The components that remain after fixation consist mostly of large molecules that do not readily dissolve, especially after treatment with the fixative. These large molecules, particularly those that react with other large molecules to form macromolecular complexes, are usually preserved in a tissue section. Examples of such large macromolecular complexes include:  nucleoproteins formed from nucleic acids bound to protein,  intracellular cytoskeletal proteins complexed with as sociated proteins,  extracellular proteins in large insoluble aggregates, bound to similar molecules by cross-linking of neighbor ing molecules, as in collagen fiber formation, and  FOLDER 1.1
Clinical Correlation: Frozen Sections
Sometimes, the pathologist may be asked to immediately evaluate tissue obtained during surgery, especially when in-stant pathologic diagnosis may determine how the surgery will proceed. There are several indications to perform such an evaluation, routinely known as a frozen section. Most commonly, a surgeon in the operating room requests a frozen section when no preoperative diagnosis was available or when unexpected intraoperative findings must be identi-fied. In addition, the surgeon may want to know whether all of a pathologic mass within the healthy tissue limit has been re-moved and whether the margin of the surgical resection is free of diseased tissue. Frozen sections are also done in combination with other procedures such as endoscopy or thin-needle biopsy to confirm whether the obtained biopsy material will be usable in further pathologic examinations. Three main steps are involved in frozen section prepa-ration:  Freezing the tissue sample. Small tissue samples are frozen either by using compressed carbon dioxide or by immersion in a cold fluid (isopentane) at a temperature of 50 C. Freezing can be achieved in a special high-efficiency refrigerator. Freezing makes the tissue solid and allows sectioning with a microtome.  Sectioning the frozen tissue. Sectioning is usually per-formed inside a cryostat, a refrigerated compartment containing a microtome. Because the tissue is frozen solid, it can be cut into extremely thin (5 to 10 m) sec-tions. The sections are then mounted on glass slides.  Staining the cut sections. Staining is done to differen-tiate cell nuclei from the rest of the tissue. The most common stains used for frozen sections are H&E, methylene blue (Fig. F1.1.1), and PAS stains. The entire process of preparation and evaluation of frozen sections may take as little as 10 minutes to complete. The total time to obtain results largely depends on the transport time of the tissue from the operating room to the pathology laboratory, on the pathologic technique used, and the expe-rience of the pathologist. The findings are then directly com-municated to the surgeon waiting in the operating room. aba b FIGURE F1.1.1 • Evaluation of a specimen obtained during surgery by frozen-section technique. a. This photomicrograph shows a specimen obtained from the large intestine that was prepared by frozen-section technique and stained with methylene blue. 160 b. Part of the specimen was fixed in formalin and processed as a routine H&E preparation. Examination of the frozen section revealed it to be normal. This diagnosis was later confirmed by examining the routinely prepared H&E specimen. 180. (Courtesy of Dr. Daniel W. Visscher.)  membrane phospholipid–protein (or carbohydrate) complexes.
These molecules constitute the structure of cells and tissues— that is, they make up the formed elements of the tissue. They are the basis for the organization that is seen in tissue with the microscope.