Autoradiographic localization of DNA synthesis in a specific ultrastructural, component of the interphase nucleus

High-resolution autoradiogaphy. II. The problem of resolution

The resolution obtainable in electron microscopic autoradiographs, using a photographic emulsion consisting of a monolayer of silver bromide crystals, was investigated theoretically and experimentally. The expected distribution of exposed crystals around a point source was calculated from the geometry of the preparation and from the range distribution of the beta particles emitted by tritium. From such a distribution an autoradiographic resolution of the order of 1000 A can be predicted. From the point source distribution, the expected distribution of grains around bacteriophages labeled with tritium was calculated. This distribution was also measured experimentally in electron microscopic autoradiographs of bacteriophages T-2 labeled with thymidine-H³. The two distributions agreed closely. It was also verified, using the nuclear region in thin cross-sections of Bacillus subtilis labeled with thymidine-H³, that resolutions of the same order were obtained for extended sources. It was concluded that an autoradiographic resolution of 1000 A could be achieved with a presently available commercial emulsion, although emulsions with finer grains might be desirable in some circumstances.

The fine structure of the DNP component of the nucleus. An electron microscopic study utilizing autoradiography to localize DNA synthesis

In the present investigation, the sites of deoxyribonucleic acid (DNA) synthesis and the fate of labeled deoxyribonucleoprotein (DNP) were studied in autoradiographs of ultrathin sections viewed with the electron microscope. Tritiated thymidine was employed as a label for DNA in the nuclei of proliferating cells of regenerating salamander limbs. In the autoradiographic method reported here, dilute NaOH was used to remove the gelatin of the emulsion after exposure and development. The exposed silver grains are not displaced by this treatment and the resolution of fine structure in the underlying section is greatly improved. Our observations suggest that the DNP component is a meshwork of interconnected filaments 50 to 75 A in diameter, which may be cross-linked to form what Frey-Wyssling would term a "reticular gel." The filamentous DNP meshwork is dispersed throughout the interphase nucleus during DNA synthesis, whereas in chromosomes, which are relatively inert metabolically, the meshwork is denser and is aggregated into compact masses. Dense chromatin centers in interphase nuclei are similar in fine structure to chromosomes and are also inert with respect to DNA synthesis. In the Discussion, the structure of the filamentous meshwork in chromatin is compared with that in chromosomes, and speculations are made as to the functional significance of the variations in DNP fine structure observed.

High-resolution autoradiography. I. Methods

Methods used in obtaining high resolution in autoradiography, with special emphasis on the technique of electron microscopic autoradiography, are described, together with control experiments designed to establish the optimum conditions or procedures. On the basis of these experiments the emulsion selected was Ilford L-4, with a crystal size slightly larger than 0.1 micron. It is applied to the specimen in the form of a gelled film consisting of a monolayer of silver halide crystals. Background, when present, can be eradicated by a simple method. The preparations can be stored, in presence of a drying agent, at room temperature or in a refrigerator. Photographic development is done in Microdol, or in a special fine grain "physical" developer. For examination in the electron microscope the sections are stained with uranyl or lead stains. These methods give a good localization of the label, at the subcellular level, and good reproducibility in relative grain counts.

Localizing tritiated norepinephrine in sympathetic axons by electron microscopic autoradiography

Following intravenous infusion of tritiated norepinephrine, rat pineals were prepared for combined autoradiography and electron microscopy. Concentrations of photographic grains were observed only over regions of preterminal autonomic axons containing granulated vesicles, thereby directly demonstrating uptake of norepinephrine into these axons and strongly suggesting that their granulated vesicles contain norepinephrine.

