Above: The tissue bank technical staff
(Left to right - Helen Cairns and Louisa McGuinness)

Earlier, it was mentioned that there are two cut-off times for retrieving tissue. If it can be retrieved within 24 hours, it is taken "fresh". (This means that it is transported to the hospital without any additional preserving steps.) Between 24 and 48 hours, it must be "fixed", by being immersed in formalin straight-away. This process prevents the tissue from degrading any further, but means that it becomes unsuitable for certain techniques that can be carried out on fresh tissue.

First of all, the brain is photographed from four aspects: superior (from the top), inferior (from the bottom) and left and right hemispheres. Digital photographs are taken using the computer-operated camera set-up pictured to the right. (These are later transferred to the online database, along with all other relevant (anonymised) case information.) The brain is then cut in half. One half (left or right is decided according to whether the donor's year or birth is odd or even) is fixed in formalin, so it can be used for neuropathological characterisation. For more information about this, see "Fixed Dissection", below.

The remaining half is cut into 1cm thick anterior slices, which are photographed on both sides before being cut into 2cm by 2cm blocks, using a grid like the one pictured below right. The blocked slices are photographed once more, and then the tissue blocks are snap-frozen in isopentane (in the freezing box pictured below) before being placed into 8-well plates. Along the top of each plate is written the following information: case number, plate number, slice(s) and "Snap frozen". Each well is labelled with the grid reference of the block in question and, for the first block of a slice, the slice number. The plates are sealed into labelled bags (two plates to a bag), which are stored in a -85^oC freezer (below left) until the tissue is requested.

As mentioned above, half of the brain (and spinal cord, if it is available) is fixed in formalin. After about four to six weeks, it is dissected in a similar manner to the fresh tissue. "Whole-fixed" brains cut in half at the start of the fixed dissection. One half is stored in formalin for future anatomical studies. The other half is processed in the same way as the fixed hemisect from a "fresh" brain.

The fixed hemibrain is cut into slices and examined by a neuropathologist, who takes samples for more detailed analysis. The sampling protocol is the same for every case, and involves taking blocks from eighteen specific regions. These blocks are placed in labelled processing cassettes and stored in formalin until processed (see below).

As with the fresh dissection, the slices are photographed on both sides, cut into blocks and then photographed again. Before being frozen in isopentane, the tissue blocks must be dehydrated by immersion in sucrose solution for a week. The tissue blocks are logged and stored in the freezer (see left and below).

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