A reliable, standardized method for ultrastructural electron microscopic radioautography

Ongoing block of Schwann cell differentiation and deployment in dystrophic mouse spinal roots

In the spinal roots of dystrophic mice, there are bundles of unensheathed axons and two populations of axon-associated cells: the typical Schwann cells of myelinated fibers and 'uncommitted' cells at the margin of the bundles. Because these 'uncommitted' cells continue to divide in adult animals but fail to ensheath the axons they appose, they can be labelled with tritiated thymidine. In the present experiments, we show that these cells may differentiate into typical Schwann cells of myelinated or unmyelinated fibers when spinal roots from [3H]thymidine-labelled dystrophic mice are grafted into the sciatic nerves of non-dystrophic animals. Thus, this study demonstrates that the 'uncommitted' cells of dystrophic spinal roots are undifferentiated Schwann cells whose differentiation in the intact spinal roots is continuously prevented by some unknown mechanism.

Biochemistry and ultrastructure of serotonergic nerve endings in the lobster: serotonin and octopamine are contained in different nerve endings

In this article we report that the distribution of serotonin in the lobster nervous system parallels the distribution of octopamine and that the same tissues that contain endogenous serotonin can synthesize it from tryptophan. Octopamine and serotonin are highly concentrated in a neurosecretory region of the second thoracic roots in association with a group of neurosecretory cells. The roots possess separate high-affinity uptake systems for both serotonin and tryptophan. Radioactive serotonin, accumulated in tissues during incubations with either tritiated serotonin or tritiated tryptophan, can be released, in a calcium-dependent manner, by depolarization with potassium. A detailed morphological examination of the second thoracic roots shows four distinct categories of nerve endings in the vicinity of the neurosecretory cells. Octopamine is synthesized in one of these types of endings and serotonin in another. The high-affinity uptake systems for serotonin and tryptophan are found only in association with the endings that make serotonin. These endings and all the biochemical parameters of serotonin metabolism in the roots are selectively destroyed by previous injection of animals with the neurotoxin 5,7-dihydroxytryptamine.

Direct in vivo demonstration by radioautography of specific binding sites for calcitonin in skeletal and renal tissues of the rat

An in vivo binding assay using radioautography was employed to visualize calcitonin receptors in rat tissues. At 2 min after intravenous injection of biologically active 125I-salmon calcitonin, free hormone was separated from bound hormone by intracardiac perfusion with lactated Ringer's followed by fixation with 2.5% glutaraldehyde. Various tissues were removed and processed for light and electron microscope radioautography. These were compared to tissues removed from animals that received identical amounts of labeled hormone with a large excess of unlabeled calcitonin. Among the tissues investigated, kidney and bone demonstrated labeling. In kidney, most silver grains were located over vesicles below the brush border of cells of theproximal convoluted tubules. These grains were still present after simultaneous injection of excess unlabeled hormone and most likely represented binding to sites involved with ingestion and degradation of hormone from the urinary filtrate. In contrast, grains localized to the basal surfaces of distal convoluted tubule cells were significantly reduced in number in control animals and represented sites of saturable, specific hormone binding. In bone, specific binding sites were found only at the periphery of osteoclasts. These labeled cells were located at resorption sites examined in tibia, humerus, and alveolar bone. This demonstration of the localization of 124I-calcitonin in situ provides a new approach for study the interaction of calcium-regulating hormones with their target cells.

Insulin binding sites localized to nerve terminals in rat median eminence and arcuate nucleus

Specific binding sites for blood-borne insulin were determined to be selectively localized on axons and axon terminals in the external median eminence and the hypothalamic arcuate nucleus by means of quantitative fine structural radioautography. This localization suggests that discrete populations of hypothalamic nerve terminals are potential targets for the direct effects of insulin and that insulin may act through synaptic mechanisms to influence hypothalamic circuits regulating energy balance and hypophyseal function.

Radioautographic identification of lactogen binding sites in rat median eminence using 125I-human growth hormone: evidence for a prolactin "short-loop" feedback site

The binding characteristics of human growth hormone were exploited to identify radioautographically lactogen binding sites in the rat median eminence. Following systemic injection 125I-human growth hormone bound preferentially to the lateral palisade zone, a region of median eminence rich in dopamine and LHRH. Coinjection of 125I-human growth hormone with an excess of unlabeled human growth hormone or ovine prolactin, but not bovine grown hormone, competitively blocked 125I-human growth hormone binding to the external median eminence. These observation provide direct evidence of recognition sites for lactogenic hormones in a discrete region of the median eminence associated with hypothalamic regulation of hypophyseal prolactin and luteinizing hormone secretion. Median eminence lactogen binding sites may mediate presumed direct effect of lactogenic hormones on the reproductive functions of the hypophysiotropic hypothalamus.

