Dissection Of Fresh Brain In Fetuses And Infants

Without an overriding need to secure unfixed samples for chemical or microbiologic examination, fetal and infantile brains are best kept intact until after proper fixation, because of their pronounced softness and ease of bruising. Our method is essentially similar to that described for adult brains. To increase consistency to fetal or infantile brains, we use as fixative 20% formalin solution containing 1% glacial acetic acid. No additional measures such as one or two changes of alcohol are needed.

DISSECTION OF FIXED BRAIN After a careful inspection of the external surface of the brain, the arteries at the base of the brain may be exposed through tears made into the arachnoid membrane and followed for a short distance distally to check for pathologic conditions such as thrombosis, embolism, or aneurysm. This procedure should be omitted when pathologic processes in this region may be disturbed. Routine removal of the arterial tree from the brain substance has no merit in a

thorough pathologic examination because this separates vascular lesions from the resulting areas of parenchymal damage.

After adequate external examination, the brain stem and the cerebellum are separated from the cerebral hemispheres. In rare instances, it is better to retain this continuity, for example, for display of the distorting effect of a supratentorial lesion on the brain stem needs. It is essential to section through the mid-brain along a flat surface perpendicular to the neuroaxis. For this purpose, with the brain placed upside down, the cerebellum should be held between the index finger of the one hand with the tip in proximity of the pineal gland and the thumb on the inferior surface of cerebellum (Fig. 6-11). With the scalpel in a pen-holding position, the cutting hand rests on the ventral aspect of the frontal lobes to provide the proper angle. The blade is held toward the prosector with its tip in front of the distant cerebral peduncle a few millimeters above the tip of the mammillary body. The blade enters the midbrain in the mid-line, aiming toward the pineal gland until the scalpel barely passes through the thickness of the brainstem; the blade is then brought toward the prosector (resulting in sectioning through half of the midbrain). The scalpel is now flipped over and moved forward along the same plane cutting the other half of the midbrain. A gentle pull with the holding hand on the brain stem and cerebellum during the procedure helps to complete the sectioning. Placing a knife over the temporal lobe and cutting the midbrain from the side should be avoided since this will result in a roof-shaped midbrain; this complicates the evaluation of the midbrain and makes its complete cross-sectional representation on histologic slides impossible.

Attempts to sever the midbrain too rostrally often result in an uneven or incomplete cut because the cerebral peduncles widen rapidly in the rostral portion. In order to avoid this, one may place a preliminary section close to the pontomesenceph-alic junction; then, under direct visualization, a parallel slice of the midbrain can be removed more rostrally.

Coronal sectioning of the cerebral hemispheres is the most common and safest method for any contingencies. We prefer free cuts, without use of a cutting apparatus. Before sectioning, the central sulci should be marked by carefully cutting, with the tip of a scalpel blade, into the leptomeninges bridging over them, without injuring the underlying brain substance. This gives a valuable point of reference on multiple coronal sections.

As an initial step we hold the brain on its convexity with the orbital lobes and occipital poles in an horizontal plane. The first section is made through the mammillary body and cut surfaces are examined for symmetry (Fig. 6-12A). Attempts to slice with a single motion of the knife often exerts undue pressure toward the cutting board, which may squash or tear various structures, while the vessels are dragged into the softer brain tissue. Multiple slicing excursions without undue downward pressure produce a clean-cut surface more effectively. The knife handle should be held lightly, as this will facilitate smooth gliding movements of the blade. A firm grip tends to cause knife marks on the surfaces of brain slices.

Alternatively, the first cut can be made just in front of the temporal poles, exposing the anterior ventricular horns. This may be important in cases of hydrocephalus, in which this view may disclose an obstruction of the foramen of Monro (e.g., by a colloid cyst or a third ventricular tumor) and still allow a change in sectioning technique to better demonstrate the obtruction (25).

Brain slices should be approx 1 cm thick. We like to section the halved brain pieces by holding them down on the cut surface and by moving the knife side to side from the inferior surface of the brain toward the convexity (Fig. 6-12B,C). A

Fig. 6-12. Sectioning of cerebral hemispheres. (A) initial cut is placed through mammillary bodies. (B,C) the halved brain pieces are held down on the cutting board and sliced from the inferior surface toward the convexity. Cloth or paper towels under the brain will prevent the board surface becoming slippery from fluid dripping from the brain. When slicing cerebral hemispheres in this fashion, the "limp" optic nerves need to be propped up to avoid cutting them longitudinally.

Fig. 6-12. Sectioning of cerebral hemispheres. (A) initial cut is placed through mammillary bodies. (B,C) the halved brain pieces are held down on the cutting board and sliced from the inferior surface toward the convexity. Cloth or paper towels under the brain will prevent the board surface becoming slippery from fluid dripping from the brain. When slicing cerebral hemispheres in this fashion, the "limp" optic nerves need to be propped up to avoid cutting them longitudinally.

Fig. 6-13. Approach to routine dissection of brain stem and cerebellum. (A) Brain stem and cerebellum are dissected together by series of cuts roughly perpendicular to neuroaxis. For consistency, base line is made through pontomedullary junction and posterior ridges of cerebellar hemispheres. This will give a flat surface on which to rest the brain stem and cerebellum, which makes the subsequent sections easy. (B) Midline incision is made in vermis, and wedge of tissue is removed from each cerebellar hemisphere. Hemispheres are further sectioned through vertical planes perpendicular to external lines of cerebellar cortex. Brain stem and rest of cerebellum are sectioned as in (A). (C) Cerebellum is separated from brain stem. Latter is sectioned as in (A). Cerebellum is sectioned either horizontally or vertically as in (B).

