Expression of cytokeratins in enamel organ, junctional epithelium and epithelial cell rests of Malassez.

BACKGROUND AND OBJECTIVE: After tooth formation is complete, it is suggested that continuity exists between the epithelial cell rests of Malassez (ERM), reduced enamel epithelium (REE) and subsequently the junctional epithelium. However, the junctional epithelium was reported to differ from REE and ERM. The developmental relationships between and among them remain controversial. Therefore, in the present study we examined the expression of cytokeratins in the three types of epithelia to investigate the epithelial phenotypes. MATERIAL AND METHODS: The maxillae of Wistar rats, 1, 2, 3 and 7 wk of age, were used, and the expression of CK14, CK17, CK19, CK10/CK13 and AE1/AE3 was detected using immunoperoxidase techniques. RESULTS: There was negative staining for CK10/CK13 in all the epithelia. ERM stained strongly for AE1/AE3, CK14, CK17 and CK19. During the transformation of inner enamel epithelial (IEE) cells into reduced ameloblasts and subsequently into junctional epithelium, strong staining for CK14 was evident in IEE, REE and junctional epithelium, whereas the expression of AE1/AE3 and of CK19 were initially negative in IEE and then strong in REE and junctional epithelium, respectively. In particular, the expression of CK17 was strongly positive in ERM and REE, but was negative in IEE and junctional epithelium. CONCLUSION: ERM are of odontogenic origin and junctional epithelium has an epithelial phenotype different from REE and ERM. This is the first report to demonstrate that CK17 can be used as a marker to distinguish junctional epithelium from ERM.

Immunohistochemical localization of connective tissue growth factor, transforming growth factor-beta1 and phosphorylated-smad2/3 in the developing periodontium of rats.

BACKGROUND AND OBJECTIVE: Connective tissue growth factor (CTGF) is a downstream mediator of transforming growth factor-beta1 (TGF-ß1), and TGF-ß1-induced CTGF expression is regulated through the SMAD pathway. CTGF is implicated in the development of cartilage, bone and tooth. However, its expression in the developing periodontium is unclear. Therefore, we aimed to investigate the immunolocalization of CTGF, TGF-ß1 and phosphorylated SMAD2/3 (pSMAD2/3) in the developing periodontium of rats. MATERIAL AND METHODS: The maxillaries of Wistar rats, 2, 3, 7 and 12 wk of age, were used and the localization of CTGF, TGF-ß1 and pSMAD2/3 was detected using immunoperoxidase techniques. RESULTS: Hertwig' s epithelial root sheath (HERS) cells were strongly positive for CTGF and TGF-ß1, but not for pSMAD2/3. Positive staining for CTGF, TGF-ß1 and pSMAD2/3 was found in bone and periodontal ligament. In cementum, most cementoblasts associated with cellular cementum and some cementocytes stained strongly for CTGF, whereas cementoblasts associated with acellular cementum did not express CTGF. No signal for TGF-ß1 was observed in cellular and acellular cementum. In addition, most cementocytes were strongly positive for pSMAD2/3. CONCLUSION: CTGF, TGF-ß1 and pSMAD2/3 are localized in bone and periodontal ligament, but are differentially expressed in HERS and cementum. The results of our study indicate that the regulation of CTGF expression by TGF-ß1 might be cell-type specific in periodontium.

Consequences for enamel development and mineralization resulting from loss of function of ameloblastin or enamelin.

Although the nonamelogenin proteins, ameloblastin and enamelin, are both low-abundance and rapidly degrading components of forming enamel, they seem to serve essential developmental functions, as suggested by findings that an enamel layer fails to appear on teeth of mice genetically engineered to produce either a truncated form of ameloblastin (exons 5 and 6 deleted) or no enamelin at all (null). The purpose of this study was to characterize, by direct micro weighing, changes in enamel mineralization occurring on maxillary and mandibular incisors of mice bred for these alterations in nonamelogenin function (Ambn(+/+, +/-5,6, -5,6/-5,6), Enam(+/+, +/- ,-/-)). The results indicated similar changes to enamel-mineralization patterns within the altered genotypes, including significant decreases by as much as 50% in the mineral content of maturing enamel from heterozygous mice and the formation of a thin, crusty, and disorganized mineralized layer, rather than true enamel, on the labial (occlusal) surfaces of incisors and molars along with ectopic calcifications within enamel organ cells in Ambn(-5,6/-5,6) and Enam(-/-) homozygous mice. These findings confirm that both ameloblastin and enamelin are required by ameloblasts to create an enamel layer by appositional growth as well as to assist in achieving its unique high level of mineralization.