The uptake and development of vaccinia virus in strain L cells followed with labeled viral deoxyribonucleic acid

Vaccinia virus which had its DNA labeled with thymidine-H³ was purified and used as inoculum for L cells growing in suspension. Samples taken over an 8-hour period after infection were studied by light and electron microscopic autoradiography. Within 20 minutes of its being taken up at the cell membrane in phagocytic vesicles, the outer coat of vaccinia becomes disrupted and the virus core containing the labeled DNA passes into the cytoplasmic matrix. Within 1 hour after inoculation the labeled material passes out of the cores into zones of viroplasm, where cores or remnants of cores are gathered and the label becomes more concentrated by 3 hours after inoculation. Most of the label is conserved in the viroplasm areas during the remainder of the experiment. However, 6 hours after inoculation a very small proportion of progeny virus in the cytoplasm, morphologically distinct from the cores of the inoculum, has associated with it labeled material, perhaps derived from the DNA of the inoculum.

ELECTRON MICROSCOPIC RADIOAUTOGRAPHY OF HELA CELLS INFECTED WITH ADENOVIRUS

Harford, Carl G. (Washington University, St. Louis, Mo.), and Alice Hamlin. Electron microscopic radioautography of HeLa cells infected with adenovirus. J. Bacteriol. 89:1540-1547. 1965.-Because intracellular particles of adenovirus are not seen by light microscopy, radioautography with tritiated thymidine was carried out with an electron microscope. Reacted grains were present over many nuclei containing viral aggregates, but only 16% of these grains were over the aggregates. Control observations with tritium-labeled bacteria indicated that this low incidence of labeling of viral aggregates could not be attributed to errors of the method. We think that deoxyribonucleic acid (DNA) labeled with tritiated thymidine in these experiments was mainly the excess viral DNA that is known to accumulate in adenovirus-infected cells. We did not exclude the possibility that some cellular DNA was labeled.

A STUDY OF THE ULTRASTRUCTURAL LOCALIZATION OF HAIR KERATIN SYNTHESIS UTILIZING ELECTRON MICROSCOPIC AUTORADIOGRAPHY IN A MAGNETIC FIELD

The sites of the incorporation of labeled cystine into keratinizing structures were studied in electron microscopic autoradiographs. The tracer used was cystine labeled with S(35) emitting long-range ionizing particles. During exposure for 1 to 2 months, according to our method of electron microscopic autoradiography, emulsion-coated specimens were exposed to a static magnetic field which appeared to result in a marked increase in the number of reacted silver grains. In young Swiss mice receiving intraperitoneal injections at 1, 3, and 6 hours before biopsy, conventional autoradiography demonstrated that S(35)-cystine was intensely localized in the keratogenous zone of anagen hair follicles, and that the radioactivity there increased in intensity progressively with time while the radioactivity in the hair bulb always remained very low. Our observations with electron microscopic autoradiography in a magnetic field appeared to indicate that at 3 and 6 hours after injection the S(35)-cystine was directly and specifically incorporated into tonofibrils in the hair cortex and into amorphous keratin granules of the hair cuticle layer, possibly without any particular concentration of this substance in the other cellular components. There seemed to be an appreciable concentration of cystine in tonofibrils of the cuticle of the inner root sheath. However, trichohyalin granules in the hair medulla and inner root sheath failed to show any evidence of cystine concentration. The improved sensitivity of the electron microscopic autoradiography with S(35)-cystine appeared to be partly due to the application of a static magnetic field. However, the reason for this could not be explained theoretically.

PROTEIN SYNTHESIS, STORAGE, AND DISCHARGE IN THE PANCREATIC EXOCRINE CELL. AN AUTORADIOGRAPHIC STUDY

The synthesis, intracellular transport, storage, and discharge of secretory proteins in and from the pancreatic exocrine cell of the guinea pig were studied by light- and electron microscopical autoradiography using DL-leucine-4,5-H³ as label. Control experiments were carried out to determine: (a) the length of the label pulse in the blood and tissue after intravenous injections of leucine-H³; (b) the amount and nature of label lost during tissue fixation, dehydration, and embedding. The results indicate that leucine-H³ can be used as a label for newly synthesized secretory proteins and as a tracer for their intracellular movements. The autoradiographic observations show that, at ∼5 minutes after injection, the label is localized mostly in cell regions occupied by rough surfaced elements of the endoplasmic reticulum; at ∼20 minutes, it appears in elements of the Golgi complex; and after 1 hour, in zymogen granules. The evidence conclusively shows that the zymogen granules are formed in the Golgi region by a progressive concentration of secretory products within large condensing vacuoles. The findings are compatible with an early transfer of label from the rough surfaced endoplasmic reticulum to the Golgi complex, and suggest the existence of two distinct steps in the transit of secretory proteins through the latter. The first is connected with small, smooth surfaced vesicles situated at the periphery of the complex, and the second with centrally located condensing vacuoles.