Persistent multiplication of axon-associated cells in the spinal roots of dystrophic mice

Axons which lack Schwann cells characterize the spinal roots of dystrophic mice. In these nerves, two types of cells are associated with large axons-typical Schwann cells which ensheath and myelinate individual axons and "uncommitted" cells which closely contact some axons but do not unsheath them normally. Although neonatal Schwann cell multiplication in dystrophic spinal roots is less than normal, the "uncommitted" cells continue to multiply in adult animals. Thus, these cells are presumably responsive to the mitogenic stimulus of axons, but are unable to extend radially and longitudinally to form a myelin sheath. This disorder of axon-sheath cell interactions could be due to an intrinsic abnormality of Schwann cells or a failure of the mechanisms whereby axons induce Schwann cell differentiation.

Ultrastructural characterization of axonal endings in the substantia gelatinosa which take up [3H]serotonin

Serotonergic axonal endings in layers I and II of the dorsal horn of the medulla were identified by autoradiography. In adult cats, pretreated with a monoamine oxidase inhibitor, tritiated serotonin ([3H]5-HT) was topically applied onto the surface of the caudal medulla. Light autoradiographs from 1 micrometer sections demonstrated silver grains in both layers I and II. In EM autoradiographs, two categories of axonal endings were labeled by [3H]5-HT uptake: dome-shaped endings which form a single synapse and scalloped endings which form multiple synapses. Each category was further divided into several types based on morphological criteria. The [3H]5-HT-labeled endings synapse primarily on small caliber dendritic shafts and spines, with the dome-shaped endings forming both symmetrical and asymmetrical synapses and the scalloped endings forming only asymmetrical synapses. Dome-shaped endings were most common and two types were found in layers I and II while a third type was found only in layer II. Layer I contained a single type of scalloped ending while layer II contained three types of scalloped endings. In a series of experiments designed to provide another approach to identifying serotonergic endings, 5,6-dihydroxytryptamine, a serotonin neurotoxin, was either topically applied onto the caudal medulla or injected into the fourth ventricle. Following treatment with the neurotoxin, blackened degenerating dome-shaped and scalloped endings similar to those labeled in the [3H]5-HT uptake experiments were found in layers I and II. The presence of serotonergic endings in layer I suggests that some of these endings synapse on the dendrites of layer I projection neurons where they may inhibit the output of the projection neuron directly. Serotonergic endings in layer II may modulate the activity of layer II interneurons by synapsing directly on these interneurons. The interneurons in layer II may function by mediating the transfer of inputs from primary endings in these layers to layer I projection neurons.

Quantitative investigation of scintillator intensification for light and electron microscope radioautography

Several reports have indicated that scintillators enhance the intensity of radioautographic reactions. If this were the case, the use of scintillators would shorten the exposure time of radioautographs and the production of radioautographs would be accelerated. It was, therefore, decided to examine the effect of scintillators on the intensity of radioautographs in a quantitative manner over uniformly labeled test specimens. Reaction intensities with or without scintillators were compared using Kodak NTB2 emulsion for light microscopy and Ilford L4 and Sakura NR-H2 emulsion for electron microscopy. In addition, the background fog was quantitatively evaluated. No increase in the intensity of the radioautographs was observed under the following conditions: pretreatment of tissue sections with scintillator solutions before emulsion coating and exposure; treatment with scintillator solutions after emulsion coating but before exposure; treatment with scintillator solutions during exposure; contact with a plastic scintillator film throughout exposure. Heightened reaction intensity was obtained only when scintillators were applied during the histological processing of tissues. The impregnation of tissue blocks with PPO (2,5-Diphenyloxazole) and POPOP (1,4-bis-2-(5-phenyloxazolyl)-benzene) during dehydration, infiltration and Epon embedding produced a 20% increase of reaction intensity in light microscope radioautographs using NTB2 emulsion and a 75% increase in electron microscope radioautographs using L4 emulsion.

Insulin binds to brain blood vessels in vivo

The brain has generally been considered an insulin-independent organ, because insulin does not apparently exert a direct effect on brain glucose consumption. Recently, however, insulin receptors have been detected throughout the central nervous system (CNS) of several species. Since important insights into the functional significance of brain insulin receptors might be provided by identification of the cell type(s) possessing these receptors, we have attempted to localise them morphologically using light and electron microscope autoradiography. We report here results indicating that blood vessels throughout the CNS of the rat bind plasma insulin rapidly and with considerable specificity.

