Stacked endoplasmic reticulum sheets are connected by helicoidal membrane motifs.

The endoplasmic reticulum (ER) often forms stacked membrane sheets, an arrangement that is likely required to accommodate a maximum of membrane-bound polysomes for secretory protein synthesis. How sheets are stacked is unknown. Here, we used improved staining and automated ultrathin sectioning electron microscopy methods to analyze stacked ER sheets in neuronal cells and secretory salivary gland cells of mice. Our results show that stacked ER sheets form a continuous membrane system in which the sheets are connected by twisted membrane surfaces with helical edges of left- or right-handedness. The three-dimensional structure of tightly stacked ER sheets resembles a parking garage, in which the different levels are connected by helicoidal ramps. A theoretical model explains the experimental observations and indicates that the structure corresponds to a minimum of elastic energy of sheet edges and surfaces. The structure allows the dense packing of ER sheets in the restricted space of a cell.

Tlx1 regulates acinar and duct development in mouse salivary glands.

Tlx1 encodes a transcription factor expressed in several craniofacial structures of developing mice. The role of Tlx1 in salivary gland development was examined using morphological and immunohistochemical analyses of Tlx1 null mice. Tlx1 is expressed in submandibular and sublingual glands but not parotid glands of neonatal and adult male and female C57Bl/6J (Tlx1+/+ ) mice. TLX1 protein was localized to the nuclei of terminal tubule cells, developing duct cells and mesenchymal cells in neonatal submandibular and sublingual glands, and to nuclei of duct cells and connective tissue cells in adult glands. Occasionally, TLX1 was observed in nuclei of epithelial cells in or adjacent to the acini. Submandibular glands were smaller and sublingual glands were larger in size in mutant mice (Tlx1-/- ) compared to wild-type mice. Differentiation of terminal tubule and proacinar cells of neonatal Tlx1-/- submandibular glands was abnormal; expression of their characteristic products, submandibular gland protein C and parotid secretory protein, respectively, was reduced. At 3 weeks postnatally, terminal tubule cells at the acinar-intercalated duct junction were poorly developed or absent in Tlx1-/- mice. Granular convoluted ducts in adult mutant mice were decreased, and epidermal growth factor and nerve growth factor expression were reduced. Along with normal acinar cell proteins, adult acinar cells of Tlx1-/- mice continued to express neonatal proteins and expressed parotid proteins not normally present in submandibular glands. Sublingual gland mucous acinar and serous demilune cell differentiation were altered. Tlx1 is necessary for proper differentiation of submandibular and sublingual gland acinar cells, and granular convoluted ducts. The mechanism(s) underlying Tlx1 regulation of salivary gland development and differentiation remains unknown.

Expression of aquaporin 5 during murine palatine glands development: a light and electron microscopic immunocytochemical study.

Although aquaporin 5 (AQP5) seems to play a role in cytodifferentiation and cell proliferation during the development of salivary glands, its distribution during minor salivary glands development has been scarcely reported. This study examined the temporal-spatial distribution of AQP5 in the developing rat palatine glands using light and electron microscopy. At embryonic (E) age E18, AQP5 labeling was observed on the cell membranes of some terminal bulb cells. After lumenization at E20, AQP5 labeled the apical membrane in acini where a lumen existed, in addition to displaying positive diffuse cytoplasmic and cell membrane staining. At the electron microscopic level, AQP5 labeled the supranuclear cytoplasm and the luminal microvilli along the apical membrane. At birth, AQP5 was also localized to the lateral membranes associated ultrastructurally with the microvilli of intercellular canaliculi. After postnatal (PN) day PN7, mucous acini and serous demilunes showed reactivity. AQP5 reached peak reactivity around PN13 with a similar staining pattern in all acini, but had reduced dramatically by PN21. Thereafter, AQP5 reactivity was mainly associated with serous cells in adults. In conclusion, the transitory expression of AQP5 during palatine glands development may reflect changing physiological functions of the secretory cells and/or AQP5 throughout the maturation of the glands.

Response of the mouse sublingual gland to spaceflight.

