How To Karyotype Chromosomes From Cells To Results?

A karyotype is a picture of all of a person’s chromosomes, organized into pairs by size. It is a standard test used to detect missing, extra, or rearranged chromosomes. The process takes a living cell sample, grows those cells, stops them mid-division, stains the chromosomes, and photographs them under a microscope. A technician then arranges the images into the classic numbered layout. The entire process, from receiving the sample to issuing a final report, typically takes several days to a few weeks.

What Is a Karyotype and Why Is It Done?

A normal human cell contains 46 chromosomes. These are arranged in 23 pairs. Twenty-two pairs are autosomes, numbered 1 through 22. The last pair is the sex chromosomes — XX in females and XY in males.

Doctors order a karyotype to find chromosome problems. Common reasons include investigating developmental delays, birth defects, infertility, or a family history of a chromosomal disorder. It is also used to diagnose conditions like Down syndrome, Turner syndrome, and Klinefelter syndrome. In cancer care, karyotypes of tumor cells can reveal specific chromosome changes that guide treatment decisions.

This test looks at the structure and number of chromosomes. It does not detect small gene mutations. If a doctor suspects a single-gene disorder, a different test like DNA sequencing is needed.

What Sample Is Needed to Start a Karyotype?

Karyotyping requires living cells that are capable of dividing. The most common sample is blood, specifically the white blood cells. Other sample types include bone marrow, skin, amniotic fluid, or chorionic villus samples from a pregnancy.

Blood is the standard for most indications. A small tube of blood is drawn and sent to the lab. The white blood cells are separated and placed in a culture medium. This medium contains nutrients that encourage the cells to grow and divide.

Bone marrow is used when investigating leukemia or other blood cancers. Skin samples may be used when blood testing is inconclusive or when mosaicism is suspected. Amniotic fluid and chorionic villus samples are used for prenatal diagnosis.

For the test to work, the cells must be alive and healthy when they reach the laboratory. Samples should be processed quickly. Delays can reduce the number of viable dividing cells, which can extend the turnaround time or require a redraw.

How Do Cells Grow and Get Stopped in Division?

Once the cells are in culture, they are incubated at body temperature, around 37°C (98.6°F). White blood cells are stimulated to divide using a mitogen, a substance that triggers cell division. This culture period usually lasts 48 to 72 hours for blood samples. Bone marrow cells divide naturally and may only need a short culture period of 24 hours.

The critical step is stopping the cells at the right moment. Chromosomes are only visible under a microscope when the cell is in metaphase. During metaphase, the chromosomes are condensed and aligned in the middle of the cell. This is the stage where they are thick enough to see and count.

To capture this moment, the lab adds a chemical called colchicine or a similar agent. This chemical stops the formation of the spindle fibers. Without these fibers, the cell cannot pull the chromosomes apart. The cells are frozen in metaphase, with their chromosomes fully condensed and visible.

Timing matters here. If the chemical is added too early, not enough cells will have reached metaphase. If added too late, many cells will have moved past the ideal stage. Skilled technicians know exactly when to add the solution based on the cell type and culture conditions.

How Are Chromosomes Spread and Stained on a Slide?

After the cells are stopped in metaphase, they must be broken open to release the chromosomes. This is done with a hypotonic solution. The solution has a lower salt concentration than the inside of the cell. Water moves into the cell, causing it to swell. The cell membrane stretches and becomes fragile.

The swollen cells are then fixed with a solution of methanol and acetic acid. This preserves the chromosomes and prevents them from degrading. The fixed cell suspension is dropped onto a glass microscope slide. The drop spreads the chromosomes out so they are not overlapping. This step is both science and art. The height of the drop, the humidity of the room, and the temperature of the slide all affect how well the chromosomes spread.

Once the slides are dry, they are stained. The most common stain is Giemsa, which produces the classic G-banding pattern. This stain creates light and dark bands along each chromosome. The banding pattern is unique to each chromosome pair, allowing technicians to identify them. Other stains, like fluorescent dyes, are used for specific questions, such as identifying the Y chromosome or detecting certain structural changes.

How Are Chromosomes Captured and Counted?

The stained slides are examined under a microscope. The technician scans the slide to find cells that are in metaphase with well-spread, non-overlapping chromosomes. These are the cells that can be analyzed.

Modern laboratories use automated imaging systems. These systems scan the slides, find good metaphase spreads, and capture digital images. The technician reviews the images and selects the best cells for analysis.

A standard analysis involves counting chromosomes in at least 20 cells. This confirms the chromosome number is correct. The technician then examines the structure of the chromosomes in 5 to 10 cells in detail. This detailed analysis looks for missing pieces, extra pieces, or rearrangements.

The images are then arranged into the standard karyotype format. The chromosomes are sorted by size, from largest to smallest, and paired up. The sex chromosomes are placed last. The final arranged image is what most people recognize as a karyotype.

What Does the Final Karyotype Report Show?

The karyotype report describes the chromosome number and any structural changes found. A normal female karyotype is written as 46,XX. A normal male karyotype is written as 46,XY.

If there is an extra chromosome, the report will note it. For example, Down syndrome is caused by an extra copy of chromosome 21 and is written as 47,XY,+21. Missing chromosomes are noted with a minus sign.

Structural changes have their own notation. A deletion is a missing piece of a chromosome. A duplication is an extra piece. A translocation is when a piece of one chromosome attaches to another. Inversions occur when a chromosome breaks and reattaches in the reverse direction. Each of these has a specific code in the report.

The report also states the number of cells analyzed and whether the result is normal or abnormal. If mosaicism is present, meaning some cells have a different chromosome makeup than others, the report will list the percentage of each cell line.

How Long Does It Take to Get Results?

The timeline depends on the sample type and the reason for testing. A standard blood karyotype usually takes 7 to 14 days. The cells need time to grow and divide in culture. The analysis and reporting add additional days.

Bone marrow samples for cancer workups are often faster. Results may be available in 3 to 7 days because the cells divide more readily. Prenatal samples from amniocentesis can take 10 to 14 days. Chorionic villus samples may take slightly longer because the cells need more culture time.

If the laboratory needs to analyze more cells or perform additional staining, the turnaround time increases. Some structural abnormalities are subtle and require extra scrutiny. The lab will not rush this step because accuracy is more important than speed.

What Are the Limitations of Karyotyping?

Karyotyping only detects large chromosome changes. It cannot see small deletions or duplications that are below the resolution of the microscope. The standard banding resolution can detect changes larger than about 5 to 10 million base pairs. Smaller changes are missed.

For these smaller changes, a test called chromosomal microarray is used. This test can detect submicroscopic deletions and duplications. It is more sensitive but does not detect balanced rearrangements like some translocations. In practice, doctors may order both tests to get a complete picture.

Karyotyping also requires living cells. If the sample is old or mishandled, the cells may not divide, and the test will fail. A failed test requires a new sample. This is why proper collection and rapid transport to the lab are essential.