In situ hybridization

Hybridization between complementary strands of nucleic acids is a fundamental technique in cell biology and is the basis of well-developed molecular biological techniques to detect mRNA or DNA sequences. The earliest uses of hybridization of complementary strands of DNA or RNA with a labelled probe were in the form of southern blots to detect specific sequences in DNA and northern blots and dot blots to detect specific mRNA species in samples transferred to nitrocellulose filters. These techniques allowed detection of specific DNA or mRNA species with a high degree of sensitivity. Later, PCR and quantitative PCR improved the sensitivity of measuring and detecting specific mRNA species in tissue samples. However, in heterogeneous tissues, the information provided by techniques based on homogenization of the sample is quite limited. Immunohistochemistry provides very useful information on the presence of particular proteins but does not unequivocally show that genes are expressed in particular tissues or cells, because proteins can be absorbed, actively transported or sequestered in tissues distant from their site of manufacture. The development of the technique of in situ hybridization (ISH) therefore provides unique information concerning expression of genes in cells and tissues, particularly where there are heterogenous cell populations present in those tissues.