The ultrastructure and immunohistochemistry of secretory proteins of sublingual glands were studied in mice flown on the US space shuttles Discovery [Space Transportation System (STS)-131] and Atlantis (STS-135). No differences in mucous acinar or serous demilune cell structure were observed between sublingual glands of ground (control) and flight mice. In contrast, previous studies showed autophagy and apoptosis of parotid serous acinar cells in flight mice. The expression of parotid secretory protein (PSP) in sublingual demilune cells of STS-131 flight mice was significantly increased compared with ground (control) mice but decreased in STS-135 flight mice. Similarly, expression of mucin (MUC-19) in acinar cells and expression of the type II regulatory subunit of protein kinase A (PKA-RII) in demilune cells were increased in STS-131 flight mice and decreased in STS-135 flight mice, but not significantly. Demilune cell and parotid protein (DCPP) was slightly decreased in mice from both flights, and nuclear PKA-RII was slightly increased. These results indicate that the response of salivary glands to spaceflight conditions varies among the different glands, cell types, and secretory proteins. Additionally, the spaceflight environment, including the effects of microgravity, modifies protein expression. Determining changes in salivary proteins may lead to development of non-invasive methods to assess the physiological status of astronauts.

Localization of cystic fibrosis transmembrane conductance regulator signaling complexes in human salivary gland striated duct cells.

The cystic fibrosis transmembrane conductance regulator (CFTR) is a cyclic AMP-dependent protein kinase (PKA)-regulated Cl(-) channel, crucial for epithelial cell regulation of salt and water transport. Previous studies showed that ezrin, an actin binding and A-kinase anchoring protein (AKAP), facilitates association of PKA with CFTR. We used immunohistochemistry and immunogold transmission electron microscopy to localize CFTR, ezrin, and PKA type II regulatory (RII) and catalytic (C) subunits in striated duct cells of human parotid and submandibular glands. Immunohistochemistry localized the four proteins mainly to the apical membrane and the apical cytoplasm of striated duct cells. In acinar cells, ezrin localized to the luminal membrane, and PKA RII subunits were present in secretory granules, as previously described. Immunogold labeling showed that CFTR and PKA RII and C subunits were localized to the luminal membrane and associated with apical granules and vesicles of striated duct cells. Ezrin was present along the luminal membrane, on microvilli and along the junctional complexes between cells. Double labeling showed specific protein associations with apical granules and vesicles and along the luminal membrane. Ezrin, CFTR, and PKA RII and C subunits are co-localized in striated duct cells, suggesting the presence of signaling complexes that serve to regulate CFTR activity.

The sld genetic defect: two intronic CA repeats promote insertion of the subsequent intron and mRNA decay.

The autosomal recessive mutation, sld, attenuates mucous cell expression in murine sublingual glands with corresponding effects on mucin 19 (Muc19). We conducted a systematic study including genetic mapping, sequencing, and functional analyses to elucidate a mutation to explain the sld phenotype in neonatal mice. Genetic mapping and gene expression analyses localized the sld mutation within the gene Muc19/Smgc, specifically attenuating Muc19 transcripts, and Muc19 knock-out mice mimic the sld phenotype in neonates. Muc19 transcription is unaffected in sld mice, whereas mRNA stability is markedly decreased. Decreased mRNA stability is not due to a defect in 3'-end processing nor to sequence differences in Muc19 transcripts. Comparative sequencing of the Muc19/Smgc gene identified four candidate intronic mutations within the Muc19 coding region. Minigene splicing assays revealed a novel splicing event in which insertion of two additional repeats within a CA repeat region of intron 53 of the sld genome enhances retention of intron 54, decreasing the levels of correctly spliced transcripts. Moreover, pateamine A, an inhibitor of nonsense-mediated mRNA decay, inhibits degradation of aberrant Muc19 transcripts. The mutation in intron 53 thus enhances aberrant splicing leading to degradation of aberrant transcripts and decreased Muc19 message stability, consistent with the sld phenotype. We propose a working model of the unique splicing event enhanced by the mutation, as well as putative explanations for the gradual but limited increase in Muc19 glycoprotein expression and its restricted localization to subpopulations of mucous cells in sld mice during postnatal gland development.

RANBP2 is an allosteric activator of the conventional kinesin-1 motor protein, KIF5B, in a minimal cell-free system.