Migration and turnover of entero-endocrine and caveolated cells in the epithelium of the descending colon, as shown by radioautography after continuous infusion of 3H-thymidine into mice

Adult male mice were given a continuous infusion of about 0.5 muCi of 3H-thymidine per gram body weight per day for periods varying from 1 to 60 days. Semithin sections of descending colon were cut from/plastic-embedded blocks and stained by a method combining silver impregnation and iron hematoxylin, by which argentaffin entero-endocrine cells and caveolated cells could be identified. From radioautographs, the labeling index of these cells was determined. One to three days after the beginning of 3H-thymidine infusion, label is observed in some of the stained entero-endocrine cells in the bottom of the crypts; the apices of these cells reach the crypt lumen and are joined to neighboring cells by terminal bars (junctional complexes). After five to seven days, labeled entero-endocrine cells are seen on the sides of the crypts, where their base stretches along the basement membrane and their apex has lost its terminal bar connections to neighboring cells. Finally, by 13 and 24 days, labeled cells are observed within the epithelium at the mucosal surface. The turnover time, which is taken to be equal to the mean time required for migration from site of origin to site of loss on the mucosal surface, has been estimated at 23.3 days. This is much longer than the 4.6 days required by the two main cell types of the epithelium -- vacuolated-columnar and mucous cells -- to travel the same route. It is likely that, after entero-endocrine cells lose their terminal bar attachment to other epithelial cells, they migrate independently and very slowly. Labeled caveolated cells are first seen in the crypt bottom one day after the beginning of 3H-thymidine infusion. By three to five days, they are on the sides of the crypts; their base is stretched along the basement membrane, but their apex retains its attachment to neighboring cells by terminal bars. By seven days, labeled caveolated cells are on the mucosal surface. Their turnover time has been assessed at 8.2 days. This is, again, longer than for the two main types to which they are bound by terminal bars throughout migration. The discrepancy is explained by the caveolated cells arising deeper in the crypts than most vacuolated-columnar and mucous cells.

The effect of colcemid on the structure and secretory activity of ameloblasts in the rat incisor as shown by radioautography after injection of 3H-proline

Enamel secretion by ameloblasts was investigated in the incisors of 100 gm normal and colcemid-injected male rats. Morphological studies were done on rats given a single intraperitoneal injection of 0.1 mg (1.25 mM) of colcemid and sacrified 1 to 4 hours after injection. Protein synthesis and secretion were investigated with radioautography in normal and colcemid-treated rats injected with 3H-proline and sacrificed at intervals between 0.5 and 3.5 hours after injection. Colcemid was injected 0.5 hours prior to 3H-proline in each experimental rat. Electron microscopic examination revealed several morphological alterations between 1 and 4 hours after injection of colcemid. These changes included fragmentation of the normally elongated rough endoplasmic reticulum into shorter profiles; a disorganization of the normally tubular configuration of the Golgi apparatus into a number of seples and profiles of smooth endoplasmic reticulum from Tomes' processes; and the accumulation of secretion granules at the mature face of the Golgi stacks, as well as in the infranuclear cytoplasm where thye are normally not found. Radioautography revealed that protein synthesis by the rough endoplasmic reticulum had continued in colcemid-altered ameloblasts. Labeled secretion granules were found at the mature surface of the Golgi stacks and in the infranuclear cytoplasm, however they did not migrate into Tomes' processes. Consequently, labeled enamel matrix did not appear extracellularly at the same time as in normal controls. Quantitative radioautography in the light microscope revealed that the effect of colcemid, although reversed within 4 hours, had temporarily inhibited normal migration, and exocytosis of secretion granules.

The cell cycle of Bacillus subtilis as studied by electron microscopy

Bacillus subtilis strain Marburg was grown exponentially with a doubling time of 65 min. To follow the time course of various cell cycle events, cells were collected by agar filtration and were then classified according to length. The DNA replication cycle was determined by a quantitative analysis of radioautograms of tritiated thymidine pulse labeled cells. The DNA replication period was found to be 45 min. This period is preceded and followed by periods without DNA synthesis of about 10 min. The morphology and segregation of nucleoplasmic bodies was studied in thin sections. B. subtilis contains two sets of genomes. DNA replication and DNA segregation seem to go hand in hand and DNA segregation is completed shortly after termination of DNA replication. Cell division and cell separation were investigated in whole mount preparations (agar filtration) and in thin sections. Cell division starts about 20 min after cell birth; cell separation starts at about 45 min and before completion of the septum.

