Introduction to histology
Introduction to histology

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Introduction to histology

3 How a histopathology department works

Pathology laboratories are very busy places; a laboratory in a medium-sized hospital with five to seven staff could be handling 100 tissue samples each day, producing up to 300 stained slides (Figure 6).

Figure 6 A histopathology department in a busy hospital handles hundreds of tissue samples each week.

Tissue samples are sent to the laboratory from a variety of sources, including operating theatres, surgeries and mortuaries. Each sample is accompanied by a form with basic patient information (full name and date of birth), a note of the tissue-type, and may also include a query as to the possible diagnosis. Each sample will be assigned a laboratory number as soon as it arrives - tracking the tissue accurately through the system is absolutely critical, because any mistake in identifying the tissue could have serious clinical consequences.

The great majority of the samples arrive in formalin. Since it takes some time for the fixative to permeate large tissue specimens, the pathologist will first assess whether the tissue is adequately fixed; if it is, they will then direct the cut-up of the specimen. Within a large piece of tissue, the important area for examination may be quite localised. By observation of the whole piece of tissue and using experience, the pathologist decides exactly which areas should be examined microscopically. In addition, the orientation of the tissue in relation to the plane of the sections is decided. For example, a blood vessel which is cut in a transverse plane (cross-section) will appear quite different and yield different information from a vessel that is sectioned in a longitudinal plane. A single piece of tissue might have several cuts taken from it in order to give the best chance of identifying affected areas. The patient number and information on the size and margin of the cut is noted on the form and on the cassettes that hold the tissue samples (Figure 7).

Figure 7 A cassette-dispensing machine. Tissue is processed and sectioned on plastic cassettes which also carry the information that identifies the patient and the type of tissue. Colour-coding of the cassettes allows the staff to identify particularly urgent specimens (red) or those that require handling in a special way.

For biopsy specimens and small pieces of tissue, a biomedical scientist or a pathologist will assess the tissue and decide how it is to be sectioned. From this point all the tissue is passed over to the laboratory staff, who take it through the processes of embedding and sectioning. Tissue samples are loaded into cassettes which are then processed by a machine which takes the tissue successively through the stages of alcohol dehydration, xylene treatment and permeation with molten wax. The entire process usually runs overnight, at which stage the cassettes are transferred to an embedding machine, which holds the tissue in molten wax until it is ready to put into tissue moulds. The moulds are then filled with additional wax and chilled so that the wax sets, and the entire block containing the tissue and the cassette (with patient information) is then mounted on the chuck of the microtome.

Sectioning of the tissues is skilled and done by hand (Figure 8). A number of slides are prepared from each block, possibly at different levels through the block, depending on the surgical specimen. The section(s) are then transferred on racks into a staining machine which does the basic H&E stain (Figure 9). Finally coverslips are added to the sections, either manually or by another machine (Figure 10).

Figure 8 (a) A medical laboratory scientist examines the tissue block to determine the location of the tissue and then (b) cuts sections from the tissue block on a microtome.
Figure 9 The staining machine has baths containing different stains. It is programmed so that the robotic arm moves racks of sections between the different baths at the correct time, so that each rack remains in each staining solution for the correct amount of time.
Figure 10 A coverslipping machine. The machine takes slides individually, places a streak of mounting medium onto the section and then lowers a coverslip onto the section, before conveying it to a rack, ready for microscopy.
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