Fig. 6-13. Approach to routine dissection of brain stem and cerebellum. (A) Brain stem and cerebellum are dissected together by series of cuts roughly perpendicular to neuroaxis. For consistency, base line is made through pontomedullary junction and posterior ridges of cerebellar hemispheres. This will give a flat surface on which to rest the brain stem and cerebellum, which makes the subsequent sections easy. (B) Midline incision is made in vermis, and wedge of tissue is removed from each cerebellar hemisphere. Hemispheres are further sectioned through vertical planes perpendicular to external lines of cerebellar cortex. Brain stem and rest of cerebellum are sectioned as in (A). (C) Cerebellum is separated from brain stem. Latter is sectioned as in (A). Cerebellum is sectioned either horizontally or vertically as in (B).

slicing guide (see below) can be used for particularly delicate specimens. It is also important to examine each new cut surface before the next slice is made so that any necessary adjustment can be made in the next plane of section. The slices are displayed on a board, with the right side of the specimen on the left side of the prosector. Although the classical pathologists' approach to the brain corresponded to viewing one's own brain from behind (and therefore, right side of the specimen on the right side of the prosector), we prefer the frontal view because of current neuroimaging practice, which is similar to that of the physician who sees the living patient face to face. A large cutting board is needed because slices should not overlap. Sufficient space for display is mandatory for adequate examination of the brain.

Several different approaches can be used in routine dissection of the brain stem and cerebellum (Fig. 6-13). The brain stem is best sectioned perpendicular to its axis, which is slightly curved. Consequently, the planes of section should be adjusted. The cerebellum can be sectioned in horizontal planes or in planes perpendicular to the folial orientation, with the converging point in front of the cerebellum. The latter method gives the best histologic orientation of the cortical structures. A combination of both methods also can be used.

Display of the brain stem and cerebellum should be consistent with the principle used for the cerebral hemispheres. There are two options to achieve this end (see below) and either method can be suitably used under different circumstances.

Since the advent of CT and MRI, sections of the brain along the planes of tomography have become important for clinico-pathologic correlation (26). For this purpose, we use a simply constructed device made of plexiglass, shown in Fig. 6-14. The table (Fig. 6-14A) has a small opening to admit the cerebellum and brain stem. The guide on top of the table can be moved up and down so that the most desirable inclination on the initial cut can be selected, based on the imaging prints. After the initial cut (Fig. 6-14B), the halved brain pieces are sectioned serially on the board (Fig. 6-14C), which has 13-mm guides on its edge. Guides half as tall as these can be attached on the other side of the board. The display slices should correspond to the printed CT images.

We consider the coronal sectioning of the cerebral hemispheres and the horizontal sectioning of the brain stem and cerebellum the best routine method for the brain in that the slices obtained will display most advantageously the pattern of vascular supplies and the relationship of the internal structures. This holds true even in the absence of corresponding neuroimages.

DISSECTION OF SPINAL CORD For routine examination, after the dura has been opened along the anterior midline and the cord surface has been examined, series of cross-sections are prepared. Marking the right side of the cord with India ink may help later when segmental and long pathway pathology need to be reconstructed. The dura should be left attached to the cord to keep the sectioned spinal cord and roots together. This allows to orient roots for cross sections during embedding. When specific radicular-level involvement has been reported premortem, the involved roots should be identified and processed separately (see "Peripheral Nerves"). With a sharp scalpel blade, the spinal cord is sectioned approx at 1-cm intervals. Occasionally, longitudinal sections can be made to emphasize the rostral-caudal extent of the lesion, such as in traumatic contusion. However, it is often difficult to get a straight plane of section. In most instances, the cross-sectional extent of the lesion at any given segmental level is more important for understanding clinical symptoms. A combination of the two methods

Fig. 6-14. Device for sectioning brain along planes of tomography. (A) Plexiglass table with opening for cerebellum and brain stem and movable guide. (B) Brain in position for initial cut. (C) Halved brain positioned on board for serial sectioning.

Fig. 6-15. Selection of tissue blocks. (A) 1 = superior and middle frontal gyri. This is an arterial border ("water-shed") zone most likely to arbor small ischemic lesions. This also may reveal atrophic or "senile" changes such as senile plaques or neurofibrillary tangles. 2 = basal ganglia. Vascular changes and their effects on parenchyma are likely to be found here, as are other "degenerative changes." (B) 2' = basal ganglia together with thalamus. 3 = hippocampus and adjacent neocortex. This is often a sensitive indicator of anoxic-ischemic changes. Neurofibrillary tangles, neuritic plaques, and the "aging" changes make their first appearance here. (C,D) 4 = pons. Vascular (particularly small arterial) changes are found more frequently here than in other portions of brain stem. 5 and 5' = cerebellum. Ischemic and toxic metabolic conditions are often reflected in cerebellar cortex.

Fig. 6-15. Selection of tissue blocks. (A) 1 = superior and middle frontal gyri. This is an arterial border ("water-shed") zone most likely to arbor small ischemic lesions. This also may reveal atrophic or "senile" changes such as senile plaques or neurofibrillary tangles. 2 = basal ganglia. Vascular changes and their effects on parenchyma are likely to be found here, as are other "degenerative changes." (B) 2' = basal ganglia together with thalamus. 3 = hippocampus and adjacent neocortex. This is often a sensitive indicator of anoxic-ischemic changes. Neurofibrillary tangles, neuritic plaques, and the "aging" changes make their first appearance here. (C,D) 4 = pons. Vascular (particularly small arterial) changes are found more frequently here than in other portions of brain stem. 5 and 5' = cerebellum. Ischemic and toxic metabolic conditions are often reflected in cerebellar cortex.

may be used by taking a cross-sectional slice at the point of maximal damage and slicing the rest longitudinally along the frontal plane. Of course, unorthodox and creative sectioning may, in rare instances, display some lesions at their best.

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