- Cell counting
The need for cell counting
Numerous procedures in biology and medicine require the counting of cells. On most occasions it is actually the concentration of the cells which is required (for example: 5,000 cells per mililiter). By counting the cells in a known volume of culture the concentration can be mediated. Here are several examples for the need for cell counting:
- In medicine, the concentration of various blood cells, such as red blood cells and white blood cells, can give crucial information regarding the health situation of a person (see: complete blood count).
- Similarly, the concentration of bacteria, viruses and other pathogens in the blood or in other bodily fluids can reveal information about the progress of an infectious disease and about the degree of success with which the immune system is dealing with the infection.
- The cell concentration needs to be known for many experiments in molecular biology, in order to adjust accordingly the amount of reagents and chemicals that are to be applied in the experiment.
- Studies that examine the growth rate of microorganisms (in other words: how fast they divide to create new cells) require cell counting.
There are several methods for cell counting. Some are primitive and do not require special equipment which is not present at any average biological laboratory, whereas others rely on sophisticated electronic appliances.
A counting chamber, also known as hemocytometer, is a microscope slide that is especially designed to enable cell counting. The slide has a sink in its middle; the area of the sink is marked with a grid. A drop of a cell culture is placed in the sink. Looking at the sample under the microscope, the researcher uses the grid to manually count the number of cells in a certain area. The depth of the sink is predefined, thus the volume of the counted culture can be calculated and with it the concentration of the cells.
Counting chambers are often used in clinical blood counts. Their advantage is being cheap and fast; this is makes them the preferred counting method in fast biological experiments in which it needs to be merely determined whether a cell culture has grown as expected. Usually the culture examined needs to be diluted, otherwise the high density of cells would make counting impossible. The need for dilution is a disadvantage, as every dilution adds inaccuracy to the measurement.
To quantify the number of cells in a culture, the cells can be simply plated on a petri dish with growth medium. If the cells are efficiently distributed on the plate, it can be generally assumed that each cell will give rise to a single colony. The colonies can then counted, and based on the known volume of culture that was spread on the plate, the cell concentration can be calculated.
As is with counting chambers, cultures usually need to be heavily diluted prior to plating; otherwise, instead of obtaining single colonies that can be counted, a so-called "lawn" will form: thousands of colonies lying over each other. Additionally, plating is the slowest method of all: most microorganisms need at least 12 hours to form visible colonies.
Cell cultures are turbid: they absorb some of the light and let the rest of it pass through. The higher the cell concentration is, the higher the turbidity. Spectrophotometers are electrical appliances that can measure turbidity very accurately. The culture is placed in a translucent cuvette, the cuvette placed in the machine, and the turbidity measured immediately. Simple mathematical formulae help convert the detected turbidity to cell concentration. Using spectrophotometry for measuring the turbidity of cultures is known as turbidometry.
In spectrophotometry, cultures usually do not need to be diluted, although above a certain cell density the results lose reliability. Of all the electrical appliances used for counting cells, a spectrophotometer is the cheapest and its operation the fastest and most straightforward. This has made spectrophotometry the methods of choice for quick measurements of bacterial growth and related applications. There are spectrophotometers in which several cuvettes can be inserted at one time, reducing work time even more. Additionally, there are spectrophotometers that require extremely small volumes of culture, as little as 1 microliter. This can be an advantage if the culture is precious and cannot be wasted. Spectrophotometry's drawback is its limited accuracy; it is the only method in which the cells are not counted directly—the machine measures light, not cells. This, combined with the stochastic nature of liquid cultures, enables only an estimation of cell numbers.
A Coulter counter is an appliance that can counts cells as well as measure their volume. It is based on the fact that cells show great electrical resistance; in other words, they conduct almost no electricity. In a Coulter counter the cells, swimming in a solution that conducts electricity, are sucked one by one into a tiny gap. Flanking the gap are two electrodes that conduct electricity. When no cell is in the gap, electricity flows unabated, but when a cell is sucked into the gap the current is resisted. The Coulter counter counts the number of such events and also measures the current (and hence the resistance), which directly correlates to the volume of the cell trapped. A similar system is the CASY cell counting technology.
Coulter and CASY counters are much cheaper than flow cytometers, and for applications that require cell numbers and sizes, such as cell-cycle research, they are the method of choice. Its advantage over the methods above is the large number of cells that can be processed in a short time, namely: thousands of cells per second. This offers great accuracy and statistical significance.
Flow cytometry is by far the most sophisticated and expensive method for cell counting. In a flow cytometer the cells flow in a narrow stream in front of a laser beam. The beam hits them one by one, and a light detector picks up the light that is reflected from the cells.
Flow cetometers have many other abilities, such as analyzing the shape of cells and their internal and external structures, as well as measuring the amount of specific proteins and other biochemicals in the cells. Therefore, flow cytometers are rarely purchased for the sole purpose of counting cells.
Wikimedia Foundation. 2010.
Look at other dictionaries:
Counting sort — In computer science, counting sort is an algorithm for sorting a collection of objects according to keys that are small integers; that is, it is an integer sorting algorithm. It operates by counting the number of objects that have each distinct… … Wikipedia
Counting of the Omer — Repentance in Judaism Teshuva Return Repentance, atonement and higher ascent in Judaism … Wikipedia
Cell growth — The term cell growth is used in two different ways in biology. When used in the context of reproduction cells of living cells the phrase cell growth is shorthand for the idea of growth in cell populations by means of cell reproduction. During… … Wikipedia
counting cell — noun or counting chamber : an accurately sized chamber in a microslide designed to accommodate a definite volume of fluid and usually ruled into divisions to facilitate the counting under the microscope of contained cells or bacteria … Useful english dictionary
counting chamber — noun see counting cell … Useful english dictionary
counting cell — hemacytometer … Medical dictionary
Thoma-Zeiss counting chamber (counting cell) — Tho·ma Zeiss counting chamber (counting cell) (toґmah tsīsґ) [R. Thoma; Carl Zeiss, German optician, 1816â€“1888] see under chamber … Medical dictionary
Chinese hamster ovary cell — Chinese hamster ovary (CHO) cells are a cell line derived from the ovary of the Chinese hamster. They are often used in biological and medical research and commercially in the production of therapeutic proteins. They were introduced in the… … Wikipedia
Third generation solar cell — Several new solar cell or photovoltaic technologies, have been proposed or discovered in recent years, due to extensive research and development with a focus on finding more efficient alternatives to traditional silicon based solar cells.… … Wikipedia
selenium cell — Elect. a photovoltaic cell consisting of a thin strip of selenium placed between two metal electrodes. [1875 80] * * * ▪ device photoelectric device used to generate or control an electric current. Selenium photocells are commonly used in… … Universalium