Influence of fluoride on secretory pathway of the secretory ameloblast in rat incisor tooth germs exposed to sodium fluoride

Fluoride, which is an environmental toxicant, is a potent inducer of mottled enamel in humans and rats. To define the influence of fluoride on the secretory pathway in enamel fluorosis, mottled enamel was induced in the incisor tooth germs of rats by subcutaneous injections of sodium fluoride for 4 days, and then morphological and cytochemical changes of the secretory ameloblast were examined in the tooth germs with HRP-labeled lectin (Con A, GS-I, SBA and PNA) and En3 antibody labeling amelogenins. The accumulation of small vesicles on the route of the secretory pathway between the rER and the Golgi apparatus, disorder of Golgi stacks, and formation of abnormal large granules in distal cytoplasm were seen in the secretory ameloblast. Lectin staining patterns of the secretory ameloblast indicated the disturbance of the vesicular transport between the rER and the Golgi apparatus, and disorganization of the Golgi stack. Immunolabeling of the cell showed disruption of the sorting and fusion process on the secretory pathway. These results suggest that the fluoride disturbs the intracellular transport in the synthesis-secretory pathway of the ameloblast, and that this effect of fluoride on the synthesis-secretory pathway participates in the formation of enamel fluorosis.

Immunolocalization of vitamin D receptor and calbindin-D28k in human tooth germ

The role of vitamin D in ameloblasts and odontoblasts has been studied experimentally in rodents. Dental dysplasias have also been reported in clinical studies of children with rickets. Vitamin D acts via a nuclear receptor which binds the major metabolite, 1,25-dihydroxyvitamin D3, and positively or negatively controls the expression of specific genes. The most extensively studied markers of 1,25-dihydroxyvitamin D3 action are calbindin-D9k, calbindin-D28k, and osteocalcin. Therefore, to study in more detail the potential role of 1,25-dihydroxyvitamin D3 in human dental development, 1,25-dihydroxyvitamin D3 receptor (VDR) was localized by immunofluorescence in forming teeth (8-26 wk of gestation). Calbindin-D28k was also mapped by immunoperoxidase in antenatal and postnatal forming and formed teeth. VDR were detected in both dental epithelium and mesenchyme of bud, cap, and bell stages of tooth germs. Nuclei of overtly differentiated ameloblasts and odontoblasts were also immunostained. Calbindin-D28k was present in differentiated ameloblasts and odontoblasts. The presence of VDR and calbindin-D28k in ameloblasts and odontoblasts suggests that 1,25-dihydroxyvitamin D3 may contribute to the regulation of enamel and dentin formation, as classically reported for bone formation. Finally, the early appearance of VDR supports the concept that 1,25-dihydroxyvitamin D3 may also control forward stages of tooth crown development in humans.

Enamel fluorosis in rat's incisor: S.E.M. and T.E.M. investigation

Findings on the alterations taking place in the enamel have demonstrated that they are generally caused by a daily use of highly fluoridated drinking waters. According to that, the Authors have carried out an ultrastructural study on lower incisors of albino rats after administering for 60 days water with a fluorine concentration five times the normal one. The samples, studied under the S.E.M., showed a general slowing of both the deposition and the maturation phase as well as the presence of some hypomineralized areas even after eruption. All this suggested the possibility that the damages observed were not due to the direct effect of fluorine on the enamel, but to the interaction between fluorine and ameloblasts. The Authors have then carried out an ultrastructural study on the enamel organ using the S.E.M. The results showed the presence of a well-evident endoplasmic reticulum, the lack in dense granules during the secretion phase, the lack in ruffle ended webs during the modulation phase, and the mitochondrial damage in the ameloblast. All this could justify the slowing of the enamel mineralization caused by fluorine effect on the ameloblasts.

