Fixation

The best routine fixative that allows the widest choice of stains for the nervous tissue is formalin, usually as a 10% solution (see Chapter 14). In fetuses and infants, the addition of acetic acid to the fixative solution appears to be helpful. Acetic acid increases the specific gravity of the fixative and allows the brain to float in the solution; it also makes the tissue firmer without altering its histologic characteristics (18).

IMMERSION METHODS For detailed anatomic studies of the nervous system it is best to fix the specimen, with a minimum of prior handling, in a large amount of freshly pre

Fig. 6-10. Removal of inner and middle ear and eye. (A) line 1 is placed near the apex of petrous bone as possible, roughly at right angle to superior edge of petrous bone. Line 2 is over mastoid region, as close to lateral wall as possible. Line 3 is placed, with blade held vertical to floor. (B) Circle indicates block to be removed with bone-plug cutter. (C) Dotted line indicates area of bone removal to approach orbital content intracranially.

Fig. 6-10. Removal of inner and middle ear and eye. (A) line 1 is placed near the apex of petrous bone as possible, roughly at right angle to superior edge of petrous bone. Line 2 is over mastoid region, as close to lateral wall as possible. Line 3 is placed, with blade held vertical to floor. (B) Circle indicates block to be removed with bone-plug cutter. (C) Dotted line indicates area of bone removal to approach orbital content intracranially.

pared 10% formalin solution. We use plastic buckets that hold 8 L. (These are readily available at local stores at a considerably lower price than traditional glass or earthenware jars, which also are heavier and break more easily). We suspend the brain to prevent distortion during fixation by passing a thread underneath the basilar artery in front of the pons. Inevitably, the vessel is slightly pulled away from the brain substance. If this is undesirable, as in the case of pontine infarcts or other lesions in this region, a thread can be passed under the internal carotid or middle cerebral arteries on both sides, provided that no pathologic lesions are suspected in these regions.

Alternatively, the dorsal dura can be used as an anchoring point. A thread is passed through the short dural flaps on either side of the falx, and the brain is suspended right-side-up. However, a minor pull may deform the parasagittal brain tissue and cause an abnormally pointed dorsal midline surface of the brain. Generally, suspension from blood vessels deforms the parenchyma less than dural suspension. In rare instances, we sus pend the brain upside down with a pair of threads tied to the edge of the entire dorsal dural flap on either side. With all these methods, the ends of the thread(s) are tied to the attachments of the bucket handle, care being taken not to allow the specimen to touch the bottom or sides of the bucket. Another safe method makes use of the plastic brain support described below for perfusion. Placing several holes in the dome-shaped receptacle will ensure proper fixation of the contact surface of the brain.

We do not recommend any method based on tying a thread around any portion of the brain substance, such as the stump of the medulla or the midbrain, nor do we recommend sectioning of the corpus callosum for alleged improved entry of fixative into the ventricles.

Formalin solution should be replaced within the first 24 h, but this not mandatory if a large amount of fixative is used. If the fixative becomes very bloody, prompt replacement with fresh solution is indicated; this also prevents undue discoloration of the specimen.

Approximately 10-14 d are required for satisfactory fixation. If the brain is dissected earlier, the central portion may still be pink, even though the consistency may be satisfactory.

PERFUSION METHODS The brain can be perfused with fixative through the arterial stumps before further fixation by immersion, as described earlier. This shortens the fixation time and ensures adequate fixation of deeper portions of the brain. When it is necessary to dissect the brain at the time of autopsy, this preliminary perfusion fixation makes the tissue firmer and thus facilitates the dissection and decreases the surface wrinkling and tissue warping that are inevitable under these circumstances.

Large volumes of formalin (for example, 1,000 mL) improve fixation but with too much fixative, large lakes of fluid may accumulate, particularly in the areas weakened by a pathologic process (e.g., infart, hemorrhage, metastasis), and the specimen may become asymmetric because of uneven perfusion. Even without these gross distortions, excessive volumes of fixative may produce annoying perivascular zones of tissue rarefaction microscopically, in addition to unnatural dilatation of small blood vessels. Obstructing emboli or thrombosis also might be obscured. The weight changes induced by perfusion fixation are described in Part III (Appendix) of this book. Injection of 150 mL of isotonic saline followed by 150 mL of 10% formalin solution causes the least problems (19). This can be done manually with a syringe connected to a simple tubing system (7). For easy handling and better preservation of the contour of the specimen, we use a plastic holder during the procedure. Satisfactory fixation for dissection can be obtained in 7-10 d. However, earlier dissection may be possible if one can tolerate some degree of incomplete fixation, which is manifested mainly by central areas of softness and pink coloration. For perfusion of a large amount of fixative, an embalmer's pump may be used. For a simple gravity-feed method, one may use an infusion bottle raised 150-180 cm above the specimen.

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Blood Pressure Health

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