The cell adhesion molecule nectin-1 is critical for normal enamel formation in mice.

Nectin-1 is a member of a sub-family of immunoglobulin-like adhesion molecules and a component of adherens junctions. In the current study, we have shown that mice lacking nectin-1 exhibit defective enamel formation in their incisor teeth. Although the incisors of nectin-1-null mice were hypomineralized, the protein composition of the enamel matrix was unaltered. While strong immunostaining for nectin-1 was observed at the interface between the maturation-stage ameloblasts and the underlying cells of the stratum intermedium (SI), its absence in nectin-1-null mice correlated with separation of the cell layers at this interface. Numerous, large desmosomes were present at this interface in wild-type mice; however, where adhesion persisted in the mutant mice, the desmosomes were smaller and less numerous. Nectins have been shown to regulate tight junction formation; however, this is the first report showing that they may also participate in the regulation of desmosome assembly. Importantly, our results show that integrity of the SI-ameloblast interface is essential for normal enamel mineralization.

Expression and characterization of a Rana pipiens amelogenin protein.

Amelogenin, the major protein of developing enamel matrix, controls enamel crystal growth via unique supermolecular features. While much has been contributed to our understanding of mammalian amelogenin function, little is known about how amelogenin and its unique physico-chemical features have evolved among vertebrates. Here we report, for the first time, amphibian amelogenin recombinant protein expression and characterization in Rana pipiens. In order to characterize R. pipiens amelogenin, the newly discovered amelogenin coding sequence was amplified, subcloned, and expressed in Eshcerichia coli. Our newly generated R. pipiens amelogenin-specific antisera resolved a major 19-kDa band on western blots of frog tooth extracts and revealed an enamel organ tissue-specific localization pattern using immunohistochemistry. Using mass spectroscopy, a single major compound with a molecular weight of 21.6 kDa was detected, which corresponded to the amino acid sequence-based molecular weight prediction of the His fusion recombinant protein. Dynamic light scattering studies resolved 41-nm radius subunits compared with 14-nm radius subunits from mouse recombinant amelogenin controls. Transmission electron microscopy revealed defined spherical subunits in R. pipiens matrix self-assembly in contrast with a homogeneous 'stippled' matrix in mouse amelogenin matrix self-assembly. Our data suggest that R. pipiens amelogenin is distinguished from mammalian amelogenins by a number of unique physico-chemical properties which may be related to specific modes of crystal formation in frog enamel.

Identification of secreted and membrane proteins in the rat incisor enamel organ using a signal-trap screening approach.

The secretome represents the subset of proteins that are targeted by signal peptides to the endoplasmic reticulum. Among those, secreted proteins play a pivotal role because they regulate determinant cell activities such as differentiation and intercellular communication. In calcified tissues, they also represent key players in extracellular mineralization. This study was carried out to establish a secretome profile of rat enamel organ (EO) cells. A functional genomic technology, based on the signal trap methodology, was applied, starting with a library of 5'-enriched cDNA fragments prepared from rat incisor EOs. A total of 2,592 clones were analyzed by means of macroarray hybridizations and DNA sequencing. Ninety-four unique clones encoding a signal peptide were retrieved. Among those were 84 matched known genes, many not previously reported to be expressed by the EO. Most importantly, 10 clones were classified as being novel, with EO-009 identified as the rat homolog of human APin protein. These data indicate that many secreted and membrane-embedded EO proteins still remain to be identified, some of which may play crucial roles in regulating processes that create an optimal environment for the formation and organization of apatite crystals into a complex three-dimensional calcified matrix.

The 17-kDa sheath protein in enamel proteins induces cementum regeneration in experimental cavities created in a buccal dehiscence model of dogs.

