An important breakthrough to address these issues has been the availability, by a variety of technologies, to perform "single cell analysis" of molecules of interest, as opposed to from a cell population. Single cell technologies are used both in research and clinical applications, including for example to study cancer by looking at tumor cells.
Within a tumor, there are different clones of malignant cells which are genetically distinct, sometimes containing different mutations with roles in malignancy. These different clones can differ also in their dividing speed, metastasis capability and sensitivity to cancer treatment. Single cell analysis allows characterization of different subclones, which can inform therapies as well as follow-up of patients' response to treatment and disease progression. Single cell analysis has also been applied to the study of circulating cells that primary and metastatic tumors shed. These circulating cells are obtained from a liquid biopsy (usually a blood sample) and can be used in early diagnosis. With the added advantage that they can be obtained and analyzed in a much less invasive manner than primary and metastatic tumor cells, they allow for more frequent disease and response-to-treatment monitoring.
Single cell analysis of heterogeneous cell samples or tissues
1) Enrichment of target cells if possible
2) Isolation of cells of interest as single cells
3) Amplification of DNA or cDNA from reverse transcription of RNA
(usually PCR-based) to be used in:
4) Sequencing studies
5) Analysis of results
Most single cell analyses are based on sequencing DNA, RNA and epigenetic modifications, for which isolated cells are first broken or "lysed" to release nucleic acids, but protein and other metabolites can also be analyzed, and cells can also be used in assays in which they are kept alive and visualized under the microscope (for more details on epigenetics and labeling/visualization of fluorescent cells under the microscope, see my home page)
Microfluidics technologies, where small amounts of liquids circulate in microchannels are widely used in single cell isolating procedures. In what is known as "droplet microfluidics" (figure below) two unmixable liquid phases (water and oil, for example) flow through microchannels leading to formation of drops of one fluid within the other (carrier) fluid, containing single cells (an aqueous droplet in oil in our example).