Specific binding sites for transferrin on ameloblasts of the enamel maturation zone in the rat incisor

During enamel maturation in rodents, an iron-containing pigment is deposited into the surface layer of the enamel. Maturation zone ameloblasts presumably are responsible for this deposition. The presence of large amounts of ferritin in the cytoplasm of these cells suggests that they receive iron, presumably from circulating transferrin. An in vivo radioautographic binding assay using iodinated transferrin was used to determine if indeed maturation ameloblasts possess transferrin receptors at their cell surfaces. Experimental rats received systemic injections of labeled transferrin while control rats received injections of labeled transferrin plus a large excess of unlabeled transferrin in order to compete with the labeled transferrin for available specific receptors. Light microscope radioautography showed that ruffle-ended ameloblasts (RAs) of the enamel maturation zone had a high density of specific receptors for transferrin relative to smooth-ended ameloblasts (SAs). Electron microscopy and energy-dispersive X-ray spectroscopy confirmed the presence of ferritin and iron, respectively, within these cells. It is postulated that the iron responsible for enamel pigmentation is transported by transferrin to maturation ameloblasts and is bound to specific transferrin receptors found mostly on RAs and that the modulation of these cells into SAs results in a loss of most of these receptors.

A radioautographic study of the effects of vinblastine on the fate of injected 45calcium and [125I]-insulin in the rat incisor

To examine the effect of vinblastine on the movement of calcium and macromolecules through the enamel organ in the secretion zone and through the odontoblast layer, 45CaCl2 and [125I]-insulin were used as radioautographic tracers. Vinblastine did not alter the localization of either labelled Ca or insulin in the enamel organ and underlying enamel, but eliminated both labels in the pulp, odontoblasts and dentine. It is concluded that vinblastine has no effect on the passage of Ca and macromolecules in the enamel organ and secretory ameloblast layers, whereas its effect on the pulp and odontoblasts prevents passage of these tracers into the predentine and dentine.

Response of the rat incisor dental tissues to penetration of the labial alveolar bone in preparation of a surgical window

The orderly formation of rod and interrod enamel containing precisely-oriented hydroxyapatite crystallites requires a high degree of cellular cooperation. This work examines the susceptibility and response of the rat incisor enamel organ and enamel to mechanically-induced trauma. Such trauma is induced by drilling through the labial alveolar bone of the rat incisor in preparation of a surgical window for a micro-injection technique (McKee and Warshawsky, 1984). The drilling ruptures the enamel organ and removes the underlying enamel. Various experiments indicate that the enamel lesion is a result of aspiration by the rotating bur. The histology of the enamel lesion is described. Several morphological changes which may indicate attempted repair are noted as the lesion moves incisally with time due to the eruption of the tooth. It is concluded that the rat incisor enamel organ is an extremely sensitive cellular system, that if sufficiently disturbed is not capable of restablishing the cellular relations necessary to produce a new enamel layer.

In vivo demonstration by radioautography of binding sites for insulin in liver, kidney, and calcified tissues of the rat

An in vivo binding assay using radioautography was employed to visualize insulin receptors in rat tissues. Two and one-half minutes after the intravenous injection of 125I-insulin, free hormone was separated from bound hormone by whole body perfusion with lactated Ringer's solution followed by perfusion with glutaraldehyde. The localization of bound hormone, fixed in situ by perfusion with glutaraldehyde, was determined. Nonspecific binding of labeled insulin was noted in the proximal convoluted tubules of the kidney cortex, prebone and adjacent bone, predentin and adjacent dentin, and enamel. Specific binding sites were observed at the periphery of hepatocytes, over osteoblasts, and in relation to the endothelial cells of fenestrated capillaries within the papillary layer of the maturation zone of the incisors.