DNA localization in nuclear fragments of apoptotic ameloblasts using anti-DNA immunoelectron microscopy: programmed cell death of ameloblasts

Ameloblasts responsible for tooth enamel formation are classified into two different phases: secretion and maturation. At the transition between these secretion and maturation stages, a considerable number of cells die. In this study, we examined the morphology of degenerating ameloblasts by conventional electron microscopy, and DNA cleavage in degenerating ameloblast nuclei by the in situ terminal transferase assay. The results suggest that apoptosis (programmed cell death) in ameloblasts, including DNA ligation is induced at the transitional stage. The nuclear fragments, chromatin condensation and DNA relocation in apoptotic nuclei were examined quantitatively by post-embedding anti-DNA immunogold electron microscopy and the in situ terminal transferase assay combined with electron microscopy. Numerical analysis revealed that immunogold labeling density in the condensed chromatin of apoptotic nuclei was comparable on the average to that in the perinuclear heterochromatin of normal nuclei, and that individual apoptotic nuclear fragments exhibited highly variable to that of normal heterochromatin, to fragments with densities twice as high as that of normal heterochromatin. The in situ terminal transferase assay combined with electron microscopy detected DNA ends exposed by ultrathin sectioning as well as DNA cleavage by a putative endonuclease. In conclusion, the state of the DNA, including its ligation and degeneration, changes gradually during chromatin condensation and nuclear fragmentation of apoptosis.

Immunohistochemical localization of connexin 43 in the developing tooth germ of rat

Distribution of gap junction protein in maxillary tooth germs of 1-day-old rats was examined by immunohistochemistry, using an affinity-purified antibody specific to residues 360-376 of rat connexin (CX) 43. In 1-day-old rats, the maxillary second molar formed the shape of the cusp, but neither dentine nor enamel was formed between the cells of the dental papilla and the inner enamel epithelium. In the tooth germ, CX 43 was expressed in the cells of the stratum intermedium and the inner enamel epithelium. Labelling in the stratum intermedium was extensive and showed an increasing gradient from peripheral to cuspal regions. CX 43 detected in the inner enamel epithelium was at cell surfaces facing the interface between the dental papilla and the inner enamel epithelium. The cells of the dental papilla and the inner enamel epithelium began differentiation as odontoblasts and secretory ameloblasts respectively, in the cusps of the first molars, where predentine and dentine were formed but enamel matrix was not secreted. CX 43 was present in the stratum intermedium, inner enamel epithelium, preodontoblasts, odontoblasts and subodontoblasts. The incisors showed the most advanced stage of development, where the enamel matrix and calcified dentine were formed in the labial part of the teeth. The CX 43 epitope was seen in the stratum intermedium, inner enamel epithelium, preameloblasts, preodontoblasts, odontoblasts, and subodontoblasts. Immunolabelling was more extensive in the stratum intermedium and subodontoblasts than in preameloblasts, preodontoblasts, and odontoblasts. The immunolabelling in preameloblasts and predontoblasts was accumulated at cell surfaces facing the predentine.(ABSTRACT TRUNCATED AT 250 WORDS)

Overview of morphological changes in enamel organ cells associated with major events in amelogenesis

The formation and mineralization of enamel is controlled by epithelial cells of the enamel organ which undergo marked, and in some cases repetitive, alterations in cellular morphology as part of the developmental process. The most dramatic changes are seen in ameloblasts which reverse their secretory polarity during differentiation to allow for extracellular release of large amounts of proteins from plasma membrane surfaces that were originally the embryonic bases of the cells. Secreted enamel proteins at first do not accumulate in a layer but, in part, percolate into the developing predentin and subjacent odontoblast layer. Appositional growth of an enamel layer begins with mineralization of the dentin, and ameloblasts develop a complicated functional apex (Tome's processes) to direct release of matrix proteins, and perhaps proteinases, at interrod and rod growth sites. Once the full thickness of enamel is produced, some ameloblasts degenerate, and the surviving cells shorten in height and spread out at the enamel surface. They reform a basal lamina to cover the immature enamel, and continue producing small amounts of enamel proteins that pass through the basal lamina into the enamel. Ameloblasts also undergo cycles of modulation where apical invaginations enriched in Ca-ATPases and other enzymes are formed and shed on a repetitive basis (ruffle-ended/smooth-ended transitions). As this happens, apatetic crystals seeded earlier expand in volume by gradual layering of new mineral at the surfaces of the preformed crystals. Ameloblasts stop modulating when the crystals almost fill existing volume formerly occupied by protein and water.(ABSTRACT TRUNCATED AT 250 WORDS)