A modified wire-loop method for quantitative electron microscopic radioautography

A simplified method of emulsion coating and development for quantitative electron microscopic radioautography without any special instrument in introduced. Seven grids carrying ultrathin sections are attached to a glass block with double-coated Scotch tape. They are coated with a wire-loop, 2.5 cm in diameter. Ten glass blocks are then stuck on a slide, exposed, developed, fixed and stained simultaneously. Employing this method, the emulsion coating and the following procedures are easily accomplished under an identical condition.

Synthesis and migration of glycoproteins in cells of the rat thymus, as shown by radioautography after 3H-fucose injection

Young male rats received a single intravenous injection of 3H-fucose and were killed after various time-intervals. Light- and electron-microscopic radioautographic studies of the thymus in animals killed shortly after injection showed that all of the different cell types present incorporated 3H-fucose label. The heaviest uptake occurred in macrophages and in hypertrophic epithelial cells located near the cortico-medullary border. Somewhat lighter incorporation was observed in medullary and cortical stellate epithelial cells and in cells designated as special cells, while the lightest reaction appeared over lymphocytes. In all cells the label was localized initially to the Golgi apparatus, where, presumably, it was incorporated into glycoproteins. With time, some of the labeled putative glycoproteins in all cell types migrated to the plasma membrane. In macrophages, much of the label migrated to lysosomal bodies, while in the special cells the label migrated to dense bodies which may also be of lysosomal nature. In stellate and hypertrophic epithelial cells much of the label migrated to characteristic vacuoles. The possible relationship between the observed glycoprotein synthesis in these cells and hormone production is discussed.

Size variations and correlation of different cell cycle events in slow-growing Escherichia coli

Cell lengths have been determined at which cycle events occur in the slow-growing Escherichia coli B/r substrains A, K, and F26. The radioautographic and electron microscope analyses allowed determination of the variations in length at birth, initiation and termination of DNA replication, and initiation of the constriction process and of cell separation. In all three substrains the standard deviation increased between cell birth and initiation of DNA replication. From there on, the standard deviation remained relatively constant until cell separation. These observations are consistent with the presence of a deterministic phase during the cell cycle in which the cell sizes at initation of DNA replication and at cell division are correlated.

DNA replication pattern and cell wall growth in Escherichia coli PAT 84

An electron microscopic radioautographic study was made of tritiated thymidine incorporation into the genome of Escherichia coli PAT 84 and of tritiated meso-D,L-2,6-diaminopimelic acid (DAP) into the cell envelope. Pulse-labeled cells growing at 30 degrees C with a doubling time of 170 min were classified according to length by the method of agar filtration. Mathematical analysis of the length distribution led to the assumption of an exponential relation between length and time. A novel DNA replication pattern was found. Within the cell cycle DNA replication terminates at 70 min; then a gap follows of 64 min, after which DNA replication is initiated at 134 min. Thus, the C period is 106 min and the D period is 100 min. Cell constriction starts at 141 min and coincides with initiation of DNA replication. Detailed quantitative analysis of the [3H]thymidine grain frequency distribution allowed the distinction of three groups of cells. The first group incorporated no label, the second group an amount C, and the third group an amount 2 X C. The relative contribution of each group to a particular length class was determined. The data fitted very well into the DNA replication pattern. The same analysis was carried out on DAP pulse-labeled cells. Again, three groups of cells could be distinguished, and their relative contributions to each length class was determined. The group with the double amount of label was especially prominent at the end of the cell cycle. The emergence of this group might represent the acquisition of new lateral growth areas.

Radioautographic and biochemical studies of secretion of venom protein in the South American rattlesnake Crotalus durissus terrificus

Protein secretion was investigated in the main venom gland of the South American rattlesnake, using radioautographic and biochemical techniques after a single intracardiac injection of L-(3,5-3H)tyrosine. All the snakes were injected at the fourth day of the secretory cycle and killed at 1/2, 1, 2, 4, 8 and 24 hours after injection. Most of the radioactive amino acid is cleared from the blood stream up to four hours after injection. On the other hand the specific activity (c.p.m./mg of protein) of the intracellular proteins reaches a peak at the 4-hour time interval decreasing afterwards. There was a good correlation between the values of the specific activity of the intracellular proteins and those of the silver grain density over the secretory cells at the several time intervals after the injection of 3H-tyrosine. The results of the quantitative analysis carried out in light- and electron-microscope radioautographs led to the conclusion that venom proteins are synthesized in the rough endoplasmic reticulum of the secretory cells, transferred to the Golgi apparatus from where they are carried to the secretory tobule lumen by the secretion granules. The fact that the values of the relative concentration of the radioactivity of he intracisternal granules double at the last three time intervals, strongly suggests that these structures are formed by the aggregation of the amorphous material present inside the cisternae of the rough endoplasmic reticulum.