BACKGROUND AND OBJECTIVE: Commercially available enamel proteins, such as Emdogain, are clinically used for periodontal regeneration. However, the real mechanisms behind the bioactivities of enamel proteins is still unclear, as enamel proteins have multicomponents. The purpose of this in vivo study was to identify the cementum regeneration-promoting factor in enamel proteins that is clinically used for periodontal regeneration to induce cementum-promotive and osteopromotive activities. MATERIAL AND METHODS: Cementum regeneration, which is an important part of periodontal regeneration, was examined in experimental cavities prepared on a buccal dehiscence model of dogs. The purification of enamel protein with cementum regeneration activity was carried out by gel filtration and ion exchange chromatographies of newly formed secretory enamel. RESULTS: Cementum regeneration activity was found in the aggregate comprising 13-17-kDa sheath proteins along with a small amount of amelogenins, found in the newly formed secretory enamel. In these proteins, cementum regeneration activity was detected upon application of the 17-kDa sheath protein, but not by other lower molecular-weight sheath proteins and amelogenins. However, the purified 17-kDa sheath protein induced cementum regeneration activity only in a small area, although the regenerated cementum was thick. The activity of the 17-kDa sheath protein was believed not to have been a result of contamination by growth factors such as transforming growth factor-beta1 (TGF-beta1) found in the enamel protein, as the application of TGF-beta1 induced weak cementum regeneration activity. CONCLUSION: It is concluded that the 17-kDa sheath protein itself exhibits cementum regeneration activity, although other factors may be needed to demonstrate its full ability.

An immunohistochemical study of the expression of heat-shock protein-25 and cell proliferation in the dental pulp and enamel organ during odontogenesis in rat molars.

OBJECTIVES: The aim of this study is to clarify the functional significance of heat-shock protein (HSP)-25 during tooth development. DESIGN: We compared the expression of HSP-25 in the dental epithelial and mesenchymal cells with their proliferative activity during odontogenesis in rat molars on postnatal days 1-100 by immunohistochemistry using anti-HSP-25 and anti-5-bromo-2'-deoxyuridine (BrdU) for cell proliferation assay. RESULTS: On day 1, BrdU-immunoreactive cells were densely located in the inner enamel epithelium in the cervical loop and intercusped areas and the dental pulp adjacent to them, whereas HSP-25-immunoractivity (IR) was restricted to the cusped area where odontoblasts and ameloblasts had already differentiated. Subsequently, BrdU-IR shifted in the apical direction to be localized around Hertwig's epithelial root sheath during days 5-30, never overlapping with concomitantly apically-shifted HSP-25-IR. On days 60-100, BrdU-immunoreactive cells were hardly recognizable in the dental pulp, where HSP-25-IR was exclusively localized in the odontoblast layer. Furthermore, the odontoblast- and ameloblast-lineage cells exhibited two steps in the expression of HSP-25 throughout the postnatal stages: first, dental epithelial and pulpal mesenchymal cells showed a weak IR for HSP-25 after the cessation of their proliferative activity, and subsequently odontoblasts and ameloblasts consistently expressed an intense HSP-25-IR. CONCLUSION: Odontoblast- and ameloblast-lineage cells acquire HSP-25-IR after they complete their cell division, suggesting that this protein acts as a switch between cell proliferation and differentiation during tooth development. The consistent expression of HSP-25-IR in the formative cells may be involved in the maintenance of their functional integrity.

Cytokeratins in epithelia of odontogenic neoplasms.

Neoplasms and tumours related to the odontogenic apparatus may be composed only of epithelial tissue or epithelial tissue associated with odontogenic ectomesenchyme. The immunohistochemical detection of different cytokeratins (CKs) polypeptides and vimentin has made it easier to explain the histogenesis of many epithelial diseases. The present study aimed to describe the immunohistochemical expression of cytokeratins 7, 8, 10, 13, 14, 18, 19 and vimentin in the epithelial components of the dental germ and of five types of odontogenic tumours. The results were compared and histogenesis discussed. All cells of the dental germ were positive for CK14, except for the preameloblasts and secreting ameloblasts, in which CK14 was gradually replaced by CK19. CK7 was especially expressed in the cells of the Hertwig root sheath and the stellate reticulum. The dental lamina was the only structure to express CK13. The reduced epithelium of the enamel organ contained CK14 and occasionally CK13. Cells similar to the stellate reticulum, present in the ameloblastoma and in the ameloblastic fibroma, were positive for CK13, which indicates a nature other than that of the stellate reticulum of the normal dental germ. The expression of CK14 and the ultrastructural aspects of the adenomatoid odontogenic tumour probably indicated its origin in the reduced dental epithelium. Calcifying odontogenic epithelial tumour is thought to be composed of primordial cells due to the expression of vimentin. Odontomas exhibited an immunohistochemical profile similar to that of the dental germ. In conclusion, the typical IF of odontogenic epithelium was CK14, while CK8, 10 and 18 were absent. Cytokeratins 13 and 19 labelled squamous differentiation or epithelial cells near the surface epithelium, and CK7 had variable expression.