[A study on human dental embryology in an endemic high fluorosis region]

In a high endemic fluorosis area in Guizhou, China, 40 human fetuses delivered by induced abortion during the 5th-8th month of gestation were utilized to study dental embryonic samples under transmission electron microscope (TEM). Compared with normal controls, ultrastructure findings in the ameloblast cell organs include swelling of mitochondria, enlargement of SER, increase in RNA granules and RER. This study suggests that the irregularities of the collagenous fibers and crystallites are due to the maldevelopment of the ameloblast Tome's processes, which explains the mechanism of motteled enamel.

Enamel matrix proteins and ameloblast biology

The paper reviews the changes in ameloblast ultrastructure, concomitant with the changes in its functions across the major stages of amelogenesis. It describes the mechanisms associated with the major events in biosynthesis and degradation of the major enamel proteins (amelogenins and tuftelin/enamelins) and with the presecretory and postsecretory mechanisms leading to the heterogeneity of these extracellular matrix proteins. The gene structure, chromosomal localization, protein, primary structure and possible function, and the involvement of the different proteins in X-linked (amelogenin) and possibly in autosomally linked (tuftelin) amelogenesis imperfecta, the most common hereditary disease of enamel, are also discussed.

The role of smooth-ended ameloblasts in enamel protein degradation in the rat incisor pigmentation

Rats were perfused with glutaraldehyde. The lower incisors were dissected free. Mid-sagittal slice was cut through the entire incisor, demineralized. The pigmentation zone was isolated and further sliced into cross sections. These sections were incubated for CMPase. CMPase reaction product were associated with the lateral cell membrane of subclass II and Subclass III. This localization confirm the role of extracellular space of SAs as a route of enamel proteins during maturation.

The spatial localization of Dlx-2 during tooth development

The spatial distribution of Dlx-2 protein during murine tooth development has been investigated using immunohistochemistry with Dlx-2 antibodies. In common with several other homeobox genes expressed in toothgerms, Dlx-2 shows a multiphasic distribution in both epithelially and mesenchymally derived structures. This localization shows a number of similarities with the expression of Msx-2 and suggests a role for Dlx-2 in tooth initiation and tissue patterning.

Ruffle-ended maturation ameloblasts in the pigmentation zone of rat incisor

Rats were perfused with glutaraldehyde and the incisors were dissected free. Midsagittal slice of the incisor were demineralized and the pigmented zone were further sliced into thin cross sections. The lysosomal activity was demonstrated utilizing CMPase. Two patterns of localization were observed. Weak local reaction product associated with the pigment granules. The second one was intense reaction product with some of the ruffled border of ameloblasts. A suggested role for these localizations were discussed in relation to pigment release into the enamel surface.

Enamel microstructure in Lemuridae (Mammalia, Primates): assessment of variability

This study describes the molar enamel microstructure of seven lemurid primates: Hapalemur griseus, Varecia variegata, Lemur catta, Lemur macaco, Lemur fulvus rufus, Lemur fulvus fulvus, and Lemur fulvus albifrons. Contrary to earlier accounts, which reported little or no prism decussation in lemurid enamel, both Lemur and Varecia molars contain a prominent inner layer of decussating prisms (Hunter-Schreger bands), in addition to an outer radial prism layer, and a thin, nonprismatic enamel surface layer. In contrast, Hapalemur enamel consists entirely of radial and, near the surface, nonprismatic enamel. In addition, for all species, prism packing patterns differ according to depth from the tooth surface, and for all species but Varecia (which also has the thinnest enamel of any lemurid), average prism area increases from the enamel-dentine junction to the surface; this may be a developmental solution to the problem of accommodating a larger outer surface area with enamel deposited from a fixed number of cells. Finally, contradicting some previous reports, Pattern 1 prisms predominate only in the most superficial prismatic enamel. In the deeper enamel, prism cross-sections include both closed (Pattern 1) and arc-shaped (Pattern 2 or, most commonly, Pattern 3). This sequence of depth-related pattern change is repeated in all taxa. It should also be emphasized that all taxa can exhibit all three prism patterns in their mature enamel. The high degree of quantitative and qualitative variation in prism size, shape, and packing suggests that these features should be used cautiously in phylogenetic studies. Hapalemur is distinguished from the other lemurids by unique, medially constricted or rectangular prism cross-sections at an intermediate depth and the absence of prism decussation, but, without further assessment of character polarity, these differences do not clarify lemurid phylogenetic relations. Some characters of enamel microstructure may represent synapomorphies of Lemuridae, or of clades within Lemuridae, but homoplasty is likely to be common. Homoplasy of enamel characters may reflect functional constraints.

