Best described as the study of the microscopic anatomy of tissue, the science of histology is of particular value to the pathologist in the diagnosis of disease. The bulk of this specialised discipline’s lab equipment is therefore associated with the preparation of tissue for microscopic examination. The samples provided for study fall into two groups. They are either tissue biopsies or organs removed during a surgical procedure, or similar items collected post mortem. In each case, the aim is to arrive at a diagnosis based upon the microscopic characteristics of the sample.

The first step in the process is known as fixation and is a preservative technique that prevents tissue decay whilst simultaneously firming it. Samples are collected directly into the fixative which is commonly a neutral solution of formaldehyde in buffered saline. Where samples are large, further dissection by the histopathologist is often necessary to select a suitable portion. From this point, the isolated material will undergo a number of pre-treatments before suitable for examination, and these processes involve a range of specialised histology lab equipment starting with an automated tissue processor.

In order to produce the near-transparently thin slices of tissue required for staining and microscopy, the sample must be embedded in a rigid medium to prevent it from tearing during the cutting process. The common choice of embedding medium is paraffin wax. Since the tissue contains water that is immiscible with wax, this must first be removed and replaced with a wax solvent, such as xylene. The automatic tissue processor achieves this by rotating specimens through progressively higher concentrations of ethanol, finally replacing the pure ethanol with xylene.

The sample is now ready for embedding in a wax block which should be chilled on ice prior to cutting. In this stage, thin sections of between 4 and 5 microns are prepared, using another item of dedicated histology lab equipment known as a microtome. The thin paraffin sections are layered on to warm water and floated on to slides, which are left to dry overnight at 37 °C. In emergencies, a special kind of microtome known as a cryotome may be used to first flash freeze samples with liquid nitrogen and then to cut the sections.

Staining is the final stage in the preparation and may employ dyes such as haematoxylin and eosin simply to differentiate the main micro-anatomical features or more complex substances designed to highlight the presence of specific biochemical compounds associated with certain pathologies. For routine batch staining, automated machines are also available to speed up and standardise the process.

Finally, the sections must be examined and evaluated. In addition to standard bright field microscopy, polarisation, dark-field, fluorescence, and phase contrast technologies offer the means to view other aspects of samples and, in addition to forming an addition to the standard histology lab equipment, are also of value in the fields of immunology, cytology, haematology, and microbiology.

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