4
EIGIL KJELDSEN and STEEN K0LVRAA
structures in cell preparations, hence the name (chromos for stain and
soma for body). It took several decades, however, before the function
of these cellular components was identified even though the chemical
composition of the structures was known. It was not until Avery and coworkers in 1944 showed that DNA was the transforming factor (Le. the
factor carrying genetic information from one cell to another), that its
role as genetic material was firmly established. Detailed characterization
of the whole chromosome complement, including the exact number of
chromosomes in man had, however, to await the development of culturing
systems together with colcimide and hypotonic treatment, before nicely
spread chromosomes were obtained as late as in 1956. It was not until this
relatively late time that it was finally established that man has 46 chromosomes and the field of cytogenetics was born.
From then on the application of chromosome analysis in medicine was
swift and the first numeric and structural chromosomal abnormalities
could be described as early as 1960. Another very important step in
the evolution of cytogenetics came with the discovery by Caspersson et
al. (1968, 1971) that certain dyes, e.g. quinacrine mustard, had the ability
to create discrete, fluorescent bands across the longitudinal axes of
the chromosomes giving each chromosome its unique identity. This
prompted fluorescence microscopy to be introduced into the field of
human cytogenetics. The ability to identify banding patterns within
each chromosome was an important step in increasing the resolution
power ofthe analysis, enabling the characterization of more discrete structural aberrations, and then to correlate these with disease.
An increasing need for physical mapping of genes resulted in 1981 in
the first gene to be localized by in situ hybridization on metaphases using
radioactive labeling of a cloned gene as probe. This development initiated
a number of gene mapping studies by in situ hybridization using radioactive labeling. At that time, radioactive labeling was the first choice of
labeling due to its higher sensitivity than fluorescence-based labeling.
However, the isotopic methods require long exposure times (weeks to
months) and the precision is low. Developments of the fluorescence based
in situ hybridization method resulted in 1986 in an improved method that
offered higher speed and spatial resolution together with sufficient sensitivity as compared to isotopic methods. Soon after, fluorescent labeling
was adopted as the label of choice, representing the birth of FISH. Various
other refinements of this technique soon followed such as whole chromosome painting (1986), reverse chromosome painting (1989), multicolor
labeling (1990) and 24-color karyotyping (1996; see Table 1 for more details). It should be remembered that these refinements of FISH have very
EIGIL KJELDSEN and STEEN K0LVRAA
structures in cell preparations, hence the name (chromos for stain and
soma for body). It took several decades, however, before the function
of these cellular components was identified even though the chemical
composition of the structures was known. It was not until Avery and coworkers in 1944 showed that DNA was the transforming factor (Le. the
factor carrying genetic information from one cell to another), that its
role as genetic material was firmly established. Detailed characterization
of the whole chromosome complement, including the exact number of
chromosomes in man had, however, to await the development of culturing
systems together with colcimide and hypotonic treatment, before nicely
spread chromosomes were obtained as late as in 1956. It was not until this
relatively late time that it was finally established that man has 46 chromosomes and the field of cytogenetics was born.
From then on the application of chromosome analysis in medicine was
swift and the first numeric and structural chromosomal abnormalities
could be described as early as 1960. Another very important step in
the evolution of cytogenetics came with the discovery by Caspersson et
al. (1968, 1971) that certain dyes, e.g. quinacrine mustard, had the ability
to create discrete, fluorescent bands across the longitudinal axes of
the chromosomes giving each chromosome its unique identity. This
prompted fluorescence microscopy to be introduced into the field of
human cytogenetics. The ability to identify banding patterns within
each chromosome was an important step in increasing the resolution
power ofthe analysis, enabling the characterization of more discrete structural aberrations, and then to correlate these with disease.
An increasing need for physical mapping of genes resulted in 1981 in
the first gene to be localized by in situ hybridization on metaphases using
radioactive labeling of a cloned gene as probe. This development initiated
a number of gene mapping studies by in situ hybridization using radioactive labeling. At that time, radioactive labeling was the first choice of
labeling due to its higher sensitivity than fluorescence-based labeling.
However, the isotopic methods require long exposure times (weeks to
months) and the precision is low. Developments of the fluorescence based
in situ hybridization method resulted in 1986 in an improved method that
offered higher speed and spatial resolution together with sufficient sensitivity as compared to isotopic methods. Soon after, fluorescent labeling
was adopted as the label of choice, representing the birth of FISH. Various
other refinements of this technique soon followed such as whole chromosome painting (1986), reverse chromosome painting (1989), multicolor
labeling (1990) and 24-color karyotyping (1996; see Table 1 for more details). It should be remembered that these refinements of FISH have very