Influence of emulsion type and atmospheric oxygen on the fading of latent image in electron microscopic radioautography

The intensity of radioautographic reactions in Ilford L4, Sakura NR-H2 and Kodak NTE emulsions was compared after exposure in either dry air or dry helium gas at 4 degrees C to test the stability of latent images in the presence or absence of oxygen. A light proof container is described in which slides bearing radioactive sections coated with the three emulsions were exposed in dry helium at a constant pressure of approximately 0.5 atm. The comparison of air and helium atmospheres during exposure of radioautographs was estimated qualitatively for 125I-labeled thyroid sections stored for several years and, in addition, quantitative data was derived from 3H-labeled methacrylate sections stored from 21 days to 1 year. With the three emulsions under study, the background fog remains low under both exposure conditions at 4 degrees C for as long as several years duration. Using L4 emulsion, similar high grain densities are obtained in air and helium, and therefore, the latent images in L4 emulsion remain stable in the presence of oxygen. In the case of NTE and NR-H2 emulsions, as the exposure time increases, substantially lower reaction intensities are observed in air than in helium. This difference in reaction intensity is evident by 3 weeks with NTE and after 4 weeks with NR-H2. Hence, there is fading of the latent images in the latter emulsions in the presence of oxygen. It is concluded that reliable results may be obtained with the L4 emulsion by exposure of radioautographs in dry air, whereas with the NR-H2 and NTE emulsions, exposure should be in an oxygen-free medium, such as is provided by a dry helium atmosphere.

Selective induction of tyrosine hydroxylase and dopamine beta-hydroxylase by nerve growth factor: comparison between adrenal medulla and sympathetic ganglia of adult and newborn rats

Administration of NGF to newborn and adult rats elicits a selective increase in TH and DBH both in sympathetic ganglia and adrenal medulla. This effect does not depend on intact preganglionic cholinergic fibers. The augmented enzyme activity results from enhanced enzyme synthesis since it can be abolished by cycloheximide and NGF has been shown to enhance the incorporation of [3H]leucine into DBH molecules. The responsiveness of the adrenal medulla to NGF is also supported by light and electron microscopic autoradiograms which show that intravenously injected 125I-NGF is accumulated with high selectivity in adrenal chromaffin as compared to adjacent adrenal cortical cells. In spite of the many similarities between the response of the adrenergic neurons and adrenal chromaffin cells to NGF, there are also two distinct differences. (a) In newborn rats the ratio between the TH increase effected by a single and 10 subsequent daily injections of NGF is 1:2 in the adrenal medulla and 1:7 in the superior cervical ganglia. (b) If adrenal medullae are transferred to organ culture after intravenous injection of NGF, maximal TH response is initiated 60-90 min after NGF administration. In superior cervical ganglia only a half-maximal response is initiated at that time. After a stationary phase a second increase starts after about 6 h to reach the maximum after 12 h. The biphasic time course of the initiation of TH induction by NGF in sympathetic ganglia is in agreement with the time course of 125I-NGF accumulation after intravenous injection27 reflecting the moiety of NGF reaching the cell bodies of the adrenergic neurons directly by the blood stream (initial accumulation) and by retrograde axonal transport (second phase).

Cell proliferation in the Lichen planus infiltrate. Investigations by light- and electron-microscopic autoradiography

In seven patients suffering from lichen planus (LP) light- and electron-microscopic 3H-thymidine (3HT) labeling studies were performed and the following results were obtained. 1. Using the light-microscope, a large number of 3HT labeled cells can be observed in the dermal infiltrate of LP. Most of the labeled nuclei belong to mononuclear, 'round cells' while isolated endothelial cells, fibroblasts, histiocytes and mast cells could be found labeled. 2. The mean labeling index of 'round cells' in our patients was 4.27 +/- 1.57%. 3. The evaluation of the electron microscopic autoradiographs showed that, as far as 'round' cells were concerned, exclusively nuclei of cells showing the morphological criteria of stimulated lymphocytes were labeled. These findings support the view that in LP, as in allergic contact dermatitis, a high proliferative activity of lymphocytes takes place.

