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Chromosome analysis and karyotyping
General Information
Humans have 46 chromosomes in all their somatic cells. The karyotype
of a female is 46,XX, that of a male 46,XY (22 pairs of autosomes and 2
sex chromosomes).
A karyogram is the ordered presentation of the 46 chromosomes which
are stained with specific techniques (e.g. GTG banding, G bands using
trypsine and giemsa stain) for visualisation and differentiation of the
chromosomes. The comparison of the banding pattern of each chromosome
pair detects numerical and structural aberrations with a resolution of
about 5 - 10 Mb.
Numerical aberrations are called aneuploidies. Normal cells have to
copies of each autosome. Trisomic cells have three copies, monosomic
cells only one copy of a chromosome. The most common viable
aneuploidies are: Down syndrome (trisomy 21), Pätau syndrome (trisomy
13), Edwards syndrome (trisomy 18), Ullrich-Turner syndrome (Monosomie
X, 45,X) and Klinefelter syndrome (47,XXY).
In triploid or tetraploid cells all chromosomes are present in triplicate and quadruple respectively.
Postnatal chromosome analysis is performed using cultivated
T-lymphoctes of a peripheral blood sample. In prenatal analysis fetal
cells from amniotic fluid, chorionic or placental villi are grown in
culture media and karyotyping is performed after harvesting and
staining the metaphase chromosomes.
Special tissue karyotyping comprise umbilical cord blood, skin fibroblasts and fetal abortion tissue.
Diagnostic
Indication
Prenatal
- advanced age of the mother (≥ age 35)
- known or presumed chromosoma aberrations in the family (e.g. balanced translokations)
- previous pregnancies with chromosomal aberrations
- pathological first trimester-/ triple-test
- pathological ultrasound
Postnatal
- known or presumed chromosoma aberrations in the family
- mental and delevopmental retardation
- phenotypic features and congenital organ malformations
- suspicion for dysmorphic syndrome
primary and secundary amenorrhoe
- reduced fertility in males and females
- recurrend abortions: balanced translocations in parents
Method
Prenatal
- Cultivating
of fetal cells from amniotic fluid or chorionic villi folloed by
mitoctic arrest, harvesting of metaphase chromosomes and subsequent
staining (GTG).
- Direct analysis of chorionic villi metaphases after overnight incubation.
Postnatal
- 72h blood culture of T-lymphocytes followed by mitotic arrest, harvesting
of metaphase chromosomes and subsequent staining (GTG, CBG).
Sample Requirement
Prenatal
- 10 - 20 ml of amniotic fluid
- 20 - 50 of chorionic or placental villi
- 2 ml of umbilical cord blood
Postnatal
- 2 - 5 ml of heparinised blood
- skin fibroblasts
- abortion tissue
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