On this page you will find repeated references to individual methods used in reproductive medicine. These include invasive procedures such as IVF, but also analytical techniques. Cytogenetics is one of the latter. It is actually a branch of genetics. It mainly involves looking at cells and their genetic material under high-resolution microscopes. Anomalies in the genetic material can be detected, which can later lead to genetic defects with sometimes serious diseases. These are therefore diagnostic procedures that are usually carried out before implantation. As the field of these diagnostic techniques is now very large and confusing, here is a brief overview of the cytogenetic methods available today.

What is cytogenetics?

Cytogenetics itself consists mainly of microscopy. In order to be able to see something under the microscope, the specimen must first be prepared. The first step is to take a sample. For example, mature oocytes can be obtained by puncturing the ovary. Sperm samples or embryonic cells before IVF can also be used. The cells of interest are fixed on a microscope slide and stained. Different colours bind to different cell components, so the appropriate stain must be chosen before each microscopic examination. Usually only the genetic material needs to be examined. Therefore, only the individual chromosomes can be removed from the nucleus of a single cell and fixed. This mapping of the chromosomes is called a karyogram.

The optical light microscope

If the sample has been stained appropriately, it does not require a particularly high resolution to see the chromosomes. The light microscope is the "normal" microscope that you may remember from your biology lessons.

In the middle of the instrument is a kind of table on which the slide with the specimen is fixed. There is a light source underneath this, so that the specimen can be illuminated by the light. The light that passes through is then focused by a lens and an intermediate image is produced. This is still upside down. At the top is the eyepiece, which looks like a pair of binoculars. It magnifies the image again and rotates it so that you can see it properly.

The standard light microscope offers 4x, 10x and 40x magnification of the specimen. This is usually sufficient to see the karyogram. This allows the chromosome set to be checked for completeness. For example, trisomies or other abnormalities in the number of chromosomes can be detected.

Unfortunately, many genetic diseases are also caused by errors in the DNA, i.e. within a chromosome. Affected people have a genetic defect but still have a normal set of chromosomes. These errors cannot be seen under a light microscope.

The Transmission Electron Microscope

The next level up is the transmission electron microscope. It works in a similar way to a light microscope, but instead of light, electrons are shone through the specimen. These are then focused by a special lens. The scattering of the atomic nuclei then produces an image that is magnified up to 100,000 times.

The Scanning Electron Microscope

When higher resolution is required, the scanning electron microscope is often used. Here the sample is bombarded with an electron beam in a vacuum. The electrons that bounce off the sample are 'collected' by an electrical voltage and produce a signal. A monitor then compiles this signal into a three-dimensional image. Magnifications of up to 100,000 times can be achieved. These images look particularly beautiful because of their three-dimensionality, but the method is relatively expensive.

The banding of chromosomes

So far, it has all been about looking at chromosomes. It is easy to find errors in the number of chromosomes, but how do you tell the chromosomes apart and how do you detect errors within a chromosome?

This is where chromosome banding comes in. The different chromosomes are coloured in a characteristic pattern. So you can be sure that you are looking at chromosome four and not fifteen. Major defects such as deletions, translocations or duplications can also be found. In these defects, whole pieces are missing, are in different places, sometimes on different chromosomes, or are present twice.

Molecular Cytogenetics

Over the past 30 years, cytogenetic methods have developed enormously. They have become increasingly precise and allow increasingly detailed diagnosis. These new methods have given rise to the field of molecular cytogenetics. The analysis is therefore carried out at the molecular level.

Sequencing and gene probes

In the early 2000s, it became possible for the first time to decode the entire genetic code of a human being. This means that the entire DNA code can be described in terms of individual bases. As a result, individual sections of the code can be assigned to different genes. Probes can then be produced. These are short synthetic sections of DNA that lie opposite a real section of DNA. This makes it possible to search specifically for individual gene variants. For example, if a child's father suffers from a hereditary disease, his genome can first be analysed to find the faulty gene that causes the disease. A probe can then be used to look for that exact sequence in the embryo's genome. If the probe binds, there is a high probability that the child will also have the disease.

FISH

Fluorescence in situ hybridisation, or FISH, is also very popular these days. This also uses probes. These are not only made of artificially produced DNA, but also have an attachment. This is a fluorescent dye. When the probe attaches, it produces a coloured light signal that can be seen under an electron microscope with special software. Several probes of different colours can be used at the same time.

The aim of cytogenetics

All cytogenetic methods are used to analyse the genome as precisely as possible. In addition to reproductive medicine, this is also extremely important in the diagnosis and treatment of tumours in order to achieve maximum therapeutic success. Genetic engineering, particularly in agriculture, also makes use of these methods, even cutting out individual genes and replacing them with others. This results in new, more resistant plant varieties.

In most cases, cytogenetics is a great gain for all concerned. But there is always a flip side. Prenatal diagnosis is therefore always hotly debated, as it is often difficult to find the right ethical position. In most European countries, for example, it is forbidden to experiment with embryos that have not been used in IVF. In other words, no changes may be made to the human genome. But even the less complex methods are often criticised. For example, it is estimated that children are more often aborted if the parents know in advance that they are likely to have a handicap. Since prenatal cytogenetic testing is generally allowed but not compulsory, it is up to you to decide what you think is right for your little family.