Biologists first tried to visualize the chromosomes in a human cell in the late nineteenth century, with estimates of the total number ranging from 30 to 80. As methods to untangle and stain chromosomes improved, the count narrowed to 46 or 48, and by 1956 was confirmed as 46, or 23 pairs. By 1959, the first chromosomal abnormalities were identified using size and crude staining patterns to distinguish the chromosomes. In the 1970s, vastly improved staining techniques enabled cytogeneticists to much more easily distinguish chromosomes, and they began amassing databases of specific chromosomal abnormalities and the clinical syndromes that they cause.
Also in the 1970s, general staining began to be replaced with in situ hybridization, an approach that links a radioactive molecule to a short sequence of DNA called a DNA probe, chosen to match a known gene of interest. When the DNA probe binds to its complementary sequence among a sample of chromosomes spread against a piece of photographic film, the radioactivity exposes the film exactly where the probed DNA sequence resides. In the 1990s, fluorescent molecules replaced the radioactive tags, and a procedure called fluorescence in situ hybridization (FISH) was born. A flash of light matches probe to chromosome. Today, FISH can use combinations of fluorescent labels and computer analysis to individually label each chromosome. The technique is called chromosome painting or spectral karyotyping. A karyotype is a picture of a person's chromosomes displayed in size-ordered pairs. FISH can be used to highlight chromosomes obtained by amniocentesis, CVS, or fetal cell sorting, described next.
Viewing fetal chromosomes requires obtaining cells from the fetus. The most common procedure is amniocentesis, first successfully performed in 1966. In amniocentesis, a needle is used to remove a sample of the amniotic fluid that surrounds the fetus. This is usually done after the fifteenth week of pregnancy. The fluid sample contains skin cells that the fetus has shed, and these are analyzed for their chromosomal content. Results from amniocentesis typically are available within two weeks. FISH is not routinely offered, but in the labs that do offer it, some preliminary information may be available more quickly than is possible with other testing procedures.
Aberrations of chromosomes 13, 18, 21, X, and Y are seen most commonly. This is not necessarily because they are affected more often, but because problems in other autosomes are so severe that development ceases long before prenatal testing can be done.
Biochemicals in the amniotic fluid can also be analyzed for signs of metabolic disorders, though this procedure is not commonly performed unless there is already a suspicion that one may be present. Chemical markers may also be sought for neural tube defects (NTDs), which are abnormalities in brain or fetal spinal cord development. Risk of amniocentesis causing miscarriage is about 1 in 200.
Chorionic villus sampling (CVS) can be performed earlier than FISH, from the tenth to twelfth week of pregnancy. A physician removes a small sample of the chorionic villi, reached either through the vagina or the abdominal wall. The chorionic villi are fingerlike projections of cells that form part of the placenta, which provides nutrients to the developing fetus. Because the chorionic villi originate from the fertilized ovum, their chromosomes and genes should be the same as those in fetal cells. However, in practice, sometimes a mutation affects only the chorionic villi, leading to a false positive test result, or only the fetus, leading to a false negative result. Maternal cells may also contaminate the sample. Because of these uncertainties, follow-up testing such as amniocentesis is required for clarification.
CVS has been linked to a fatal limb defect, and carries a risk of miscarriage of about 1 percent. It is typically recommended for women over the age of thirty-five, for those who have already had a child with a detectable genetic or chromosomal defect, if there is a family history of a genetic disorder, or when abnormalities are detected by ultrasound. For example, CVS is often used if there is a family history of Duchenne muscular dystrophy or Tay-Sachs disease. Unlike amniocentesis, CVS cannot detect NTDs because it does not sample biochemicals. Its advantage is that it can be performed earlier in the pregnancy.
A third technique, called fetal cell sorting, is being studied and may eventually replace amniocentesis and CVS in obtaining fetal cells. This approach isolates the rare fetal cells that enter the mother's blood stream and analyzes them for gene and chromosome abnormalities. A device called a fluorescence-activated cell sorter detects and isolates fetal cells by their different surface features compared to cells from the pregnant woman. Because fetal cell sorting requires only a blood sample from the pregnant woman, it cannot endanger the fetus.
Medicine EncyclopediaGenetics in Medicine - Part 3Prenatal Diagnosis - Viewing Chromosomes, Less Invasive Methods, Preimplantation Genetic Diagnosis, Genetic Counseling And The Ethics Of Prenatal Diagnosis