Block Selection for Microscopy

If gross lesions are found during sectioning of the brain, selecting blocks becomes relatively easy. If a tumor is involved, a correct impression of the cytology and architecture can usually be obtained from

Fig. 5.2a-f. Frontal sections through the cerebrum and block selection. The sections were cut parallel to the frontal plane and marked blocks are selected for histological examination

blocks obtained from a well-defined border of the lesion. In the case of cortical contusion injuries or vascular lesions, it is necessary to take a sizeable block extending the entire width of the glass slide and including at least one edge of the contusion injury and adjacent cortex. If there has been hemorrhaging in the brain, the presence of possible degenerative or inflammatory changes in the vessels can only be determined if the blocks originate from the ganglionic or medial side of the lesion and from the contralateral portion of the brain. The hemorrhagic margins in such cases should be scrutinized for small, easily overlooked anomalous vessels.

Proper selection of blocks is more difficult in brains with no apparent gross lesions. The clinical record can sometimes provide information on the topography of the lesions (Courville 1964). If the deceased has suffered from mental deterioration, corti-cal-subcortical blocks should be obtained from the midfrontal area; if there was known paralysis, blocks should be taken from the contralateral motor cortex or the motor pathways; blocks from patients with epilepsy should include the motor cortex, uncus or hippocampus; if there was documented visual failure, tissue from the calcarine cortex, optic chiasm, and optic tract must be examined; in cases of ataxia, blocks from the cerebellar cortex (including dentate nucleus), medulla (including olivary nucleus), and spinal cord are needed; if the victim has died of sudden death in a febrile state, blocks should be made of the upper spinal cord, pons, medulla, thalamus, and midbrain.

If the clinical record indicates no specific area of the brain and no gross lesions are present, multiple blocks may be necessary. They can be of various sizes and shapes (triangular, rectangular, elongated, square, etc.) and should enable identification of the topography on the histological slide, as described by Courville (1964). Notches can be made on the block to distinguish the right and left hemispheres. Our own experience with routine cases has shown that the following areas of the brain must be preserved for microscopic examination (see Figs. 5.2, 5.3):

1. Frontal cortex (first and second frontal gyrus including white matter)

2. Lenticular nucleus

3. Thalamic nuclei

4. and 5. Hippocampus (right and left)

6. Corpus callosum (including caudate nuclei)

Fig. 5.3a-c. Sections cut parallel to the hat line. The marked blocks are selected for histological investigation

7. Cerebellum (including cortex and dentate nucleus)

8. Pons

9. Medulla in formalin. Frozen sections may subsequently be needed for metallic methods or special stains.

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