A karyotype is an actual photograph of the chromosomes from one cell. Karyotypes are usually done using blood cells, fetal skin cells (from amniotic fluid or the placenta) and occasionally bone marrow cells. While a karyotype is used to confirm that a person has Down syndrome due to an extra chromosome number 21, it actually gives much more information.
From a patient’s perspective, a karyotype is a simple blood test. But what happens to the blood after it is collected is actually quite complex. This step by step guide will help you understand why it takes so long (up to a week) to get karyotype results.
The first step in performing a karyotype is collecting the “sample.” In newborns, a blood sample which containes red bloods cells, white blood cells, serum and other fluids is collected. A karyotype will be done on the white blood cells which are actively dividing (a state known as mitosis). During pregnancy, the sample can either be amniotic fluid collected during an amniocentesis or a piece of the placenta collected during a chorionic villi sampling test (CVS). The amniotic fluid contains fetal skin cells which are used to generate a karyotype.
Transport to the Laboratory
A karyotype is a specialized test that is done in a specific laboratory called a cytogenetics lab. Not all hospitals have cytogenetics labs. If your hospital or medical facility doesn’t have it’s own cytogenetics laboratory (most don’t), the test sample will be sent to a lab that specializes in karyotype analysis. The test sample is analyzed by specially trained cytogenetic technologists, Ph.D cytogeneticists, or medical geneticists. ‘Cytogenetics' is a word for the study of chromosomes.
Separating the Cells
In order to analyze chromosomes, the sample must contain cells that are actively dividing (or in mitosis). In blood, the white blood cells are actively dividing cells. Most fetal cells are actively dividing. Once the sample reaches the cytogenetics lab, the non-divided cells are separated from the dividing cells using special chemicals.
In order to have enough cells to analyze, the dividing cells are grown in special media or a cell culture. This media contains chemicals and hormones that enable the cells to divide and multiply. This process of “culturing” the cells can take 3 to 4 days for blood cells, and up to a week for fetal cells.
Chromosome are long string of human DNA. In order to see chromosomes under a microscope, chromosomes have to be in their most compact form. This compact form occurs at a specific stage of mitosis called metaphase. In order to get all the cells to this specific stage of cell division, the cells are treated with a chemical which stops cell division at the point where the chromosomes are the most compact.
Releasing the Chromosomes from their Cells
In order to see these compact chromosomes under a microscope, the chromosomes have to be out of the white blood cells. This is done by treating the white blood cells with a special solution that causes them to burst. This is done while the cells are on a microscopic slide. The leftover debris from the white blood cells is washed away, and the chromosomes are now fixed (or stuck) to the slide.
Staining the Chromosomes
Chromosomes are naturally colorless. In order to be able to tell one chromosome from another, a special dye called Giemsa dye is applied to the chromosomes on the slide. Giemsa dye stains regions of chromosomes that are rich in the bases adenine (A) and thymine (T). When stained, the chromosomes look like strings with light and dark bands. Each chromosome has a specific pattern of light and dark bands which enables cytogeneticist to tell one chromosome from another. Each dark or light band actually encompasses hundreds of different genes.
Once chromosomes are stained, the slide is put under the microscope and the analysis of the chromosomes begins. A picture is taken of the chromosomes and at the end of the analysis, the total number of chromosomes will be known and there will be a picture of the chromosomes arranged by size.
The first step of the analysis is counting the chromosomes. Most humans have 46 chromosomes. People with Down syndrome have 47 chromosomes. It is also possible for people to have missing chromosomes or more than one extra chromosome. By looking at just the number of chromosomes, it is possible to diagnose different conditions including Down syndrome. However, cytogeneticist don’t stop there.
After determining the number of chromosomes present, the cytogeneticist will start sorting the chromosomes. To sort the chromosomes, the cytogeneticists will compare chromosome length, the placement of centromeres (the areas where the two chromatids are joined), and the location and sizes of G-bands. The chromosomes pairs are numbered from largest (number 1) to smallest (number 22). There are 22 pairs of chromosomes, called autosomes, which match up exactly. There are also the sex chromosomes, two X‘s is a female and an X and a Y is a male.
Looking at the Structure
In addition to looking at the total number of chromosomes and the sex chromosomes, the cytogeneticist will also look at the structure of the specific chromosomes to make sure that there is no missing or additional material, no structural abnormalities like translocations and a variety of other possible chromosome abnormalities.
The Final Result
In the end, the final karyotype test shows the total number of chromosomes, the sex of the person being studied, and if there are any structural abnormalities with any of the individual chromosomes. A digital picture of the chromosomes is generated with all of the chromosomes arranged by number.
When diagnosing Down syndrome, the focus tends to be on the total number of chromosomes, but in reality, a karyotype gives you information on the number, the structure and many other facets of an individual’s chromosomes.