The association of cargoes to kinesins is thought to promote kinesin activation, yet the validation of such a model with native cargoes is lacking because none is known to activate kinesins directly in an in vitro system of purified components. The RAN-binding protein 2 (RANBP2), through its kinesin-binding domain (KBD), associates in vivo with kinesin-1, KIF5B/KIF5C. Here, we show that KBD and its flanking domains, RAN GTPase-binding domains 2 and 3 (RBD2/RBD3), activate the ATPase activity of KIF5B approximately 30-fold in the presence of microtubules and ATP. The activation kinetics of KIF5B by RANBP2 is biphasic and highly cooperative. Deletion of one of its RBDs lowers the activation of KIF5B threefold and abolishes cooperativity. Remarkably, RBD2-KBD-RBD3 induces unfolding and modest activation of KIF5B in the absence of microtubules. Hence, RANBP2 is the first native and positive allosteric activator known to jump-start and boost directly the activity of a kinesin.

A contractile nuclear actin network drives chromosome congression in oocytes.

Chromosome capture by microtubules is widely accepted as the universal mechanism of spindle assembly in dividing cells. However, the observed length of spindle microtubules and computer simulations of spindle assembly predict that chromosome capture is efficient in small cells, but may fail in cells with large nuclear volumes such as animal oocytes. Here we investigate chromosome congression during the first meiotic division in starfish oocytes. We show that microtubules are not sufficient for capturing chromosomes. Instead, chromosome congression requires actin polymerization. After nuclear envelope breakdown, we observe the formation of a filamentous actin mesh in the nuclear region, and find that contraction of this network delivers chromosomes to the microtubule spindle. We show that this mechanism is essential for preventing chromosome loss and aneuploidy of the egg--a leading cause of pregnancy loss and birth defects in humans.

Distribution of dendritic cells in normal human salivary glands.

Dendritic cells (DC) are believed to contribute to development of autoimmune sialadenitis, but little is known about their distribution in normal salivary glands. In this study, DC were identified and their distribution was determined in normal human parotid and submandibular glands. For light microscopy, salivary gland sections were stained with H&E or immunocytochemically using antibodies to DC markers. Transmission electron microscopy (TEM) was used to evaluate the ultrastructural characteristics of DC. In H&E sections, elongated, irregularly shaped nuclei were occasionally seen in the striated and excretory duct epithelium. Immunolabeling with anti-HLA-DR, anti-CD11c and anti-S100 revealed DC with numerous processes extending between ductal epithelial cells, often close to the lumen. Morphometric analyses indicated that HLA-DR-positive DC occupied approximately 4-11% of the duct wall volume. Similar reactive cells were present in acini, intercalated ducts and interstitial tissues. TEM observations revealed cells with indented nuclei containing dense chromatin, pale cytoplasm with few organelles, and lacking junctional attachments to adjacent cells. These results indicate that DC are abundant constituents of normal human salivary glands. Their location within ductal and acinar epithelium suggests a role in responding to foreign antigens and/or maintaining immunological tolerance to salivary proteins.

Odontoblast processes of the mouse incisor are plates oriented in the direction of growth.

Odontoblast processes are thin cytoplasmic projections that extend from the cell body at the periphery of the pulp toward the dentin-enamel junction. The odontoblast processes function in the secretion, assembly and mineralization of dentin during development, participate in mechanosensation, and aid in dentin repair in mature teeth. Because they are small and densely arranged, their three-dimensional organization is not well documented. To gain further insight into how odontoblast processes contribute to odontogenesis, we used serial section electron microscopy and three-dimensional reconstructions to examine these processes in the predentin region of mouse molars and incisors. In molars, the odontoblast processes are tubular with a diameter of ~1.8 µm. The odontoblast processes near the incisor tip are similarly shaped, but those midway between the tip and apex are shaped like plates. The plates are radially aligned and longitudinally oriented with respect to the growth axis of the incisor. The thickness of the plates is approximately the same as the diameter of molar odontoblast processes. The plates have an irregular edge; the average ratio of width (midway in the predentin) to thickness is 2.3 on the labial side and 3.6 on the lingual side. The plate geometry seems likely to be related to the continuous growth of the incisor and may provide a clue as to the mechanisms by which the odontoblast processes are involved in tooth development.

Regulation of meiotic prophase arrest in mouse oocytes by GPR3, a constitutive activator of the Gs G protein.

The arrest of meiotic prophase in mouse oocytes within antral follicles requires the G protein G(s) and an orphan member of the G protein-coupled receptor family, GPR3. To determine whether GPR3 activates G(s), the localization of Galpha(s) in follicle-enclosed oocytes from Gpr3(+/+) and Gpr3(-/-) mice was compared by using immunofluorescence and Galpha(s)GFP. GPR3 decreased the ratio of Galpha(s) in the oocyte plasma membrane versus the cytoplasm and also decreased the amount of Galpha(s) in the oocyte. Both of these properties indicate that GPR3 activates G(s). The follicle cells around the oocyte are also necessary to keep the oocyte in prophase, suggesting that they might activate GPR3. However, GPR3-dependent G(s) activity was similar in follicle-enclosed and follicle-free oocytes. Thus, the maintenance of prophase arrest depends on the constitutive activity of GPR3 in the oocyte, and the follicle cell signal acts by a means other than increasing GPR3 activity.

