Hemocytometer
The haemocytometer is a device usually used and originally designed to count blood cells.
The haemocytometer was invented by Louis-Charles Malassez and consists of a thick glass microscope slide with a rectangular indentation that creates a chamber. This chamber is engraved with a laser-etched grid of perpendicular lines. The device is carefully crafted so that the area bounded by the lines is known, and the depth of the chamber is also known. It is therefore possible to count the number of cells or particles in a specific volume of fluid, and thereby calculate the concentration of cells in the fluid overall.
The gridded area of the haemocytometer consists of nine 1 x 1 mm (1 mm2) squares. These are subdivided in 3 directions; 0.25 x 0.25 mm (0.0625 mm2), 0.25 x 0.20 mm (0.05 mm2) and 0.20 x 0.20 mm (0.04 mm2). The central square is further subdivided into 0.05 x 0.05 mm (0.0025 mm2) squares. The raised edges of the hemocytometer hold the coverslip 0.1 mm off the marked grid, giving each square a defined volume (see figure on the right).
To use the hemocytometer, first make sure that the special coverslip provided with the counting chamber is properly positioned on the surface of the counting chamber. When the two glass surfaces are in proper contact Newton's rings can be observed. If so, the cell suspension is applied to the edge of the coverslip to be sucked into the void by capillary action which completely fills the chamber with the sample. The number of cells in the chamber can be determined by direct counting using a microscope, and visually distinguishable cells can be differentially counted. The number of cells in the chamber is used to calculate the concentration or density of the cells in the mixture the sample comes from. It is the number of cells in the chamber divided by the chamber's volume, which is known from the start, taking account of any dilutions and counting shortcuts:
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