Modification of the enamel maturation pattern by vinblastine as revealed by glyoxal bis(2-hydroxyanil) staining and 45calcium radioautography

Patterns characteristic of enamel maturation can be visualized at the surface of the rat incisor by staining with glyoxal bis(2-hydroxyanil) (GBHA) and radioautography following 45calcium injection. In this study, the effects of vinblastine on enamel maturation were monitored by these two methods. At 4 h after injection of vinblastine, the darkly-stained GBHA bands had widened incisally into the interband regions when compared to normal, control teeth. Radioautography at 5 min after calcium injection in vinblastine-treated animals (4 h) showed a modified maturation pattern of weaker labeling and less distinct banding. At 8 h after vinblastine injection, most of the enamel stained uniformly with GBHA, and bands and interband regions could not be resolved. Radioautography at 5 min after calcium injection showed that the 8 h vinblastine treatment removed the banding pattern, leaving only a weakly-labeled area. Vinblastine is known to destroy and prevent the formation and turnover of microtubules, and hence the formation of ruffled borders of ruffle-ended ameloblasts (Akita et al. 1983). The concomitant decrease in calcium incorporation implies that events taking place in relation to the ruffled border may affect calcium exchange or accretion within the enamel.

Penetration of various molecular-weight proteins into the enamel organ and enamel of the rat incisor

During enamel maturation, most of the organic matrix is removed as the mineral content increases; it is postulated that proteolytic enzymes within enamel break down large proteins into more mobile fragments. To predict how such fragments might leave the enamel, the entry and penetration of various proteins into it was examined. Rats (100 g) were injected via the external jugular vein with 125I-iodinated calcitonin (3600), insulin (5700), epidermal growth factor (EGF; 6100) and albumin (68,000). They were killed after 10 min and radioautographs made to visualize these molecules in the incisor enamel organ and enamel. In addition, dissected incisors were wiped free of their enamel organs, dipped in the iodinated protein solutions for 10 min, and processed for radioautography. In all dipped teeth, except those exposed to albumin, there was a gradient of silver-grain density over the entire thickness of enamel in both the secretion and maturation zones. In all injected animals, enamel labelling in the secretion zone was only slightly above background. In the maturation zone of animals injected with calcitonin and insulin, many grains were over enamel adjacent to smooth-ended ameloblasts but not ruffle-ended ones. Animals injected with EGF and albumin had no labelled enamel in the maturation zone. Thus dipped rat incisor enamel was permeable to proteins with molecular weights as high as 6100. Localization of injected proteins indicates that the enamel organ restricts their passage into enamel, but proteins with molecular weights as high as 5700 may pass into enamel through or between smooth-ended ameloblasts. As exogenous proteins readily diffused into the enamel, it seems likely that enamel proteins of similar size can leave enamel by a similar route.

Effects of various agents on staining of the maturation pattern at the surface of rat incisor enamel

In the maturation zone, two types of ameloblasts are arranged as bands across the rat incisor; these corresponded with a staining pattern at the surface which reflected the alternating pattern of ruffle-ended and smooth-ended ameloblasts. PAS stain showed bands and stripes similar to those following glyoxal bis(2-hydroxyanil) (GBHA) staining; these stains visualize the organic (PAS) and inorganic (GBHA) components of the maturation pattern. To further elucidate the nature of these bands, dissected rat incisors were treated with various agents prior to staining with GBHA or PAS. Guanidine for 2 h at room temperature showed no maturation pattern when stained with GBHA, as did teeth treated with EDTA and a bisophosphonate (HEBP). Hydrochloric acid and nitric acid removed the layer of outer enamel and incisors did not stain with GBHA, suggesting that the staining is a surface-related phenomenon. As staining was abolished by either the removal of mineral (EDTA) or protein (guanidine), the concurrent localization of non-crystal-bound calcium by GBHA staining, and of glycoprotein by PAS staining, indicates that calcium is associated with glycoprotein at the surface of the enamel.

