Cytogenetics

Cytogenetics is an important branch of human genetics. It involves the examination (usually under a light microscope) of the genetic material, i.e. the chromosomes in the cell nucleus. The number, shape and structure of the set of chromosomes is analyzed. The cell material can be of various origins (amniotic fluid cells, blood, abortion material, fibroblasts).

Chromosome analysis is used in prenatal diagnostics as well as in postnatal diagnostics. It is the dominant technique for detecting numerical chromosome aberrations such as trisomies 21, 13, 18 (Down syndrome, Pätau syndrome, Edwards syndrome), Ullrich-Turner syndrome (monosomy X), Prader-Willi syndrome and Klinefelter syndrome. There are many indications for cytogenetic testing, including infertility diagnosis, infertility diagnosis (in case of stillbirths), suspected genetic disorders, certain malformations, known chromosomal abnormalities in the family, physical and mental developmental delays in children and age indication in expectant mothers.

These indications suggest the importance of reliable and, above all, rapid results for those affected. Long waiting times and the associated uncertainty are often a great burden for those affected and their relatives. In order to be able to obtain clear analysis results in a short time, the quality of cytogenetics products must therefore be correspondingly high.

To generate a karyogram (visualization of all chromosomes in a cell in an ordered fashion), cells are cultured in a nutrient medium until metaphase is reached. The metaphase chromosomes are then isolated on a glass slide. Using image processing software, the chromosomes are ordered by size, centromere region and banding pattern. This allows changes in the number as well as the structure of the chromosomes to be detected; the so-called numerical or structural chromosome aberrations.

A typical workflow looks as follows:

Patient cells (e.g. lymphocytes, amniocytes, chorionic villus cells) are allowed to grow in a medium that stimulates cell division (phytohaemagglutinin is often used). After a defined time – the cells should be in the metaphase – cell division is interrupted by adding a mitotic inhibitor (e.g. colcemid, the poison of the autumn crocus). The cells are centrifuged, the media supernatant is discarded and the remaining cell sediment is collected in a hypotonic, 0.075 M potassium chloride solution.

The hypotonic conditions swell the cells and facilitate cell bursting when the cells are mounted on the slide. Cells in metaphase do not have a nuclear membrane, so chromosomes are released directly onto the slide. Metaphase chromosomes are highly condensed and can be readily examined by light microscopy – especially if they have been additionally stained (Giemsa-banding, FISH).