Not harmful to nearby tissues/cells

Destroys where it cuts

Not appropriate for hard target tissues

Appropriate for hard target tissues

In the case of gravity-based collection the operator can observe the target cells through the microscope at an appropriate magnification that enables the selection and cutting of the target with the aid of specific imaging software. The last laser pulse will allow the target sample to be collected under gravity into the tube cap placed immediately under the microscope. The successful removal of the sample can be confirmed by re-focusing the microscope onto the tube cap (Plate 10.5). A disadvantage of this system is that it does not allow for the possibility that the wrong sample can be cut off, which would be extremely important in forensic medicine where non-repeatable analyses are carried out.

The micro-isolation device based on the adhesive polymer allows visualization of the target sample through an inverted microscope and then, after cutting, collection by direct contact between the specimen and the tube cap where the polymer is located; in this way the target sample is torn away from the specimen. The adhesiveness of the polymer ensures that the sample is removed and its presence is confirmed by microscopy. A disadvantage of this system is that the contact between the slide and the cap could cause transfer of contaminating material. With catapulting microdissection, an inverted microscope is coupled to a high-resolution CCD camera showing the collected sample on the collection tube cap where a laser pulse has pushed it (Plate 10.6). There is no contact between specimen and collection tube and hence no need for any adhesive polymer to retain the cut sample, which can be easily visualized by the microscope. Moreover, the collection tube is positioned within a few microns of the specimen from which the target sample will be cut, which makes sample collection a very precise and contamination-free activity.

The choice of method will depend on the specific biological problem under investigation and individual preferences. The choice of microdissecting device/ collection method is the starting point and there are several additional steps that must be taken into consideration in order to achieve good results in forensic medicine. These are summarized as follows:

• Requirement for a dedicated 'isolated' laboratory exclusively for the use of microdissecting activity.

• Requirement for personal protective equipment (PPE).

• Specific collection tubes that are sterilized and, if possible, made of low-binding plastic.

• Ultraviolet-sterilized environment.

• Isolated air supply in microdissection room.

• Requirement for humidity and temperature monitoring.

• Rigorous quality control of instrumentation.

• Dedicated histological reagents.

• DNA typing of all operators involved in the analysis chain.

• Dissection and processing of control material not concurrent with target sample.

• Performing analysis on duplicate samples if possible.

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