we know all there is to know, but the fact is that the more experiments we do, the more mysteries we face. Doing research requires the energy and drive to act immediately on an idea when it comes to mind. If you do so, you will find that this will lead to a new discovery or a fresh idea. The important thing is to take action as soon as you think of something, before the idea fades forever into oblivion. I look forward to sharing with you the thrill of making new discoveries.