Thyroid hormone binding to adult rat alveolar type II cells. An autoradiographic study

Hypothyroid adult rats were injected intravenously with [3, 5, 3'-125I]triiodothyronine (T3) or [125I]thyroxine (T4). Others were similarly treated except for the addition of a 500-fold excess of the appropriate unlabeled hormone. Light microscopic autoradiography of semithin sections of lung from these animals demonstrated labeling localized over alveolar parenchymal tissue. Analysis of the labeling studies of type II cells revealed high affinity, low capacity binding of T3 and, to a much lesser extent, T4 to both the nuclear and cytoplasmic compartments. Adult lung in organ culture was also treated with 125I-T3 or 125I-T4 and electron microscopic autoradiography performed. These studies revealed that, when expressed as grains per square micrometer, there was substantially more labeling over the lamellar inclusion bodies and the mitochondria than over the nucleus. The results of this study provide morphologic evidence of specific uptake and binding of thyroid hormones by the nucleus and cytoplasm of alveolar type II cells, and suggest that the lamellar inclusion bodies and mitochondria may be the primary location of the cytoplasmic binding. These findings also substantiate the view of a direct effect of thyroid hormones on the lung and, in particular, on the type II alveolar cells.

An alternative to the flat substrate method of preparing electron microscope autoradiographs

Difficulty with flat substrate methods of preparing electron microscope autoradiographs has prompted reconsideration and refinement of a technique in which an electron microscope grid is placed beneath the specimen prior to dipping. This technique avoids the problems commonly associated with the direct application of emulsions to specimen grids, and should be considered as an alternative to flat substrate techniques when difficulty with these methods is encountered.

The prepupal salivary glands of Drosophila melanogaster

Evidence is presented in support of the concept that the larval salivary gland of Drosophila melanogaster continues to function as an important secretory organ throughout prepupal stages and after pupation. Just after puparium formation, and at other later periods, the glands appear to be in the process of disintegration, but each time they recover until after pupation. Nuclear blebbing occurs through the time of survival of the glands, but is shown not to involve transport of RNA out of the nucleus. Transport in and out of the nucleus is clearly rapid and in a steady state as compared to the massive and intermittent export of cytoplasmic substance into the lumen of the gland.

Coating sections for electron microscopic autoradiography: a stripping technique using liquid emulsion

A coating technique for electron microscope autoradiography is described which combines the advantage of forming an emulsion film by a dipping method with the ease of coating sections already on grids. Sections are coated so that a formvar support film separates the section and the emulsion crystals. This intermediate layer of formvar ensures a random distribution of the emulsion crystals. Using light gold sections, Ilford L-4 emulsion and Microdol-X development, the resolution of this technique, as determined by the half distance method, was 150 nm. The additional layer of formvar slightly reduced the image quality with biological samples in the electron microscope. This technique has a minimal loss of resolution and image quality for moderate resolution electron microscope autoradiography.