Selective trans-synaptic migration of tetanus toxin after retrograde axonal transport in peripheral sympathetic nerves: a comparison with nerve growth factor

Adult rats were injected unilaterally into the anterior eye chamber and the submandibular gland with either [125I]tetanus toxin or [125I]nerve growth factor (NGF). Fourteen and 24 h later in electron microscopic autoradiographs of the superior cervical ganglia of the injected side the labeling was confined to a limited number (15-20%) of adrenergic ganglion cells and the silver grains were localized over axons, perikarya and dendrites providing evidence for a retrograde intraaxonal transport of the two macromolecules. Moreover, after injection of [125]tetanus toxin there was a very marked labeling of the presynaptic cholinergic nerve terminals. In contrast, after [125I]NGF these terminals were free of label. In both cases no specific labeling could be detected over glia and extracellular space. In the postganglionic axons the radioactivity seemed to be mainly associated with vesicles and smooth endoplasmic reticulum, in the perikarya and dendrites of the adrenergic neurons with secondary lysosomes, vesicles and smooth endoplasmic reticulum. The Golgi cisternae and the nuclei were free of radioactivity. The specific labeling of presynaptic terminals after injection of [125I]tetanus toxin together wirans-symaptic migration of [125I]tetanus toxin from the adrenergic ganglion cell to its innervating presynaptic terminals following retrograde intraaxonal transport.

Formation and turnover of plasma membrane glycoproteins in kidney tubules of young rats and adult mice, as shown by radioautography after an injection of 3H-fucose

The formation and turnover of the glycoproteins of the plasma membrane have been investigated by quantitative radioautography in the kidney tubules of young rats and adult mice killed at various time intervals after an intravenous injection of 3H-fucose. In young (40 g) rats killed five to ten minutes after the injection, radioautographs of distal tubule cells show that the Golgi apparatus contained about 85% of the cell label. By 30 hours, only 8% of the label remained in this organnele, whereas 67% was in the plasma membrane, indicating that most of the label had migrated from Golgi apparatus to this membrane. Similarly, in proximal tubule cells, about 82% of the label was initially in the Golgi apparatus, but less than 2% remained at 30 hours, at which time 78% was in the plasma membrane. In the latter cells, the apical tubules and vacuoles became heavily labeled before the apical microvilli did and, therefore, may be involved in the transit of label from the Golgi apparatus to the microvillous membrane. The results are interpreted to mean that, in kidney tubule cells, the Golgi apparatus is the site of a continuous incorporation of fucose into glycoproteins and that these migrate to the plasma membrane. In fully formed cells, such a conclusion would imply a continuous turnover of plasma membrane glycoproteins. However, in the rapidly growing kidney of young rats many new cells are added daily, the growth of which might involve net addition as well as turnover of glycoproteins. Accordingly, the experiment has been repeated in adult mice, in which the cells are assumed to be fully formed. Furthermore, since turnover implies eventual decrease of incorporated label, some of the animals have been killed at longer intervals, up to 27 days after injection. In these adult mice, as in young rats, prompt Golgi uptake and subsequent migration of label to the plasma membrane were observed in distal and proximal tubules cells. With time the label content of the plasma membrane decreased gradually, and by 27 days had virtually disappeared. From grain counts, it is concluded that the mean half-life of glycoproteins in the apical membrane of distal tubule cells is about two days, whereas in both the apical and basal membranes of proximal tubule cells, it is slightly over three days.

Potential of Schwann cells from unmyelinated nerves to produce myelin: a quantitative ultrastructural and radiographic study

In adult mice, most fibres in the cervical sympathetic trunk (CST) are unmyelinated whereas a large proportion of sural nerve fibres are myelinated. This study of nerve grafts in syngeneic mice was designed to determine if Schwann cells originating from the unmyelinated CST would produce myelin when in contact with regenerating axons of the sural nerve. Quantitative microscopy of triated thymidine-labelled CST segments grafted to unlabelled sural nerve stumps revealed that, one month after grafting, previously unmyelinated grafts contained many myelinated fibres. By phase and electron microscope radioautography, nearly 40% of the myelin-producing cells in the reinnervated graft were shown to have originated in the unmyelinated CST. These findings indicate that Schwann cells originating from unmyelinated fibres are able to differentiate into myelin producing cells.

