Hepatocyte growth factor stimulates root growth during the development of mouse molar teeth

BACKGROUND AND OBJECTIVE

It is well known that tooth root formation is initiated by the development of Hertwig's epithelial root sheath (HERS). However, relatively little is known about the regulatory mechanisms involved in root development. As hepatocyte growth factor (HGF) is one of the mediators of epithelial-mesenchymal interactions in rodent tooth, the objective of this study was to examine the effects of HGF on the root development of mouse molars.

MATERIAL AND METHODS

The HERS of mouse molars and HERS01a, a cell line originated from HERS, were used in this study. For detection of HGF receptors in vivo and in vitro, we used immunochemical procedures. Root development was assessed by implanting molar tooth germs along with HGF-soaked beads into kidney capsules, by counting cell numbers in HERS01a cell cultures and by performing a 5'-bromo-2'-deoxyuridine (BrdU) assay in an organ-culture system.

RESULTS

HGF receptors were expressed in the enamel epithelium of molar germs as well as in HERS cells. HGF stimulated root development in the transplanted tooth germs, the proliferation of HERS01a cells in culture and HERS elongation in the organ-culture system. Examination using BrdU revealed that cell proliferation in HERS was increased by treatment with HGF, especially that in the outer layer of HERS. This effect was down-regulated when antibody against HGF receptor was present in the culture medium.

CONCLUSION

Our results raise the possibility that HGF signaling controls root formation via the development of HERS. This study is the first to show that HGF is one of the stimulators of root development.

Origin-associated features of chondrocytes in mouse Meckel's cartilage and costal cartilage: an in vitro study

Using a cell culture method, we histochemically and immunohistochemically investigated whether chondrocytes deriving from different origins, such as Meckel's or costal cartilages, express similar phenotypic characteristics. Chondrocytes isolated enzymatically from Meckel's and costal cartilages of 17-day embryonic mice both actively proliferated and formed cartilage nodules consisting of toluidine blue-positive proteoglycans and type II collagen. Both deposited calcified cartilaginous matrix as revealed by alkaline phosphatase (ALPase) activity and alizarin red staining throughout 3 weeks in culture. Immunostaining for osteopontin (OP), osteocalcin (OC), and osteonectin (ON) revealed that chondrocytes from both cartilages were positive for their proteins, but type I collagen was detected only in cells transforming from Meckel's chondrocytes late in the culture. Electron microscopy demonstrated that although costal and Meckel's chondrocytes had typical chondrocytic features during 2 weeks in culture, Meckel's chondrocytes transformed into osteocytic cells that produced thick, banded type I collagen fibrils. In contrast, costal chondrocytes maintained typical hypertrophic morphology throughout the final stage of culture. The present study suggests that Meckel's chondrocytes derived from neural crest-ectomesenchyme retain osteogenic potential, and differ from costal chondrocytes originating from mesoderm.

Formation of the sphenomandibular ligament by Meckel's cartilage in the mouse: possible involvement of epidermal growth factor as revealed by studies in vivo and in vitro

In mammals, the midportion of the soft tissue of Meckel's cartilage at the degenerating stage forms a ligament known as the sphenomandibular ligament. To clarify the mechanism of formation of this ligament by Meckel's cartilage in mouse, we examined the effects of epidermal growth factor (EGF) on the chondrocytes in terms of the proliferation and differentiation of cells and calcification of the matrix in vivo and in vitro. The effects of EGF were examined by immunohistochemical staining, with EGF-soaked beads, by electron microscopy, and by general histochemical analysis of proteoglycans and calcification. Analysis of labeling with bromodeoxyuridine (BrdU) and the rate of cell growth revealed that EGF enhanced DNA synthesis and the proliferation of Meckel's chondrocytes. Histological findings in organ culture and in cell culture, with and without the application of EGF-soaked beads, revealed that EGF inhibited the differentiation of cells to chondrocytes and induced phenotypic changes in fibroblastic cells. The inhibition of alkaline phosphatase activity that resulted from exposure to EGF was accompanied by prolonged calcification of the matrix. Whole-mount staining revealed that subcutaneous injection of EGF enhanced the disappearance of Meckel's cartilage. Our results suggest a possible mechanism whereby the midportion of Meckel's cartilage remains uncalcified and is rapidly transformed into the sphenomandibular ligament.