Transient expression of heat shock protein (Hsp)25 in the dental pulp and enamel organ during odontogenesis in the rat incisor.

The expression of heat shock protein (Hsp) 25 during odontogenesis in the dental pulp and enamel organ of rat incisors was investigated by immunocytochemistry and confocal microscopy. In the process of dentin formation, immature odontoblasts first exhibited Hsp 25-immunoreactivity, and increased in immunointensity with the advance of their differentiation. In the dental pulp, in contrast, intense immunoreaction in the mesenchymal cells became weak or negative in parallel with the progress of cell differentiation. The immunoreaction for Hsp 25 in the enamel organ revealed a characteristic stage-related alteration during amelogenesis. In secretory ameloblasts, the immunoreaction for Hsp 25 was found throughout their cell bodies, intense reactivity being located near the proximal and distal terminal webs. At the maturation stage, ruffle-ended ameloblasts (RA) consistently showed Hsp 25-immunoreactivity throughout the cell bodies, whereas smooth-ended ameloblasts (SA) lacking a ruffled border were weak in immunoreaction at the distal cytoplasm. Other cellular elements of the enamel organ were negative. The subcellular localization of Hsp 25-immunoreactivity in this study appeared essentially identical to that of actin filaments as demonstrated by confocal microscopy using rhodamine-labeled phalloidin. These immunocytochemical data suggest that the Hsp 25 molecule is involved in reinforcement of the cell layer following cell movement during odontogenesis and in the formation and maintenance of the ruffled border of RA.

Expression of cytokeratins in human enamel organ.

Cytokeratin (CK) is a filament which plays a central role in epithelial tissue and, like the polypeptides of intermediate filaments in general, shows a high degree of tissue specificity. The CK expression patterns of odontogenic epithelia are still poorly described. We studied the distribution of individual CK polypeptides in the human enamel organ at bell stage and in remnants of the dental lamina. Our immunohistochemical study showed that epithelial cells stained for CKs 7, 13, 14 and 19 with slight changes in their pattern during the differentiation phase of odontogenesis. There was negative staining for all other CK polypeptides tested (CKs 8, 10, 16, 17 and 18). Most of the CKs in the enamel organ epithelia did not show differences related to the stage-specific state of differentiation, except for CKs 14 and 19 at the inner enamel epithelium. A strong label for CK 14 was present at the inner dental epithelium at early bell stage, and this was substituted by CK 19 at the late bell stage when the ameloblasts were fully differentiated.

Immunohistochemical localization of the differentiation marker E11 in dental development of rats.

E11 antigen, originally characterized in a rat osteosarcoma cell line, is known to be expressed during late stages of the osteogenic cell lineage both in vitro and in vivo. The aim of the present study was to monitor the occurrence and distribution patterns of the E11 antigen using monoclonal antibodies (mAb E11 and MEP-1) during different stages of tooth germ development of new-born rats by means of immunohistochemistry. Both antibodies strongly bound to plasma membranes of ameloblasts in presecretory and secretory stages. In addition, odontoblasts and cells of the periodontium were immunoreactive for E11 and MEP-1. During maturation, the immunoreactivity of ameloblast plasma membranes decreased significantly. Our data suggest that E11 and MEP-1 might be important as markers for cell differentiation and mineralization processes during tooth germ development.

Fine structure of dental epithelial cells and the enameloid during the enameloid formation stages in an elasmobranch, Heterodontus japonicus.

The structural features of the dental epithelial cells and the enameloid in tooth germs of the Japanese Port Jackson shark, Heterodontus japonicus, in the stages of enameloid formation, were investigated by light and transmission electron microscopy. At the enameloid matrix-formation stage, tall columnar inner dental epithelial cells contained large numbers of glycogen particles. At the enameloid mineralization stage, when many sharply outlined crystals appeared throughout the enameloid, the inner dental epithelial cells exhibited well-developed Golgi apparatuses and many mitochondria in the proximal cytoplasm, and abundant vesicles and vacuoles in the distal cytoplasm. Marked interdigitations of the lateral membrane were visible in the inner dental epithelial cells. The outer dental epithelial cells contained many mitochondria, lysosomal bodies, vesicles and microtubules, and the capillaries usually approached the outer dental epithelial cells. At the enameloid maturation stage, large numbers of crystals occupied the enameloid, and most of the organic matrix had disappeared from the enameloid area after demineralization. The organelles in the inner and outer dental epithelial cells decreased in number, but there were still widely distributed Golgi apparatuses, abundant intermediate filaments and granules containing an electron-dense substance in the inner dental epithelial cells. It is probable that the dental epithelial cells are involved in the removal of organic matrix from the enameloid and in the process of mineralization at the later stages of enameloid formation, i.e., the mineralization and the maturation stages.