Correlated alkaline phosphatase histochemistry and quantitative backscattered electron imaging in the study of rat incisor ameloblasts and enamel mineralization

The different functional conditions of pre-ameloblasts, secretory ameloblasts, and maturation ameloblasts in 9 day rat incisors were recognized using high resolution light microscopic alkaline phosphatase histochemistry: Digital backscattered electron imaging was performed using the block surfaces from which thin sections were taken for histochemical study. It was possible to correlate exact locations in histochemical sections with positions in the block face at all stages of enamel mineralization from early secretion through late maturation. The first steep increase in the rate of mineralization of completed enamel matrix occurs after the first transition from smooth ended ameloblasts to ruffle ended ameloblasts. In the 9 day rat incisors used for this purpose, there are only two smooth to ruffle cyclical transitions, and the width of successional smooth ended bands of ameloblasts in the maturation cycling process is always narrow. Nevertheless, there seems to be a good correlation between mineralization increase and the acquisition of the high alkaline phosphatase activity in the deeply enfolded distal cytoplasm of the ruffle-ended maturation stage ameloblasts.

The three-dimensional structure of Tomes' processes and their relationship to arrangement of enamel prism in dog teeth

The relationship between the course of enamel prisms and the direction of secretory face in Tomes' process of ameloblast was examined three-dimensionally in the dog tooth by light and electron microscopy, as well as by reconstruction. Scanning electron microscopic observation on the developing enamel surface after the dissolution of the enamel organ showed numerous groups of pits with bulbous faces inclined in the same sidewards direction, while those faces in neighboring groups were inclined in an opposite direction. Tangential semithin sections from demineralized tooth germs were serially cut from the enamel surface to the enamel-dentin junction. These sections showed numerous belt-like zones arranged perpendicular to the meridian of the tooth. A straight row of enamel prisms perpendicular to the boundaries of the belt-like zones was selected at 142 microns from the enamel-dentin junction. The row of prisms and their successive Tomes' processes were reconstructed from micrographs with a personal computer. The cut ends of the enamel prisms initially perpendicular to the boundaries appeared as a sine curve, viewed from the enamel-dentin junction. In a single belt-like zone, the horizontal tilt angles of the enamel prisms towards the enamel-dentin junction tended to be largest at the center of the zone, and smallest near the boundaries. The long axis of each enamel prism was at right angles to the secretory face of the Tomes' process. The secretory faces in a single belt-like zone were inclined in the same sidewards direction and the secretory faces in neighboring zones were inclined in opposite directions. The results suggest that the direction of the sidewards displacement in ameloblasts is related to that of the secretory face of Tomes' processes. Further, one group of ameloblasts with their secretory faces inclined in the same sidewards direction form one belt-like zone.

Ultrastructural studies and immunolocalization of enamel proteins in rodent secretory stage ameloblasts processed by various cryofixation methods

BACKGROUND

Cryofixation rapidly immobilizes cell and tissue components in their native state, thereby resulting in an ultrastructural preservation very close to the living situation. We have applied this approach to examine the morphology of secretory stage ameloblasts and the distribution of enamel proteins in these cells.

METHODS

Molar and incisor tooth germs from newborn mice and/or rats were quickly dissected and divided into segments. The segments were then rapidly frozen using slam, plunge or pressure freezing, freeze-substituted and embedded in Epon. In addition, incisors from older rats were chemically fixed by vascular perfusion and also dehydrated by freeze-substitution.