The biosynthesis and secretion of precursor enamel protein by ameloblasts as visualized by autoradiography after tryptophan administration

The incorporation of 3H-tryptophan into the inner enamel epithelium of newborn mouse incisor tooth organs has been studied in situ by light and electron microscopic autoradiography to determine the sites and kinetics of biosynthesis, migration, and secretion of precursor enamel protein during newborn mouse incisor tooth formation maxillary and mandibular incisor tooth amelogenesis was studied 5, 30, 60, 120, 240 minutes and 24 hours following the intraperitoneal injection of 3H-tryptophan. By 5 minutes, 40% of the total silver grains associated with the secretory ameloblasts were localized over the rough endoplasmic reticulum and 50% of the silver grains were localized over the Golgi apparatus. By 30 minutes, silver grains were observed predominately over condensing vacuoles and secretory granules within the forming Tomes' processes, and were also localized over the extracellular "granular" pre-enamel matrix. The enamel proteins were synthesized on membrane-bound polysomes, transferred within the cisternae of the rough endoplasmic reticulum and then accumulated in the inner saccules of the Golgi apparatus. The enamel proteins were than packaged in condensing vacuoles which subsequently became secretory granules which migrated to the lateral and apical secretory regions of the forming Tomes' processes. It was concluded from these in vivo studies that enamel protein were synthesized and subsequently secreted within 30 minutes. The initially secreted precursor enamel protein was localized over a material which demonstrated a granular or stippled ultrastructure. The labeled protein then was localized over the amorphous enamel matrix per se which contained the forming calcium hydroxyapatite crystals. We assumed, therefore, that there are two different ultrastructural forms of 3H-tryptophan containing extracellular enamel proteins and suggest that the granular or "stippled" form represents newly secreted precursor enamel protein.

An autoradiographic identification of Purkinje axon terminals in the cat

The Purkinje axon boutons terminating in nuclei fastigii and interpositus in the cat have been identified after injection of 3H leucine into the cerebellar cortex overlying the nuclei. The animals survived from 4-48 h after injection of the isotope. Semithin and ultrathin sections were coated and exposed for 3 and 14 weeks, respectively. The electron micrographs showed labelling over myelinated axons down to a diameter of 0.8 mum, and over boutons. Fifty labelled boutons were used for identification of the shape of their synaptic vesicles. The statistical analysis including the test for skewness showed that 39 boutons (78%) fall in one group. Most of the synaptic vesicles in this group are elliptical (ration from 1:1.1 - 1:1.7). Slightly ovoid vesicles (ration up to 1:1.3) are frequent, but flattened vesicles (ration above 1:1.7) are relatively few in this group of boutons. 8% of the boutons have a rather homogeneous vesicle population (prevalence of round vesicles, ratio 1:1). Synaptic specializations of Gray's type II or of an intermediate type were found in the boutons belonging to the first group (78% of the boutons). Specializations of Gray's type I were found in the other bouton groups (8% of the boutons).

Electron microscopic evidence for a transsynaptic migration of tetanus toxin in spinal cord motoneurons: an autoradiographic and morphometric study

Adult rats were injected intramuscularly (musculus deltoideus) with 125I-labeled tetanus toxin. First signs of tetanic rigidity became visible 12-13 h after injection: death occurred 1-2 h later. Electron microscopic autoradiography of the ventrolateral spinal cord gray matter showed a large part of the radioactivity to be located in motoneurons (perikaryon, axon and dendrites) at 7 and 14 h after injection. A great majority of the rest of the label was found over synaptic terminals, most of them were afferent to motoneurons. The percentage of labeled nerve terminals increased significantly (P less than 0.05) between 7 h and 14 h after injection. Glial elements surrounding the motoneurons were mostly unlabeled. These observations strongly favor the assumption of a transsynaptic migration of tetanus toxin. Dorsal root, dorsal horn and cell bodies of spinal cord interneurons were free of label.

Serotonin axons in the supra- and subependymal plexuses and in the leptomeninges; their roles in local alterations of cerebrospinal fluid and vasomotor activity

