As used in histology, a cryostat is an apparatus consisting of a refrigerated or cooled chamber which contains a precision cutting device called a microtome. The cryostat is capable of maintaining a constant low temperature, especially below 0.degree. C. (e.g. by means of mechanical refrigeration). The microtome is used to cut both thick and very thin slices or sections of specimens, especially frozen specimens, such as body tissue. Other types of samples such as teeth, bones, or sections of body organs are frequently sectioned. Tissue sections must be uniform in thickness and are cut as thin as 2.mu. (microns or micrometers) or less (i.e. about 40 millionths of an inch or less). The prepared specimen sections are mounted on a microscope slide and can be stained, and then examined microscopically.
In a normal procedure for preparing a frozen histological specimen for microscopic examination, a tissue sample is brought into the laboratory for diagnosis. Embedding medium (e.g. aqueous saline solution, viscous aqueous solutions or gels) is routinely dispensed onto a specimen holder usually in the form of a small metal block. The specimen is either placed on top of or submerged into the embedding medium, and then is frozen, typically to about -15.degree. C. to about -30.degree. C., by any number of means. Once the specimen is frozen, the specimen holder is clamped tightly in the jaws of the microtome chuck (i.e. clamp). For sectioning frozen samples, the specimen holder is also maintained at a low temperature to avoid sample thawing. The low temperature is usually provided by the cryostat.
For cryogenic sectioning (also called cryotomy), a crank is either turned by the operator or is motor-driven and the microtome mechanism moves the specimen holder in an up and down motion. For each up-down cycle, the specimen holder or the microtome knife is advanced a pre-determined distance usually near the top of the up-cycle, typically between about 2 microns and about 20 microns. During the downward movement of the microtome specimen holder, the surface of the frozen tissue, as well as the surrounding embedding medium, are cut by the sharp microtome knife which is aligned at an appropriate angle. The surface produced by this cutting action is referred to as the cutting plane.
Typically, a sufficient quantity of frozen material must be trimmed away to form a flat surface exposing the section of interest. The trimming action is often accomplished by using a coarse setting of the microtome advance. When the appropriate level for collecting sections is reached, the microtome is reset for a smaller advance. The tissue section of interest is then cut and retrieved by the operator.
During the ordinary course of cryotomy, much waste and debris material is generated during the trimming phase of the procedure. This material is in the form of shavings, residual tissue debris, waste particles, and the like, frequently associated with the knife edge and/or generally dispersed throughout the cryostat during the cutting action. These shavings and pieces of specimen debris are frozen due to the low temperature of the cryostat, and are very thin and lightweight. The presence of this flimsy waste material is a source of several problems for operators of the microtome, for those who clean the cryostat, and when attempting to recover clean specimens, uncontaminated by pieces of the waste material.
First, depending on the source and type of the tissue specimen being sectioned, the production of the loose shavings and debris is potentially dangerous, especially when sectioning tissue specimens which may contain infectious agents. Loose or airborne shavings can easily come into contact with the operator. For example, if debris from a fresh tissue section contacts the skin, the section melts and leaves an invisible deposit. Thus, the operator may be unaware of this potential hazard. Such tissue debris or fragments can be transferred unknowingly from the hand to the mouth or other body parts of the operator, as well as to others with whom the operator has contact. If the shaving or section contacts the skin where the skin is cut or broken, infectious agents contained in the specimen may enter the tissues of the operator. A further potential hazard exists, since some airborne pieces of the shavings may be inhaled by the unsuspecting operator or by others in the vicinity.
Second, because the tissue debris is lightweight and plentiful, shavings and fragments generally become distributed throughout the interior of the cryostat after only a few minutes of operating the microtome. Moreover, the bulk of the debris and shavings accumulate in the immediate vicinity of the cutting edge of the knife blade. The presence of the waste material is also detrimental to the performance of the microtome, because loose particles can become trapped between sliding elements of the microtome and can create excess friction.
Third, the residual shavings can become lodged under inaccessible surfaces of both the cryostat and the microtome devices, making the process of cleaning-up very difficult. In order to clean the cryostat and microtome thoroughly, it is often necessary to remove the entire microtome from the cryostat chamber. This is a burdensome task and is frequently deferred until the build-up of the debris is pronounced and/or interferes with the operation of the microtome.
An ordinary vacuum cleaner device can be used to collect the loose shavings after sample sectioning. However, such vacuum cleaners pose several problems. Freshly cut shavings which have not dried out prior to the vacuum process will melt when they contact the inner walls of the hose or the warm filter medium situated inside of a typical vacuum cleaner. In such a case, after only a short period of operation, the melted debris will be aspirated into, collapse within, and seal off the pores of the filter as it dries, thereby making the vacuum cleaner inoperable. Even before the filter clogs, it is not uncommon for bacteria and mold to grow overnight on the inside wall of the hose and on the filter, producing an odor of rotting meat and an unsightly appearance to the hose. If a conventional vacuum cleaner is used at some time after the shavings and debris have already been generated, it is difficult to collect all of the shavings. Indeed, airborne shavings may have escaped much earlier during sample preparation and may have already dried onto the exposed skin of the operator, or may have been inhaled before the residual material is vacuumed away.
An ordinary vacuum cleaner, called a "HISTOVAC".TM., has been equipped with a disposable wand, vacuum hose, and a filter trap in the hose, and is offered as a product to clean a cryostat. Such a vacuum cleaner unit and its hose-containing filter trap are located outside of the cryostat chamber. This type of vacuum cleaner suffers from all of the shortcomings described above. Specifically, since the vacuum cleaner and its filter are outside of the cryostat, the frozen flakes of waste material melt inside the warm or room-temperature vacuum hose and on the filter which is also warm or at room temperature. The filter is quickly clogged. Since vacuuming is done after the debris is generated, operator exposure can still occur and health risks due to accumulation and difficult clean-up, as mentioned above, are encountered. Furthermore, because of the melting of debris on the inner wall of the hose and on the filter during use, a considerable amount of hazardous material may adhere to and become lodged inside of the hose. As a consequence, the entire hose assembly must be discarded quite frequently. Repeated replacement of the entire hose assembly is inefficient and expensive.
The prior art does not adequately or cost-effectively solve the above-related problems associated with rapid clogging of the filter, or with the hazards of airborne waste material produced during the sectioning procedures. The prior art vacuum cleaner arrangement addresses after-the-fact clean-up of specimen waste material at room temperature and does not satisfy the need for trouble-free clean-up and removal of frozen debris during or immediately after its production.