Unsourced material may be challenged and removed. Malpighi analysed several parts of the organs of bats, frogs and other animals under the microscope. Malpighi, while studying histology of nervous tissue pdf structure of the lung, noticed its membranous alveoli and the hair-like connections between veins and arteries, which he named capillaries.
His discovery established how the oxygen we breathe enters the blood stream and serves the body. In the 19th century, histology was an academic discipline in its own right. They had dueling interpretations of the neural structure of the brain based in differing interpretations of the same images. Cajal won the prize for his correct theory and Golgi for the staining technique he invented to make it possible. The tissues from plants, fungi, and microorganisms can also be examined histologically. Their structure is very different from animal tissues.
These fixatives preserve tissues or cells mainly by irreversibly cross-linking proteins. This can be detrimental to certain histological techniques. Formalin fixation leads to degradation of mRNA, miRNA, and DNA as well as denaturation and modification of proteins in tissues. However, extraction and analysis of nucleic acids and proteins from formalin-fixed, paraffin-embedded tissues is possible using appropriate protocols.
The aim of Tissue Processing is to remove water from tissues and replace with a medium that solidifies to allow thin sections to be cut. For light microscopy, paraffin wax is most frequently used. Since it is immiscible with water, the main constituent of biological tissue, water must first be removed in the process of dehydration. Paraffin wax does not provide a sufficiently hard matrix for cutting very thin sections for electron microscopy. Again, the immiscibility of most epoxy and acrylic resins with water necessitates the use of dehydration, usually with ethanol. After the tissues have been dehydrated, cleared, and infiltrated with the embedding material, they are ready for external embedding. The acrylic resins are polymerised by heat, ultraviolet light, or chemical catalysts.
The hardened blocks containing the tissue samples are then ready to be sectioned. FFPE tissues are an important resource for historical studies in medicine. Embedding can also be accomplished using frozen, non-fixed tissue in a water-based medium. Pre-frozen tissues are placed into molds with the liquid embedding material, usually a water-based glycol, OCT, TBS, Cryogel, or resin, which is then frozen to form hardened blocks. Sections can be cut through the tissue in a number of directions. Fixed or unfixed tissue may be frozen and sliced using a microtome mounted in a refrigeration device known as a cryostat. The frozen sections are mounted on a glass slide and may be stained to enhance the contrast between different tissues.