Extensive plexuses of serotonin axons form a supra- and subependymal system in the walls of the ventricles, in the arachnoid sheath around major cerebral blood vessels, and in the pia over the spinal cord. These have been demonstrated by autoradiography after continuous intraventricular perfusions of exogenous [3H]5-HT in rats and monkeys. The axons accumulate 5,6-DHT rendering them electron opaque, but have no uptake systems for [3H]NE. After treatment with MAO inhibitors and [3H]5-HT, the axonal boutons contain large (70nm) variably dense synaptic vesicles, and small (35 nm) vesicles each equipped with a dense dot. The latter vesicles are not seen in untreated controls. Electrical stimulation in the raphe nuclei causes significant increases in axonal [3H]5-HT uptake indicating that the fibers originate in the raphe. Quantitatively, the supraependymal plexus is variable, profuse over the dorsal and ventral aqueductal surfaces, sparse over the lateral aspects. Individual raphe neurons have their specific uptake affinities for [3H]5-HT that are independent of tracer concentration or diffusion gradient. It is suggested that raphe neurons with low 5-HT uptake may utilize other neurotransmitters. Two new functional roles are proposed: (1) the serotonin ventricular and pial axons are probably important modifiers of local cerebrospinal fluid (CSF) composition so that regional CSF variations in 5-HT and its metabolites are highly probable; (2) the subarachnoid plexus around major cerebral vessels may contribute to local vasomotor action, thus affecting the cerebral blood flow. The possible significance of these serotonin systems for an understanding of certain neurological entities such as migraine and hemodynamic regulation in cerebral vascular disease is indicated.

A comparison of various procedures for fine grain development in electron microscopic radioautography

Fine grain development for electron microscopic radioautography was investigated with two types of radioactive specimens: sections of tritiated methacrylate, which provide a homogeneously labeled source for quantitative evaluation of the radioautographic reaction, and sections of 125I-labeled thyroid. Radioautographs were prepared with Ilford L4, Sakura NR-H2, Agfa-Gevaert NUC 307 or Kodak NTE emulsions. The radioautographs were developed with one of several "solution physical" development procedures (Agfa-Gevaert, phenidone-ascorbic acid, p-phenylenediamine developers) or with arrested "direct" developments (D-19b, Elon-ascorbic acid developers). By arresting each development at an early stage of the reaction and at progressively longer time intervals, it was possible to examine the sequence of shapes in the growth of developed silver deposits for each emulsion-development combination. Thus, conditions which resulted in the development of small, round, compact silver deposits were defined for each emulsion. These developments were used in conjuction with gold latensification, a treatment which increases the sensitivity of the emulsions and thus compensates for the lowered sensitivity of fine grain development procedures. The location of the silver deposits in relation to the silver bromide crystals from which they derive was investigated. The emulsion gelatin surrounding the crystals was stained whereas the spaces, which remained after the crystals were dissolved in the photographic fixer, appeared transparent. This analysis permitted the selection of development procedures in which the single or multiple round silver deposits originating from a single crystal will remain within or on the boundary of this crystal. By this method, quantitation of radioautographic reactions composed of small, round silver deposits was studied by using the uniformly labeled 3H-methacrylate sections as a standard source of radiation. The conditions under which grain counting is feasible are discussed.

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.

Synthesis, migration, and release of precursor collagen by odontoblasts as visualized by radioautography after (3H)proline administration

The elaboration of dentin collagen precursors by the odontoblasts in the incisor teeth of 30-40-g rats was investigated by electron microscopy, histochemistry, and radioautography after intravenous injection of tritium-labeled proline. At 2 min after injection, when the labeling of blood proline was high, radioactivity was restricted to the rough endoplasmic reticulum, indicating that it is the site of synthesis of the polypeptide precursors of collagen, the pro-alpha chains. At 10 min, when the labeling of blood proline had already declined, radioactivity was observed in spherical portions of Golgi saccules containing entangled threads, and, at 20 min, radioactivity appeared in cylindrical portions containing aggregates of parallel threads. The parallel threads measured 280-350 nm in length and stained with the low pH-phosphotungstic acid technique for carbohydrate and with the silver methenamine technique for aldehydes (as did extracellular collagen fibrils). The passage of label from spherical to cylindrical Golgi portions is associated with the reorganization of entangled into parallel threads, which is interpreted as the packing of procollagen molecules. Between 20 and 30 min, prosecretory and secretory granules respectively became labeled. These results indicate that the cylindrical portions of Golgi saccules transform into prosecretory and subsequently into secretory granules. Within these granules, the parallel threads, believed to be procollagen molecules, are transported to the odontoblast process. At 90 min and 4 h after injection, label was present in predentin, indicating that the labeled content of secretory granules had been released into predentin. This occurred by exocytosis as evidenced by the presence of secretory granules in fusion with the plasmalemma of the odontoblast process. It is proposed that pro-alpha chains give rise to procollagen molecules which assemble into parallel aggregates in the Golgi apparatus. Procollagen molecules are then transported within secretory granules to the odontoblast process and released by exocytosis. In predentin procollagen molecules would give rise to tropocollagen molecules, which would then polymerize into collagen fibrils.