Plasma membrane transformations in spermatogenesis revealed by aldehyde fixatives containing tannic acid

The use of different fixatives and hydrophilic embedding media (Historesin and Unicryl) for the study of embryonic tissues

The effects of different fixatives, dehydration procedures, and embedding media on the structural and ultrastructural preservation of young chick embryos (Hamburger and Hamilton stages 18-24) have been studied by means of light and electron microscopy techniques. Under the light microscope, the results obtained with the use of Bouin, glutaraldehyde, or glutaraldehyde-paraformaldehyde mixtures, followed by partial dehydration of the samples and the embedding with two different polar resins (Historesin and Unicryl), were compared with the results obtained using conventional paraffin-embedding methods. Cell and tissue shrinkage was determined by comparing blood cells from those embryos embedded in either of resins with those embedded in paraffin. Samples were also compared with blood smears, either methanol-fixed or unfixed, obtained from embryos at the same Hamburger and Hamilton stages. The results obtained when Unicryl and Araldite were used for electron microscopy have also been compared. When ultrastructural images from glutaraldehyde-tannic acid/osmium tetroxide fixed, Unicryl embedded samples were compared with those from araldite embedded samples, the same good results were observed with either of the resins. Araldite embedding requires a complete dehydration of the samples, while Unicryl allows the embedding of partially dehydrated embryos with optimal ultrastructural results. We suggest that these polar resins can be considered as complementary tools for embedding delicate embryonic tissues, allowing partial dehydration of the specimens with an excellent cell and tissue preservation.

Some unusual staining properties of tannic acid in plants

Maize root tips were fixed in glutaraldehyde fixatives containing tannic acid and then processed for electron microscopy. Under these conditions, tannic acid selectively stained the contents of the Golgi apparatus secretory vesicles of some outer root cap cells, the cell walls of all cells, and substances in, and adjacent to, intercellular connections of mature primary walls and of secondary walls. Intercellular connections of the young primary walls were not stained. Plasma membranes, and substances associated with the outer leaflets of the plasma membranes, were also stained. Tannic acid-positive material was associated with the cell plate vesicles of forming walls but very little, or none, was associated with the Golgi apparatus vesicles of dividing cells.

Tannic acid-glutaraldehyde fixation reveals calcium ionophore-induced changes in rabbit polymorphonuclear leukocyte membranes

TAG fixation of normal and Ca2+ ionophore-treated rabbit polymorphonuclear leukocytes (PMN) has revealed membrane components not apparent with conventional glutaraldehyde fixation. These included a 30 A external electron-dense coating on untreated cells. A somewhat thicker coat (40 A) was observed in ionophore-treated, nondegranulating PMN. In ionophore-treated, degranulating PMN, a 65 A cell membrane coat was observed. A similar coat was observed on the inner side of the membrane of some azurophil-type granules, but the electron density and thickness were not so pronounced. Cytoplasmic granules were often closely apposed and often protruded outward at the plasma membrane. Extracellular lamellae, sometimes stacked apposed to the plasma membrane, possibly represent remnants of intense granule extrusion. Sequential degranulation of the respective granules was not apparent.

Tannic acid as an electron microscope tracer for permeable cell membranes

To recognize damaged cells in preparations for transmission electron microscopy, high molecular weight (1700 MW) tannic acid (1-4%) has been added to glutaraldehyde fixing solutions. During fixation, the tannic acid penetrates only those cells whose plasma membranes were previously damaged. It enhances the electron density of the injured cells, which become clearly distinguishable from the undamaged ones. As a tracer tannic acid shows great advantages over either lanthanum hydroxide, ruthenium red, or horseradish peroxidase. It diffuses evenly throughout the tissue block and is not removed by preparative steps. Furthermore, it is also a good tracer at the light microscope level.