Amylase and cyclic amp receptor protein expression in human diabetic parotid glands.

BACKGROUND: Salivary dysfunction and oral disorders have been described in both type 1 and type 2 diabetes mellitus. However, the cellular and molecular consequences of diabetes on oral tissues remain to be ascertained. The purpose of this investigation was to study, by means of electron microscopy, the morphologic and molecular changes that occur in salivary glands during diabetes. METHODS: Biopsy samples of parotid glands were excised from non-diabetic and diabetic (type 1 and type 2) consenting patients and processed by standard methods for routine morphology and electron microscopic immunogold labeling. Specific antibodies were used to determine and quantify the expression of secretory proteins (alphaamylase and the regulatory subunit of type II protein kinase A). RESULTS: Morphologic changes in the diabetic samples included increased numbers of secretory granules, and alterations in internal granule structure. Quantitative analysis of immunogold labeling showed that labeling densities were variable among the parotid gland samples. In type 1 diabetes amylase expression was greater than in non-diabetic glands, whereas in type 2 diabetes it was not significantly changed. Expression of type II regulatory subunits was slightly, although not significantly, increased in acinar secretory granules of type 1 diabetic samples and was unchanged in type 2 diabetic samples. CONCLUSIONS: Our data show that diabetes elicits specific changes in secretory protein expression in human salivary glands, thus contributing to the altered oral environment and oral disease associated with diabetes.

Ultrastructural localization of salivary mucins MUC5B and MUC7 in human labial glands.

As a result of their presence throughout the mouth in the submucosa or between muscle fibers, minor salivary glands secrete directly and continuously into the oral cavity, providing mucosal surfaces with highly glycosylated proteins that are active in bacterial aggregation and in oral tissue lubrication. In this study, we investigated the ultrastructural localization of the MUC5B and MUC7 mucins in human labial glands by means of a postembedding immunogold technique. Thin sections of normal human labial glands, obtained during surgery, were incubated with polyclonal antibodies to human salivary mucins MUC5B and MUC7, and then with gold-labeled secondary antibodies. Specific MUC5B reactivity was found in the secretory granules of mucous cells of all glands examined, and was associated with the luminal membrane of duct cells. MUC7 labeling was observed in the granules of both mucous and seromucous secretory cells of the glandular parenchyma. Quantitative analyses demonstrated that seromucous granules have higher immunogold labeling densities for MUC7 than mucous granules. Our immunohistochemical data extend the results of previous light microscopic studies of MUC5B and MUC7 localizations, pointing out the significant contribution of human labial glands in the secretion process of these two mucins.

Effects of spaceflight on the mouse submandibular gland.

OBJECTIVE: This study was conducted to determine if the morphology and biochemistry of the mouse submandibular gland is affected by microgravity and the spaceflight environment. DESIGN: Tissues from female mice flown on the US space shuttle missions Space Transportation System (STS)-131 and STS-135 for 15 and 13 d, respectively, and from male mice flown on the 30 d Russian Bion-M1 biosatellite, were examined using transmission electron microscopy and light and electron microscopic immunohistochemistry. RESULTS: In contrast to the parotid gland, morphologic changes were not apparent in the submandibular gland. No significant changes in protein expression, as assessed by quantitative immunogold labeling, occurred in female mice flown for 13-15 d. In male mice, however, increased labeling for salivary androgen binding protein alpha (in acinar cell secretory granules), and epidermal growth factor and nerve growth factor (in granular convoluted duct cell granules) was seen after 30 d in space. CONCLUSION: These results indicate that spaceflight alters secretory protein expression in the submandibular gland and suggest that the sex of the animals and the length of the flight may affect the response. These findings also show that individual salivary glands respond differently to spaceflight. Saliva contains proteins secreted from salivary glands and is easily collected, therefore is a useful biofluid for general medical analyses and in particular for monitoring the physiology and health of astronauts.

Expression of Muc19/Smgc gene products during murine sublingual gland development: cytodifferentiation and maturation of salivary mucous cells.

