Chapter 1: Introduction

1.1 Introductory Remarks

As the name implies, flow cytometry is the measurement of cells in a flow system, which delivers the cells singly past a point of measurement. In theory, there are many types of measurements that could be made but, in practice, the name refers to instruments in which light is focused at the point of measurement. The scattered light and fluorescence of different wavelengths are then recorded. Typically, light scatter at two different angles and from one to six or more fluorescences will be measured. Although cells (mammalian, plant, algae, yeast, bacteria) are usually analysed, other particles, such as nuclei, chromosomes or small beads, can also be studied.

Some organisms, such as marine algae, are inherently fluorescent but, in general, the fluorescences arise from different labels. Fluorescent chemicals may used to label cell components, such as DNA, directly; others are attached to antibodies against a wide variety of cellular proteins. 

As will be emphasised throughout this book, the value of the technique lies in the ability to make measurements on large numbers of single cells within a short period of time (tens of seconds to minutes). The heterogeneity of populations can be revealed and different subsets of cells identified and quantified. Selected cell populations can also be physically sorted for further study.

The major disadvantage of flow cytometry is that it requires a suspension of single cells or other particles, with minimum clumps and debris. This means that the tissue architecture and any information about the spatial relationship between different cells are lost when single cells or nuclei are prepared.

1.2 Data presentation

Data are usually shown either as single parameter histograms (Figure 1.1) or as two parameter correlated plots, often called cytograms. A cytogram may be presented as a dot plot, a density or a contour plot (Figure 1.2). In a dot plot, each cell recorded is shown as a single dot. This is the form in which data are shown during data acquisition.

Figure 1.2 shows forward scatter on the x-axis and side scatter on the y-axis. The significance of these parameters is discussed in Chapter 4.

Figure 1.1. A histogram showing mammalian cells stained for DNA content using the red fluorescent dye, propidium iodide.Data file