Binding of intact parathyroid hormone to chicken renal plasma membranes: evidence for a second binding site with carboxyl-terminal specificity

We studied the binding of bovine PTH (bPTH) to chicken renal plasma membranes using an intact hormone radioligand, [125I]bPTH-(1-84). In contrast to previous studies using amino-terminal radioligands, our experiments revealed the presence of two distinct binding sites for intact bPTH. Scatchard analysis of competition curves consistently gave a biphasic curve, and computerized nonlinear regression analysis indicated a high affinity [dissociation constant (Kd) = 1.21 nM] and a low affinity (Kd = 333 nM) site (P less than 0.001). Analysis of binding of [125I] bPTH-(1-34) using identical techniques revealed only one site, similar to the high affinity sites seen with intact hormone tracer. The low affinity site for [125I]bPTH-(1-84) had carboxyl-terminal specificity since analysis of competition curves with unlabeled human PTH-(53-84) gave virtually identical binding parameters to the low affinity site obtained by competition with unlabeled native hormone. Binding to the low affinity site was inhibited in the presence of 10 mM Mg2+ and was reduced after storage of membranes for over 1 month at -70 C. Association of [125I]bPTH-(1-84) to the low affinity site took distinctly longer (approximately 4 h) than to the high affinity site (essentially complete by 2.25 h). Our data suggest that the high affinity site is coupled to adenylate cyclase in these membranes, while the low affinity site is not. The physiological significance of the low affinity carboxyl-terminal PTH binding is not known, and further studies are indicated.

In vivo experimentation on rat incisor enamel organs through a surgical window

Experimental agents administered systemically are costly and often toxic to animals. An in vivo technique has been developed whereby a surgical window in the alveolar bone allows selected areas of the rat incisor enamel organ and underlying enamel to be exposed to various drugs, radiolabeled molecules, and molecular weight markers. Sherman rats weighing 100 gm were anesthetized and the inferior surface of each hemimandible was surgically exposed. A slow-speed dental hand drill was used to drill a small hole through the alveolar bone overlying the secretion or maturation zones of the enamel organ. The wound was closed and during recovery the mechanical trauma to the underlying tissue moved away from the hole due to the continuous eruption of the tooth. Two to 5 days later the hole was reexposed and microinjections of 3H-proline, 125I-salmon calcitonin, vinblastine sulphate, and normal saline (as control) were administered through the hole with a micromanipulator and a microliter syringe. Radioautographic detection of 3H-proline incorporation in secretory ameloblasts and enamel at 10 minutes, 30 minutes, 1 hour, 4 hours, 1 day, and 2 days after microinjection was identical to that obtained previously by systemic injection. Two hours after microinjection of vinblastine sulphate the cellular response was again identical to that following systemic injection; 125I-salmon calcitonin (M.W. approximately 3,600D) was used as a molecular weight marker and was seen to diffuse into the enamel of the maturation zone at 10 minutes after microinjection. This study has demonstrated the feasibility of this new technique for experimentation on rat incisor enamel organs.

Quantitative analysis of rough endoplasmic reticulum approaches to the cell membrane in the secretory ameloblast of the rat incisor

The distribution of approaches of rough endoplasmic reticulum to the cell membrane within the supranuclear region of the secretory ameloblast of the rat incisor was quantitated using a Zeiss MOP-3. In ameloblasts cut in cross section, most of these approaches appear as circular profiles representing cross sections of elongated cisternae, which are aligned parallel to the long axis of the cell. Because of their position, orientation, and distribution of ribosomes, these approaches were consistent with the appearance of subsurface cisternae. Using cross-sectioned ameloblasts, the lengths of apposed plasma membranes either between or within rows of cells were measured from electron micrographs. Along these lengths, matched approaches of rough endoplasmic reticulum from opposite sides of the apposed plasma membranes were counted. Approaches from either side that were unmatched were also counted. Thirteen percent of the approaches were matched between rows of ameloblasts, and 13.5% of the approaches were matched within rows, demonstrating no significant difference between the two sites. Furthermore, mathematical analysis showed that the theoretical probability of two approaches coinciding is 17%. The experimental values are not statistically different from the theoretical probability, and it is concluded that the matching of rough endoplasmic reticulum approaches to the plasma membrane, or subsurface cisternae, occurs at random.