Further evidence for secretion of matrix metalloproteinase-1 by Meckel's chondrocytes during degradation of the extracellular matrix

We examined the possibility that chondrocytes in Meckel's cartilage might secrete matrix metalloproteinase-1 (MMP-1) during degradation of the extracellular matrix. Evidence for the secretion of MMP-1 was obtained by immunohistochemical staining and immunoelectron microscopy, in addition to general histochemical staining for proteoglycans. Not only staining with toluidine blue and alcian blue but also immunostaining for chondroitin sulfate proteoglycan (CSPG) revealed that levels of glycoproteins are rapidly reduced at the late stage of degradation. MMP-1 was detected continuously in cells from chondrocytes at the early stage to hypertrophic chondrocytes at the late stage. Immunoelectron microscopy revealed that the deposition of colloidal golds shifted from an intracellular localization in chondrocytes at the early stage to pericellular spaces at the late stage. The localization of tissue inhibitor of the metalloproteinase-1 (TIMP-1) at the early stage was similar to that of MMP-1, but the level of TIMP-1 decreased significantly in hypertrophic cartilage. These findings suggest that MMP-1 is present continuously in Meckel's chondrocytes but that the active form, which degrades the extracellular matrix, is the MMP-1 that accumulates in the pericellular spaces around hypertrophic chondrocytes.

Histochemical and immunohistochemical analysis of the mechanism of calcification of Meckel's cartilage during mandible development in rodents

It is widely accepted that Meckel's cartilage in mammals is uncalcified hyaline cartilage that is resorbed and is not involved in bone formation of the mandible. We examined the spatial and temporal characteristics of matrix calcification in Meckel's cartilage, using histochemical and immunocytochemical methods, electron microscopy and an electron probe microanalyser. The intramandibular portion of Meckel's cartilage could be divided schematically into anterior and posterior portions with respect to the site of initiation of ossification beneath the mental foramen. Calcification of the matrix occurred in areas in which alkaline phosphatase activity could be detected by light and electron microscopy and by immunohistochemical staining. The expression of type X collagen was restricted to the hypertrophic cells of intramandibular Meckel's cartilage, and staining with alizarin red and von Kossa stain revealed that calcification progressed in both posterior and anterior directions from the primary centre of ossification. After the active cellular resorption of calcified cartilage matrix, new osseous islands were formed by trabecular bone that intruded from the perichondrial bone collar. Evidence of such formation of bone was supported by results of double immunofluorescence staining specific for type I and type II collagens, in addition to results of immunostaining for osteopontin. Calcification of the posterior portion resembled that in the anterior portion of intramandibular Meckel's cartilage, and our findings indicate that the posterior portion also contributes to the bone formation of the mandible by an endochondral-type mechanism of calcification.

Expression of osteopontin in Meckel's cartilage cells during phenotypic transdifferentiation in vitro, as detected by in situ hybridization and immunocytochemical analysis

The localization of osteopontin (OP) was examined in Meckel's cartilage cells that bipotentially expressed cartilage and bone phenotypes during cellular transformation in vitro. Cultured cells were analyzed by in situ hybridization, immunostaining followed by light and electron microscopy, electron microscopy, and electron probe microanalysis. The combination of ultrastructural analysis and immunoperoxidase staining indicated that OP-synthesizing cells were cells that were autonomously undergoing a change from chondrocytes to bone-forming cells at the top of nodules. Double immunofluorescence staining of 2-week-old cultures revealed that OP was first synthesized by chondrocytic cells at the top of nodules. After further time in culture, the distribution of OP expanded from the central toward the peripheral regions of the nodules. Electron probe microanalysis revealed that the localization of OP was associated with matrices of calcified cartilage and osteoid nodules that contained calcium and phosphorus. Immunoperoxidase electron microscopy revealed that, in addition to the intracellular immunoreactivity in chondrocytes and small round cells that were undergoing transformation, matrix foci of calcospherites and matrix vesicles, in particular, included growing crystals that were immunopositive for OP. An intense signal due to mRNA for OP in 3-week-old cultures was detected in nodule-forming round cells, while fibroblastic cells, spreading in a monolayer over the periphery of nodules, were only weakly labeled. These findings indicate that OP might be expressed sequentially by chondrocytes and by cells that are transdifferentiating further and exhibit an osteocytic phenotype, and moreover, that expression of OP is closely associated with calcifying foci in the extracellular matrix.