Muc19/Smgc expresses two splice variants, Smgc (submandibular gland protein C) and Muc19 (mucin 19), the latter a major exocrine product of differentiated murine sublingual mucous cells. Transcripts for Smgc were detected recently in neonatal sublingual glands, suggesting that SMGC proteins are expressed during initial salivary mucous cell cytodifferentiation. We therefore compared developmental expression of transcripts and translation products of Smgc and Muc19 in sublingual glands. We find abundant expression of SMGC within the initial terminal bulbs, with a subsequent decrease as Muc19 expression increases. During postnatal gland expansion, SMGC is found in presumptive newly formed acinar cells and then persists in putative acinar stem cells. Mucin levels increase 7-fold during the first 3 weeks of life, with little change in transcript levels, whereas between postnatal days 21 and 28, there is a 3-fold increase in Muc19 mRNA and heteronuclear RNA. Our collective results demonstrate the direct transition from SMGC to Muc19 expression during early mucous cell cytodifferentiation and further indicate developmentally regulated changes in Muc19/Smgc transcription, alternative splicing, and translation. These changes in Muc19/Smgc gene expression delineate multiple stages of salivary mucous cell cytodifferentiation and subsequent maturation during embryonic gland development through the first 4 weeks of postnatal life.

Protein expression in salivary glands of rats with streptozotocin diabetes.

Diabetes mellitus (DM) is a widespread disease with high morbidity and health care costs. An experimental animal model was employed, using morphological and biochemical methods, to investigate the effects of DM on the expression and compartmentation of salivary gland proteins. The distribution of proline-rich proteins (PRP), submandibular mucin (Muc10) and the regulatory (RI and RII) subunits of cyclic AMP-dependent protein kinase type I and type II was determined in the parotid and submandibular (SMG) glands of rats treated with streptozotocin. Quantitative immunocytochemistry of secretory granules in diabetic glands revealed decreases of 30% for PRP in both the parotid and SMG, and a 40% decrease in Muc10 in the SMG. Immunogold labelling showed that RII decreased in nuclei and the cytoplasm in diabetic acinar cells while labelling of secretory granules was similar in control and diabetic parotid. Electrophoresis and Western blotting of tissue extracts of two secretory proteins showed that the response to DM and insulin treatment was gland specific: PRP showed little change in the SMG, but decreased in the parotid in DM and was partially restored after insulin treatment. Photoaffinity labelling showed only RI present in the SMG and mainly RII in the parotid. The results of this and previous studies demonstrating highly specific changes in salivary protein expression indicate that the oral environment is significantly altered by DM, and that oral tissues and their function can be compromised. These findings may provide a basis for future studies to develop tests using saliva for diabetic status or progression in humans.

Nuclear envelope breakdown in starfish oocytes proceeds by partial NPC disassembly followed by a rapidly spreading fenestration of nuclear membranes.

Breakdown of the nuclear envelope (NE) was analyzed in live starfish oocytes using a size series of fluorescently labeled dextrans, membrane dyes, and GFP-tagged proteins of the nuclear pore complex (NPC) and the nuclear lamina. Permeabilization of the nucleus occurred in two sequential phases. In phase I the NE became increasingly permeable for molecules up to approximately 40 nm in diameter, concurrent with a loss of peripheral nuclear pore components over a time course of 10 min. The NE remained intact on the ultrastructural level during this time. In phase II the NE was completely permeabilized within 35 s. This rapid permeabilization spread as a wave from one epicenter on the animal half across the nuclear surface and allowed free diffusion of particles up to approximately 100 nm in diameter into the nucleus. While the lamina and nuclear membranes appeared intact at the light microscopic level, a fenestration of the NE was clearly visible by electron microscopy in phase II. We conclude that NE breakdown in starfish oocytes is triggered by slow sequential disassembly of the NPCs followed by a rapidly spreading fenestration of the NE caused by the removal of nuclear pores from nuclear membranes still attached to the lamina.

Immunocytochemical analysis of cyclic AMP receptor proteins in the developing rat parotid gland.