RESULTS

Well-preserved ameloblasts were obtained with all four tissue processing methods. However, slam freezing often showed mechanical damage to the ameloblasts, particularly at the level of the distal portion of Tomes' processes which appeared severed or distorted. Plunging into liquid nitrogen-cooled liquid propane resulted in comparatively less tissue distortion. High pressure freezing gave a relatively higher yield of well-preserved specimens, although displacement of organelles in ameloblasts was sometimes observed, probably resulting from hydrostatic pressure. Minimal ice crystal and mechanical damage was observed in chemically fixed tooth samples processed by freeze-substitution since such specimens are cryoprotected and their examination is not restricted to a surface layer. With all of the above cryopreparation methods, the ultrastructure of well-preserved ameloblasts was, in general, similar to that obtained following conventional chemical fixation, and immunocytochemistry with an anti-amelogenin antibody indicated no profound differences in the distribution of enamel proteins.

CONCLUSIONS

These results indicate that, despite some limitations, it is possible to adequately cryofix tooth organs while preserving the architecture of ameloblasts and permitting immunolocalization of enamel proteins. Furthermore, they confirm the general morphology of secretory stage ameloblasts as currently derived from conventional chemical tissue processing.

Ultrastructure of quick-frozen secretory ameloblasts of the rat molar tooth

Ameloblasts in molar tooth germs from 4 to 12-day-old neonatal rats were quick-frozen and then freeze-substituted in acetone-tannic acid and acetone-osmium tetroxide. Following quick freezing and freeze-substitution, the preservation of ameloblasts was principally the same as seen in conventionally-fixed ones. Several differences, however, became apparent between two methods. Concerning enamel formation, the quick-freezing procedure greatly improved the preservation including that of the extrusion of secretory granules into the extracellular space. The exocytosis is a type of merocrine secretion. The precursor stippled material was demonstrated at the front of Tomes' processes and in the intercellular space of the distal terminal junction. In the more advanced stage, the stippled material tended to decrease gradually in amount. Prior to the formation of the typical Tomes' process, large secretory granules fused directly with the distal plasma membrane, which plays a role in the formation of prismless enamel. After the typical Tomes' process had formed, secretory granules containing electron-dense material instead of the large secretory granules accumulated in the distal portion of the Tomes' process. These latter secretory granules had two different destinations; i.e. the granules discharged their contents into the infoldings of the distal part of Tomes' process and into the infoldings of the lateral cell membrane. These types of secretion seem to have a functional role in the rod or inter-rod enamel formation. The ultrastructure of the stippled material consisted of 3-dimensionally interconnected strands after quick freezing, which is different from the appearance of granules embedded in amorphous material after conventional fixation. The stippled material was seen in the intercrystal space among the newly formed crystals. Also, an extensive tubulo-vesicular structure was preserved in both the cell body and Tomes' process. The present data suggest that the membrane of this structure showed a smooth contour opening to the extracellular space, and also an intimate relationship with the secretory granules.

Observations on the enamel of odontomas

The morphological study of odontomas provides an alternative model for observing the formation of dental tissues, since different maturing stages are present simultaneously. Investigations were performed on decalcified samples (using light microscopy and transmission electron microscopy) and on undecalcified samples of complex odontoma enamel (using transmission electron microscopy). Simultaneous presence of prismatic enamel at various maturing stages with different structural characteristics was observed. Such enamel was sometimes associated with layers of ameloblastic cells with characteristics of cells in functional activity. In other sites, the enamel did not present a prismatic structure but it appeared as unstructured material clusters with abundant organic component. It was concluded that the theory according to which an ecto-mesenchymal inductive failure occurs in odontomas is not confirmed. The defect seen at the beginning of the differentiated and anomalous tissue maturation may be related to latest events in the development of the enamel organ. In this regard, it was concluded that such events involve the efficiency of the ameloblasts and the possible alterations in the organic matrix.

An immunohistochemical study of the effects of fluoride on enamel development in the rat incisor

A monoclonal antiamelogenin antibody was used to investigate the effects of fluoride on enamel development in the rat incisor. The results suggested that during secretion the enamel matrix molecules are arranged in such a way as to mask the epitope recognized by the monoclonal antibody. However, during the transition stage of development as the matrix begins to be degraded the epitope becomes exposed and labelling intensity increases to reach a maximum at the end of transition/start of maturation. The effect of fluoride is to delay the appearance of labelling within the enamel matrix until the end of transition. This suggests that the fluoride may inhibit enzymatic degradation or disaggregation of the matrix, the resulting residual matrix then inhibiting crystal growth.