Immunolocalization of epithelial and mesenchymal matrix constituents in association with inner enamel epithelial cells.

After crown formation, the enamel organ reorganizes into Hertwig's epithelial root sheath (HERS). Although it is generally accepted that HERS plays an inductive role during root formation, it also has been suggested that it may contribute enamel-related proteins to cementum matrix. By analogy to the enamel-free area (EFA) in rat molars, in which epithelial cells express not only enamel proteins but also "typical" mesenchymal matrix constituents, it has been proposed that HERS cells may also have the potential to produce cementum proteins. To test this hypothesis, we examined the nature of the first matrix layer deposited along the cervical portion of root dentin and the characteristics of the associated cells. Rat molars were processed for postembedding colloidal gold immunolabeling with antibodies to amelogenin (AMEL), ameloblastin (AMBN), bone sialoprotein (BSP), and osteopontin (OPN). To minimize the possibility of false-negative results, several antibodies to AMEL were used. The labelings were compared with those obtained at the EFA. Initial cementum matrix was consistently observed at a time when epithelial cells from HERS covered most of the forming root surface. Cells with mesenchymal characteristics were rarely seen in proximity to the matrix. Both the EFA matrix and initial cementum exhibited collagen fibrils and were intensely immunoreactive for BSP and OPN. AMEL and AMBN were immunodetected at the EFA but not over the initial cementum proper. These two proteins were, however, present at the cervical-most portion of the root where enamel matrix extends for a short distance between dentin and cementum. These data suggest that epithelial cells along the root surface are likely responsible for the deposition of the initial cementum matrix and therefore, like the cells at the EFA, may be capable of producing mesenchymal proteins.

Immunolocalization of CD44 and the ezrin-radixin-moesin (ERM) family in the stratum intermedium and papillary layer of the mouse enamel organ.

We studied the immunohistochemical localization of CD44 and the ezrin-radixin-moesin (ERM) family of actin binding proteins in mouse enamel organ, using confocal laser scanning microscopy and transmission electron microscopy to clarify their role in cytoskeletal organization. At the differentiation stage of ameloblasts, immunoreactivity to CD44 was detected on the plasma membrane of the inner enamel epithelium, the stellate reticulum, the stratum intermedium, and the external enamel epithelium. In accordance with the differentiation of preameloblasts into secretory ameloblasts, immunoreactivity increased in the stratum intermedium cells. At the maturation stage, intense immunoreactivity was observed on the papillary layer cells. For the ERM family, the stratum intermedium and the papillary layer cells were stained with anti-ezrin and -radixin monoclonal antibodies but not with the anti-moesin antibody. Electron microscopic observations revealed that CD44, ezrin, and radixin were localized in the region at which preameloblasts came into contact with the stratum intermedium at the differentiation stage. At the secretory and maturation stages, they were concentrated in the microvilli of the stratum intermedium and the papillary layer cells. These findings suggest that the CD44-ezrin-radixin-actin filament system is involved in cell-cell interaction between preameloblasts and the stratum intermedium, and in the cytoskeletal organization of the cells in the stratum intermedium and the papillary layer.

Immunohistochemical localization of connexin43 in the enamel organ of the rat upper incisor during ameloblast development.

We examined the localization of connexin (Cx)43 in the enamel organ during ameloblast development. The specificity of monoclonal anti-Cx43 antibody was elucidated by immunoblot analysis and immunoelectron microscopy. Gold particles for Cx43 were detected by immunoelectron microscopy on gap junctions but not on other structures such as desmosomes. Punctate and intense immunofluorescence for Cx43 was detected in all cell types of the enamel organ. Cx43 expression in ameloblasts showed a transient decrease and then increase during ameloblast development. Double staining of Cx43 and amelogenin, one of the enamel proteins, revealed that immunofluorescence for Cx43 markedly decreased in some late presecretory ameloblasts just prior to enamel formation. Moreover, the localization of Cx43 changed during enamel formation. Cx43 was distributed randomly on the lateral plasma membranes of presecretory ameloblasts, but tended to gather on those corresponding to the supranuclear regions of secretory ameloblasts. Immunofluorescence for Cx43 in maturation ameloblasts appeared linear rather than punctate. These results suggest that Cx43 in the late presecretory ameloblasts is degraded just before enamel formation and then newly synthesized Cx43 is redistributed during the secretory stage. These changes in Cx43 expression may be related to the cellular differentiation of ameloblasts.