UNLABELLED: Previous studies showed that regulatory subunits of type II cyclic AMP-dependent protein kinase (RII) are present in adult rat parotid acinar cells, and are secreted into saliva. If the synthesis and intracellular distribution of RII exhibit developmental specificity, then RII can be an indicator of secretory and regulatory activity of salivary glands. OBJECTIVE: To determine the expression and distribution of RII in the rat parotid at specific ages representing defined developmental stages. METHODS: Parotid glands of fetal, neonatal and adult rats were prepared for morphologic and immunocytochemical study. The cellular distribution of RII was studied using light microscopic immunogold silver staining with anti-RII, and its intracellular distribution using electron microscopic immunogold labeling. RESULTS: In utero, parotid RII levels were low; 5-18 days after birth, labeling of secretory granules and cytoplasm rose to a peak, followed by a rapid decrease in both compartments at 25 days. At 60 days, granule labeling increased to levels near those at 18 days, whereas cytoplasmic labeling remained low. Nuclear labeling was highest during the first 3 weeks after birth, and then declined. CONCLUSIONS: The higher nuclear and cytoplasmic labeling during the neonatal period may reflect RII involvement in acinar cell differentiation. The accumulation of RII in secretory granules is similar to the pattern of the major salivary proteins, amylase and PSP. The redistribution of RII in these compartments during development may reflect changing gene expression patterns, and may be useful for identification of genetic or metabolic abnormalities.

Developmental Morphology of the Palatine Glands in Rats: An Electron Microscope Study.

Although minor salivary glands play a significant functional role in the oral cavity, their developmental morphology and cell differentiation has been scarcely studied. This study aimed to describe the development of rat palatine glands with regard to the ultrastructural morphology of the secretory cells and surrounding myoepithelial cells (MECs). Palatine glands from rats at embryonic ages (E) 18 and 20 days, and postnatal days (PN) 0, 3, 7, 10, 13, 21, 30, 42, and 60 were fixed and prepared for morphological analysis and immunocytochemical labeling of alpha-smooth muscle actin (α-SMA). At E18, epithelial cords were observed extending from the palatal epithelium and showed negative reactivity to α-SMA. After luminization at E20, the cells of immature acini accumulated secretory granules of various densities: electron-dense, electron-lucent and some empty-appearing granules. MECs were poorly differentiated at E20 and exhibited only slight α-SMA expression. At birth, mucous and serous cells were typically located around a common lumen. Thereafter, serous cells began to move to the periphery to form demilunes by PN7. The mucous secretory granules of intermediate electron density became predominant around PN13. At PN21, these granules were dramatically reduced in number and most of the acini in adults contained acinar cells with numerous electron-lucent granules, and a few serous demilune cells with electron-dense granules. After birth, MECs progressively accumulated actin microfilaments until prominent α-SMA expressing MECs invested the acini and the proximal part of the intercalated ducts in the adult. Anat Rec, 301:1820-1833, 2018. © 2018 Wiley Periodicals, Inc.

Responses to spaceflight of mouse mandibular bone and teeth.

OBJECTIVE: To determine if spaceflight and microgravity affect non-weight bearing bones and development and mineralization of teeth, reasoning that combining an organ and a cellular level approach can lead to greater insights about these effects. DESIGN: Mandibles and incisors of mice flown on the US STS-135 space shuttle mission and the Russian Bion-M1 satellite were studied using micro-computed tomography and immunohistochemistry. Ground controls were mice housed in standard vivarium cages and flight habitats. RESULTS: Incisor length was greater in the 13-day STS-135 flight mice than in either control group. Initial incisor mineralization occurred more posteriorly, and incisor, enamel and dentin volumes and enamel and dentin thicknesses were greater in the 30-day Bion-M1 flight and habitat control mice than in vivarium control mice. Mandibular bone volume (BV) was increased in STS-135 flight and habitat groups and decreased in Bion-M1 flight and habitat groups compared to vivarium controls. No significant histological alterations occurred, but changes were seen in the bone and tooth proteins dentin sialoprotein, amelogenin and the type II regulatory subunit of protein kinase A. The percentage of sclerostin positive osteocytes was greatest in flight mice, and greater in STS-135 flight and habitat control mice than in the corresponding Bion-M1 groups. TRAP staining, representing osteoclastic bone remodeling, differed between the two flights and corresponded with changes in BV. Interpretation of the findings was limited by a small number of flight mice, different sex and ages of the mice in the two missions, and different habitats and diets. CONCLUSIONS: Microgravity has measurable effects on mandibular bone physiology and incisor development and mineralization. The results also showed that the habitat had an effect either in flight or ground control samples, as demonstrated by the changes in BV and apparent slowing of incisor eruption. Therefore, developing appropriate habitats is critical for future spaceflight missions.