Single-cell sequencing is an emerging field that allows scientists to analyze individual cells. By studying cells one by one, researchers can gain insights into heterogeneous cell populations that were previously unobtainable by traditional bulk cell analysis methods. Single-cell sequencing offers an unprecedented view into cellular diversity and function at the single cell level.
Background and History
The development of Single-cell sequencing technologies has revolutionized our understanding of biology. Prior to the 21st century, researchers could only study average behaviors of large groups of cells. Individual cell characteristics were obscured in bulk measurements. Starting in the late 1990s, advances in microfluidics and instrumentation enabled the isolation and analysis of single cells. Some of the earliest single cell studies looked at gene expression patterns in immune cells and stem cells. These initial experiments demonstrated that cells within populations are not identical as previously assumed, but instead exhibit significant cell-to-cell variability in their gene expression and phenotypes. Over the past two decades, Single-cell sequencing methods have rapidly advanced and diversified. Today researchers have a wide array of techniques to probe the DNA, RNA, proteins and metabolites of individual cells.
Single Cell Genomics
One of the most powerful applications of Single Cell Analysis is single cell genomics. By measuring the genome or transcriptome of each cell individually, researchers can identify rare cell subpopulations and discover new cell types that may comprise only a small percentage of a tissue. Single cell RNA sequencing has been particularly influential, revealing vast cellular heterogeneity in tissues previously thought to be homogeneous. For example, studies have found over 100 distinct cell types in the human cortex based on unique patterns of gene expression. Single cell genomics has also accelerated our understanding of developmental biology by enabling 'snapshots' of cellular differentiation trajectories during processes like embryonic development and immune cell maturation. At the same time, single cell genome sequencing is uncovering genetic variations between equivalent cells and the factors underlying clonal evolution in diseases like cancer.
Proteomics and Epigenomics at Single Cell Resolution
Just as genomics transformed our ability to interrogate heterogeneity at the Single Cell Analysis, new proteomic and epigenomic techniques are advancing our view of cellular diversity beyond just the genome and transcriptome. Mass cytometry, also known as CyTOF, utilizes heavy metal isotopes instead of fluorescence to simultaneously detect over 40 proteins per cell. Applied to cell surface marker analysis, CyTOF enables high dimensional immune profiling at an unprecedented level of resolution. Advancing in parallel, single cell chromatin accessibility assays and single cell DNA methylomics are shedding light on epigenetic variability between equivalent cells by measuring alterations to chromatin structure and DNA methylation patterns on a cell-by-cell basis. Such techniques are providing new insight into gene regulation, cell fate decisions, and how cells interpret environmental cues differently based on their unique epigenomic profiles.
New Frontiers in Single Cell Multi-omics
With the ability to concurrently profile different -omics layers at the single cell resolution, researchers are now pursuing integrative 'multi-omic' approaches. By combining genomic, epigenomic and proteomic datasets from the same single cells, scientists aim to develop a more complete understanding of cellular identities and states. Such multi-omic studies also hold promise to better elucidate gene regulatory networks and pathways operating within individual cells. Though technically challenging, initial multi-omic experiments on thousands to tens of thousands of single cells are yielding intriguing insights into cell type classification, developmental trajectories, and disease heterogeneity. Going forward, integrating multi-omic profiles with spatial information, machine learning analyses and modelling will likely further transform our ability to dissect the complex interplay between a cell's genome, epigenome and proteome.
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