The rat amelogenin gene--some aspects of evolution and expression.

This study presents data to support the hypothesis that a major portion of the coding sequence of the amelogenin gene may have arisen by tandem duplication of internal sequences which as a consequence has introduced several additional potential RNA splice acceptor sites into the sequence. This duplication of splice sites has led to an increase in the heterogeneity of amelogenin forms found in developing enamel. By screening a rat enamel organ cDNA library for alternatively spliced products, it appears that as much as 20% of the amelogenin mRNA molecules may be alternatively spliced forms.

In vitro effects of retinoic acid on mouse incisor development.

The developing dentition is known to express the complete set of retinoic acid (RA) nuclear receptors and cytoplasmic RA-binding proteins (CRABPI and II), and RA is required for in vitro mouse molar morphogenesis, so the role of RA during in vitro mouse incisor development was investigated. Histological procedures, immunocytochemical detection of proliferating cells, immunofluorescence detection of laminin, and in situ hybridization with RNA probes for CRABPI and II were done on the tooth-germ cultures either in the presence or in the absence of RA. RA appeared to control initial morphogenesis, particularly the asymmetrical growth of the cervical loop, and to regulate required differential mitotic activity. RA seemed also to be involved in asymmetrical laminin deposition. The distribution of the CRABP gene transcripts was similar during in vivo and in vitro incisor development. However, CRABPI gene transcript distribution in the labial part of the epithelial loop was detected in vitro only in the presence of RA. A direct role of the CRABPs during tooth development is, however, unlikely because Ch55, a synthetic RA analogue that does not bind to CRABP, had the same effects as RA on in vitro incisor development.

Immunohistochemical demonstration of bcl-2 protein in human tooth germs.

This study sought to detect patterns of bcl-2 protein expression that could provide more insight into the cellular dynamics of tooth development. As bcl-2 serves to prevent cell death, its occurrence in odontogenic tissues might be helpful in identifying cell populations from which odontogenic tumours may arise. The bcl-2 protein was found only in the epithelial part of the tooth germ and was present in all parts of the enamel organ except the ameloblast. This suggests that bcl-2 protein plays a part in maintaining the viability of the enamel organ. The presence of bcl-2 in the fully matured tooth germ and adjacent dental lamina might indicate that epithelial odontogenic tumours may originate from various parts of the enamel organ.

A2a adenosine receptor gene expression in developing rat brain.

Adenosine is a neuromodulator in the adult central nervous system. Membrane-bound receptors for adenosine have been identified and cDNAs for A1, A2a, A2b, and A3 adenosine receptor subtypes have been cloned recently. The present study documents the developmental appearance of A2a adenosine receptor gene expression in the rat brain. In situ hybridization using 35S-labeled RNA probes generated from the rat A2a adenosine receptor cDNA revealed receptor gene expression in the striatum on gestational day (GD) 14. Developmental alterations in the pattern of receptor gene expression within the striatum suggest that this receptor mRNA is expressed by striatal neurons soon after they complete neurogenesis. Transient expression of the A2a adenosine receptor mRNA was observed in cerebral cortex, subiculum, parafascicularis nucleus of the thalamus, facial nucleus, trigeminal nucleus, locus coeruleus, area postrema, anterior pituitary gland and in the fetal cerebral vasculature. The ganglia of cranial nerves V, VII, VIII, IX and X expressed A2a adenosine receptor mRNA in fetuses; adults have not been examined. A2a adenosine receptor mRNA was expressed in the carotid body and intermediate lobe of the pituitary during development and also in adult rats. Northern blot analysis revealed that the A2a adenosine receptor transcript is consistent in size (ca 2.5 kb) across the developmental period examined (GD 14 through adult). Previous studies in adult rats have reported that A2a adenosine receptor gene expression is limited to a population of striatal medium spiny neurons. This study documents early developmental expression of the A2a adenosine receptor gene in the striatum and its transient expression elsewhere in the brain and cerebral vasculature. If the A2a adenosine receptor mRNA is translated into receptor protein shortly after the mRNA is expressed, adenosine could influence neuronal differentiation, migration, synaptogenesis, and angiogenesis. Expression of A2a adenosine receptor mRNA in cranial ganglia, carotid body, and intermediate lobe of the pituitary gland similarly suggests novel sites of adenosine action during development and in the adult.