Changes of lectin staining pattern of the Golgi stack during differentiation of the ameloblast in developing rat molar tooth germs

Changes of lectin staining patterns in the Golgi stack during cell differentiation were examined in the ameloblasts of developing rat molar tooth germs, using HRP-labeled lectins: Canavalia ensiformis (Con A), Griffonia simplicifolia I (GS-I), Glycine max (SBA), Ulex europeus I (UEA-I), Triticum vulgaris (WGA), and Arachis hypogaea (PNA). The Golgi stacks of the inner enamel epithelial cells and the presecretory ameloblasts were stained with the lectins, although the staining strength and pattern varied among the stacks with each lectin. In some cases, the reaction products for the lectins were observed in most or all saccules of the Golgi stack. In the secretory ameloblasts, however, discrete staining patterns of the Golgi stack were found for each lectin. The reaction products deposited in definite saccules of the Golgi stack of the secretory ameloblast, especially for UEA-I and PNA which stained only the trans Golgi saccules of the stack. The reaction-positive saccules distributed more extensively in the Golgi stack of the inner enamel epithelial cell and the presecretory ameloblast than in the secretory ameloblast. These findings suggest that the Golgi stack is not fully compartmentalized in the inner enamel epithelial cell and the presecretory ameloblast. It is proposed that, in the differentiating ameloblast, various glycosyltransferases may coexist in most saccules of the Golgi stack.

High-resolution scanning electron microscopy of rat incisor ameloblasts

The internal three-dimensional organization of secretory and maturation stage ameloblasts was examined using field emission scanning electron microscopy. Particular attention was given to the structure of the Golgi apparatus, the distribution of smooth membrane tubulo-vesicular elements and their relationship with endosomal/lysosomal components. Rat incisors were fixed by intracardiac perfusion with glutaraldehyde, decalcified and divided into segments. The tissues were cryoprotected with glycerol and freeze-fractured. They were then macerated in osmium, and after conductive staining with osmium/tannic acid, the samples were critical-point dried and sputtered with gold. High-resolution scanning electron microscopy showed that ameloblasts contain a well-developed Golgi apparatus, even in the maturation stage where cells are generally believed not to be actively involved in protein secretion. Smooth tubulo-vesicular structures formed a complex network which extended throughout the cell. In secretory stage ameloblasts, this network reached into Tomes' process and consisted of small and large tubules, and distended portions. The smaller tubules radiated from a central core of organelles towards the plasma membrane. Numerous lysosomal/endosomal elements were observed in the Golgi region, and in some cases smooth tubular portions were seen at the surface of multivesicular bodies. These data show that high-resolution scanning electron microscopy can be applied to correlate three-dimensional structural detail with the secretory and resorptive functions of ameloblasts.

The presence of vesicles containing lactate dehydrogenase in the ameloblast layer of bovine enamel organ

Ameloblast layers were removed from bovine enamel organs, followed by the separation of the extracellular matrix-vesicle fraction after collagenase digestion. Lactate dehydrogenase (LDH)-containing vesicles were found in that fraction. LDH in these vesicles did not result from cell lysis and vesicle capture during the preparation of the fraction. The isoenzyme pattern of LDH in LDH-containing vesicles was similar to that of cytosolic LDH of ameloblasts, suggesting the presence of a mechanism for specific uptake of cytosolic LDH during the in vivo formation of the vesicles. The existence of LDH-containing vesicles in enamel organ, where mineralization had been believed not to be initiated by matrix vesicles, suggests the possibility that LDH-containing vesicles have a specific function different from that of matrix vesicles.

Presence of multinucleate cells in the papillary layer of the Macaca fuscata enamel organ

The enamel organ of Macaca fuscata from post-secretory transition to the early maturation stage of was investigated by means of light and electron microscopy. Unusual, large multinucleated cells were observed in the papillary layer. These cells contained organelles characteristic of the maturation stage ameloblast and often extended to the enamel surface, suggesting a possible origin from the ameloblast layer.

