These automatic tissue processors are essentially a series of vats, arranged in a circle with a timing device (Figure 37.4). These containers hold reagents and paran wax. The transfer arm changes the position of the basket through the processing reagents. An aempt is made to complete the tissue processing during the night (Table 37.2a). To achieve this schedule, the timing disc is cut, screwed on the plate to be held in place by the disc-holder frame. With the timing lever set at zero, the machine is started at about 4.30 p.m. (Figure 37.5). The basket with the cassees automatically changes position and takes a bath in dierent reagents kept in beakers to accomplish dehydration, clearing and inltration. The nal dip is in the warm paran wax until the technician arrives in the morning. The cassees are then opened and the tissues are embedded. The same schedule is completed through day and night shis, as shown in Table 37.2b. This can be adapted to the prevailing conditions of developing countries.

The inltrated-impregnated tissue is then placed in warm liquid paran, commonly known as embedding. Here the tissues solidify into a rm block when it cools down to room temperature. The other names for this process is casting or blocking. Embedding helps the tissue to be cut on a microtome. During this process, the tissue samples are placed into moulds along with liquid embedding material which is then hardened. This is achieved by cooling in case of paran wax and heating in case of epoxy resins (curing). The acrylic resins are polymerized by heat through ultraviolet light or chemical catalysts. Tissue samples are in the hard blocks and ready to be sectioned. Embedding can also be accomplished using frozen, non-xed tissue in a water-based medium. Pre-frozen tissues are placed into moulds with the liquid embedding material, usually a water-based glycol or resin, which is then frozen to form hardened blocks. This procedure is adapted in case of emergency. For routine embedding, paran wax with a higher melting point (56–58°C) is used. Commercially available paran wax, however, should be ltered before being used for embedding. This will protect the edge of the knife. The cakes and trimmings from blocks are put in a clean enamel pitcher and kept in the oven. The molten paran is ltered inside the oven through a coarse lter paper into another container, which is used in embedding. Embedding centres (Figure 36.3) are available nowadays for dispensing the hot paran into the metal moulds which hold the tissue specimen and cool the tissue blocks.

Microscope slides are rst prepared before aaching the section on the slide. Since the subsequent manipulation of the slide involves many movements of the slide in and out of various uids and stains, an adhesive must be used to keep the section rmly xed to the slide. Otherwise the section may wash away. A thin, at section of a tissue will usually adhere to a clean glass surface and stay in place through the common preparative procedures of staining, washing, dehydration, and clearing. The close contact between the two at surfaces gives the anity of adhesion between the slide and the tissue section. It comes through ionic aractions between sections (protein) and the clean surface of the slide (silica). But what happens when all sections are not perfectly at or the slides surfaces are not perfectly clean? In addition, there are some staining techniques that are harsh enough to remove the aest section from the cleanest slide. There are two ways to prevent or reduce the loss of sections subjected to harsh treatments. These are as follows: (a) by using an adhesive, and (b) by inclusion in a permeable protective lm. One can also use special slides described in Chapter 36 of this Volume.