64 Freezing Method (chap. G) 



The pieces to be cut should be no more than 2-4 mm. thick for rapid 

 and uniform freezing. One surface should be flat to provide sufficient 

 contact with the tissue carrier. If, during freezing, there is a tendency 

 for the tissue to break loose from the carrier, cut a piece of filter paper 

 to fit the top of the carrier and place it, wet, under the tissue. This may 

 help to hold the tissue in place while freezing and sectioning. 



Consider the shape and type of tissue when it is oriented on the car- 

 rier. If one side is narrower than the other, place that side, or any 

 straight side, parallel to the knife. If there is a tough membrane on one 

 side of the tissue, place that side toward the knife, otherwise the mem- 

 brane can break away from the rest of the block during sectioning. 

 Usually the amount of water carried over ^vith the tissue is sufficient 

 for firm attachment to the carrier. (Use normal saline for fresh tissues.) 

 Avoid too much water and do not allow it to settle around the tissue 

 in a wall of ice that can deflect the knife or tear throus^h the tissue dur- 

 ing sectioning. Uneven or torn sections result. If, however, gum is be- 

 ing used, it is necessary to build the gum around the tissue while freez- 

 ing it. In this case the tissue must be completely encased by the medium. 



With a finger, gently press the tissue upon the tissue carrier. Slowly 

 freeze the tissue by turning on COo for a moment or two, and then turn 

 it off. A series of successive jets of gas freezes better and wastes less gas 

 than a continuous stream. When the tissue is frozen fast and the finger 

 can be removed, move the knife over the block so it too is in the path 

 of the cooling gas. This aids in uniform freezing of the tissue by deflect- 

 ing some of the gas onto the top of it, and at the same time cools the 

 knife. 



Good sections depend on the proper temperature in the tissue. Ma- 

 terial frozen too hard forms white brittle fragments on the knife; if too 

 soft it forms a mushy mass. In both cases, the sections break up when 

 placed in water. Difficulties may be avoided by slightly over-freezing 

 the block and then as it warms and reaches the correct temperature, a 

 number of sections can be cut in close succession. When these fall on 

 the tissue carrier, allo^v several to accumulate and pick up the group 

 with one sweep of the finger. Or, as soon as the section is cut, remove it 

 from the knife with a finger tip and place it in a dish of water. Do not 

 allow water to collect on the knife. Keep both knife and fingers dry. 

 Some tissues (adipose for one) tend to stick to the finger and will not 

 shake loose in water. Transfer them to 70 or 80% alcohol. 



Occasionally, when there is no time for gelatine embedding, one en- 

 counters a tissue difficult to section because it tends to fall apart. Bush 