Proliferative and functional stages of rat ameloblast differentiation as revealed by combined immunocytochemistry against enamel matrix proteins and bromodeoxyuridine

A double-staining immunocytochemical technique was used for the simultaneous detection, at the light- and electron-microscopical level, of proliferating bromodeoxyuridine (BrdU)-labelled cells and enamel protein (EP)-producing cells in the inner enamel epithelium (IEE) of rat tooth germ. BrdU-positive cells were found in the region of the IEE close to the cervical loop and never displayed EP-like immunoreactivity. BrdU-immunoreactivity was confined to the nucleus of replicating cells. In contrast, epithelial cells displaying EP-like immunoreactivity were found in the region of the forming dental cusp and were consistently BrdU-negative. EP-like immunoreactivity was detectable in the cytoplasmic compartments involved in the exocrine secretion pathway and in the extra-cellular matrix close to EP-immunoreactive cells. These data support the view that withdrawal from the cell cycle in the IEE is a temporal prerequisite for acquiring the functional competence of secreting EP. Moreover, cycling cells and secretory cells in the IEE constitute two separate compartments that are spatially defined, and that exhibit clear-cut staining patterns with respect to BrdU- and EP-immunoreactivity, respectively. We thus propose that BrdU-incorporation and EP-production may be used as specific markers of the differentiation of the IIE cells in studies of the possible role of growth factors, their receptors and oncoproteins in this tissue.

Changes of cytochemical properties in the Golgi apparatus during in vivo differentiation of the ameloblast in developing rat molar tooth germs

The cytochemical changes of the Golgi stacks occurring concomitantly with cell differentiation were examined in ameloblasts of developing rat molar tooth germs using osmium impregnation and cytochemistry with nicotinamide adenine dinucleotide phosphatase (NADPase), thiamine pyrophosphatase (TPPase), and acid phosphatase (Acpase). NADPase, TPPase, and Acpase activities were already present in the Golgi stacks of the inner enamel epithelial cells, the undifferentiated form of the ameloblast: NADPase activity existed in the medial Golgi cisternae, TPPase activity in the trans Golgi cisternae, and Acpase activity in almost all cisternae and strongly in the trans-most cisterna of the Golgi stack. At this stage, however, osmium deposits after impregnation were not observed in the cisterna of Golgi stacks but were present in some small vesicles. These vesicles were located throughout the cytoplasm. Osmiophilic cisternae in the Golgi stacks were apparent for the first time at the stage when the Golgi apparatus developed and migrated to the region distal to the nucleus with the progression of cell differentiation. These findings indicate that the cis subcompartment of the Golgi apparatus was incomplete in the inner enamel epithelial cells with regard to appearance of its cytochemical property, as compared with the medial and trans subcompartments. It is suggested that the cis compartment of the Golgi stack may be completed only in the last stage of the compartmentalized Golgi organization during differentiation of the ameloblast.

Growth hormone regulates nucleolar organizer regions during odontogenesis in the rat

Nucleolar organizers are major sites of ribosomal RNA synthesis and provide an index of transcriptional activity. In order to further define growth hormone actions on nucleolar organizer regions in tooth forming cells, hypophysectomized rats treated with growth hormone for 4 and 24 h, hypophysectomized and sham-operated animals were used. After demineralization and standard paraffin embedding, longitudinal sections of maxillary incisors were stained by a silver stain technique to reveal nucleolar organizer regions. The area of these regions per nucleus was measured using a modified microdensitometer. Analyses of variance of the resulting data showed that preameloblasts and preodontoblasts have greater silver stained nucleolar organizer region values than ameloblasts and odontoblasts. Hypophysectomy reduced and growth hormone partly restored the level of nucleolar organizer regions in preameloblasts and preodontoblasts, but not in mature ameloblasts or odontoblasts. In the case of the younger preameloblasts and preodontoblasts, the effect of growth hormone was seen within 4 h of growth hormone injection. In conclusion, rRNA synthesis, as revealed by the specific silver staining of nucleolar organizer regions in tooth forming cells, appears to be regulated by growth hormone over a relatively short time frame.