Tissue Cutting Preparation for Histological Examination

An essential point in  preparing  tissues  for section  is to get them as fresh as possible. So that anything in the nature of retrogressive changes in the cells may be absolutely avoided tissue cutting.   On this account other animals are, for the most part, used  instead of man. As there is always more or less delay in obtaining  anything from the post mortem room. The examination of  the corpse for histological examination being not usually made till many hours after death. Neither can the organs be relied upon as being healthy.

The usual method of procedure when organs,  such  as  the liver or kidneys of a small mammal, as the cat or  rabbit,  arc required to be hardened, may be shortly described as follows.­

Step 1 – Dead body

The animal  is  first killed, usually by placing it under a bell jar. This jar is  just large enough to accommodate it. Therefore, with a piece of tow soaked in chloroform or ether. When dead it is removed, and body is washed including the abdomen thoroughly moistened with a wet sponge before making an incision.

Step 2 – Losing the skin for tissue cutting

In order to avoid the inconvenience arising from the loose fur if the skin  is  cut without  this  pre­ caution  being taken. The abdominal wall may now be cut with blunt pointed scissors  in  the middle  line  from  the sternum   to just above  the  pubes.  And again  transversely  about  the  level  of the umbilicus.

Step 3 – Cutting opening the body cavity

The contents of the cavity are next exposed by holding back the flaps. The liver  and  kidneys  are removed. These must  then  be  moved  rapidly to and  from in a  basin of  water, to remove the superfluous blood. Water  not being  an  indifferent fluid quickly affects the tissues. So that this rough process  of cleansing  is made  as short as  possible. But in all cases where the tissue cutting is such that contact with  ordinary  water will be injurious,   normal  saline should   be  used  instead.

Step 4 – Separating the body organs

The organs are now cut with a sharp  scalpel  or  razor  into  suitable pieces. In ordinary  cases should  be  less  than half an inch in their narrowest diameter. The smaller the pieces can be made consistently with the sections subsequently derived from them demonstrating the structure of the organ. The better as the penetration  of  the  hardening  reagent  is  the  more rapid.  It is sometimes convenient to cut the pieces rather large  at first, re-cutting   them   more  carefully   the  following   day.

Step 5 – Washing, Tissue cutting and organ preserving

Rapidly wash the pieces  as  soon  as  cut in  water or normal  saline,  to again remove the blood.  Place  them  in  the fluid  in  which they are to be hardened. Of these fluids there is a considerable choice.  It  depends  greatly on  what  the  organ  is, as  to  which is selected. Some hardening reagents, however, may almost be termed general ones. That is to say, they are applicable  to  the treatment of nearly  all tissues.

Whereas others, on the other hand, are only used for special purposes. Whichever reagent is used,  there  are  certain rules  to be attended  to in the  process  of  hardening, which  hold  good for all. It  is of primary  importance that  there  should   be  an  abundance of  the fluid  used.  It  should  be  largely in  excess  of  the   pieces of tissue placed in it, for if only sufficient to cover them  in  the vessel in which  they are  placed. It  very soon  loses  its strength, and if not quickly replaced by fresh fluid, changes in the tissue rapidly  supervene.

Step 6 – Preserving the tissue for future tissue cutting

On  the  second day  the  fluid,  which   will have become more  or  less  turbid,  should  be  poured  off. The bottle containing the tissues rinsed out  with  water, and a fresh supply of the reagent added. It will be noted we are now speaking of the commoner reagents, such as MUiler’s fluid, MUiier and spirit, etc. After the second day, this should bechanged as often as it  becomes turbid.  How often  this occurs will depend largely upon  the  relative quantity of the  fluid and the tissue. The greater the proportion of the former the less frequently will  it  require to be changed. Also on  the  nature of  the tissue dealt with.  Pieces of  the  lung  require the fluid to be comparatively seldom changed. Those of the liver more frequently. Roughly speaking, every third or fourth day would suffice in most cases.

Step 7 – Harding of cut tissues

Though as the  hardening progressed, once in five, six, or more days, would be sufficient. The  process usually occupies from  three to five weeks, though  in some cases a shorter time suffices. After hardening, the  tissues are  pre­ served in methylated spirit for future  tissue cutting.

Step 8 – Tissue Cutting

Various methods are adopted to ensure every piece being thoroughly exposed to the action of the reagent. To this end a layer of tow or cotton wool may be placed at the bottom of the vessel for them to rest upon. Sometimes it is convenient to suspend them separately by means of thread, the ends of the threads being attached  to  a  glass  rod  crossing  the  mouth of the vessel. Or  they may be fixed  in any other convenient  way. As a general  rule this is unnecessary if there is  plenty of fluid, and too many pieces are not placed in the same vessel.