Fetal monkey absorption of 55Fe from amniotic fluid

A single dose of 0.5 mCi radioiron (55Fe) was injected directly into the amniotic cavity of each of four pregnant monkeys near term. At 6 or 24 hours after 55Fe adminstration, samples of fetal jejunum, ileum, blood, spleen, liver, lung and fetal membranes were taken for radioautography and scintillation counts. Counts were also made on corresponding maternal tissues. Intestinal radioiron levels and localization indicated that fetuses swallowed and absorbed 55Fe within 6 to 24 hours after injection. Silver grains over the meconium bodies of fetal ileum suggested that intestinal epithelium retained at least some of the absorbed radioiron. However, elevated 55Fe levels in fetal blood and hematopoietic organs indicated that transport of radioiron to the circulation had occurred, presumably through fetal intestine, extraembryonic membranes and/or fetal lungs. Transfer of 55Fe across the placenta from fetus to mother occurred at a slow rate, i.e., samples of maternal organs obtained at 24 hours gave low counts.

Frozen thin sections of fresh tissue for electron microscopy, with a description of pancreas and liver

A simple method has been developed that allows frozen thin sections of fresh-frozen tissue to be cut on a virtually unmodified ultramicrotome kept at room temperature. A bowl-shaped Dewar flask with a knifeholder in its depths replaces the stage of the microtome; a bar extends down into the bowl from the microtome's cutting arm and bears the frozen tissue near its lower end. When the microtome is operated, the tissue passes a glass or diamond knife in the depths of the bowl as in normal cutting. The cutting temperature is maintained by flushing the bowl with cold nitrogen gas, and can be set anywhere from about -160 degrees C up to about -30 degrees C. The microtome is set for a cutting thickness of 540-1000 A. Sections are picked up from the dry knife edge, and are placed on membrane-coated grids, flattened with the polished end of a copper rod, and either dried in nitrogen gas or freeze-dried. Throughout the entire process the tissue is kept cold and does not come in contact with any solvent. The morphology seen in frozen thin sections of rat pancreas and liver generally resembles that in conventional preparations, although freezing damage and low contrast limit the detail that can be discerned. Among unusual findings is a frequent abundance of mitochondrial granules in material prepared by this method.

Thymidine-3H electron microscope radioautography of osteogenic cells in the fetal rat

The proximal tibial epiphyses of 18-21-day-old fetal rats have been studied by thymidine-(3)H radioautography. The results reveal that the label is incorporated into two types of osteogenic cells: (a) a spindle cell type (A cells) with characteristics generally associated with matrix production, including an extensive development of the endoplasmic reticulum and the presence of large intracellular accumulations of a dense, finely granular material, morphologically identifiable as glycogen; and (b) a rounded cell type (B cells) with morphological features similar in degree and kind to those of the developing neutrophilic leukocyte, including an abundance of free ribosomes and mitochondria and a complex Golgi apparatus associated with dense specific granules, morphologically identifiable with primary lysosomes. These results, along with the occurrence of recognizable, labeled, immature, perivascular forms of both of these A and B type cells, lead to the conclusion that the specialization of osteogenic cells into osteoclasts and osteocytes may involve separate pathways of cytodifferentiation.

Sites of glycogen synthesis in rat liver cells as shown by electron microscope radioautography after administration of glucose-H3

Glycogen synthesis was investigated by giving tritium (H(3))-labeled glucose with carrier to fasted rats in vivo or incubating liver slices from fasted rats in vitro using a glucose-H(3)-containing medium. After 15 min or 1 hr, pieces of liver were fixed and radioautographed for light and electron microscopy. In vivo and in vitro, radioautographic reactions appeared over "glycogen areas" and over zones transitional between these areas and ergastoplasm. Treatment of sections by alpha amylase removed all but about 5% of the radioactivity, so that about 95% of it consisted of glycogen (synthesized during the 15 min or 1 hr elapsing after administration of glucose-H(3)). Within glycogen areas and transitional zones, most silver grains were over or very close to glycogen granules and smooth (or partly smooth) vesicles. Presumably, much of the label was added onto growing glycogen granules, in accord with the biochemical view that glycogen may serve as substrate for further glycogen synthesis. The few silver grains located far from glycogen granules-15% at the 15 min interval in vivo-approximated smooth (or partly smooth) vesicles of endoplasmic reticulum. This observation raised the possibility that smooth membranes play a role in glucose uptake at an early stage in de novo formation of glycogen granules.