Characterization of the inner enamel epithelium in the enamel-free area based on the ability to secrete enamel protein demonstrated by in situ hybridization and immunohistochemistry

Both the expression of amelogenin mRNA and secretion of amelogenin were investigated in rat molars by in situ hybridization and immunohistochemistry. Probes were designed by multiple-labeling of oligonucleotide probes for in situ hybridization. Amelogenin mRNA first appeared in differentiating ameloblasts of the distal region and some inner enamel epithelial cells of enamel-free area (EFA cells) of the second cusp at postnatal day 0. At the same time, amelogenin protein was detected in the extracellular matrix between dentin and differentiating ameloblasts and in some EFA cells of the second cusp. At postnatal day 1-3, amelogenin was expressed in the secretory ameloblasts, and in the matrix beneath these cells. Both amelogenin mRNA and amelogenin were detected in the EFA cells and their extracellular matrix. After postnatal day 5, amelogenin mRNA and amelogenin were detected in the secretory ameoloblasts and extracellular matrix in the enamel-forming region, respectively. At this time, amelogenin mRNA was not detected in the EFA cells, but a small amount of amelogenin was found in the matrix beneath the EFA cells. These findings suggest that EFA cells differentiate into amelogenin-secreting cells, i.e. ameloblasts, but that the secretion lasts for only a short period at the early stage of tooth development.

Inhibition of endogenous expression of connective tissue growth factor by its antisense oligonucleotide and antisense RNA suppresses proliferation and migration of vascular endothelial cells

Previously, we cloned an mRNA predominantly expressed in hypertrophic chondrocytes by differential display-PCR from a human chondrosarcoma-derived chondrocytic cell line (HCS-2/8) that is identical to that of connective tissue growth factor (CTGF). In the present study, we investigated the roles of CTGF in the proliferation and migration of vascular endothelial cells using its antisense oligonucleotide and antisense RNA, because angiogenesis into the hypertrophic zone of cartilage occurs at the final step of endochondral ossification. Immunohistochemical and immunofluorescence techniques revealed that not only hypertrophic chondrocytes but also endothelial cells in the cost-chondral junctions of mouse ribs were stained with an anti-CTGF antibody in vivo. Northern blot analysis revealed that CTGF was strongly expressed in chondrocytic cells as well as bovine aorta endothelial (BAE) cells in culture, but not in other types of cells such as osteoblastic cells. Its expression in BAE cells was greater in the growing phase than in the confluent phase. When one-half of a monolayer of a confluent culture of BAE cells had been peeled off, only the cells proliferating and extending into the vacant area were stained with the anti-CTGF antibody. The addition of an antisense oligonucleotide inhibited the proliferation and extension of the BAE cells into the vacant area. The antisense oligonucleotide also inhibited the proliferation of BAE cells in the rapidly proliferating phase. In a Boyden chamber assay, pretreatment with the antisense oligonucleotide markedly inhibited the migration of BAE cells. Furthermore, the abilities to proliferate and migrate of BAE cells, which were stably transfected with expression vectors that generate the antisense RNA of CTGF cDNA, were markedly lower than those of the control. These findings suggest that endogenous CTGF expression is involved in the proliferation and migration of BAE cells.

Evidence for transformation of chondrocytes and site-specific resorption during the degradation of Meckel's cartilage

It is unknown whether cells in the midportion of Meckel's cartilage undergo transformation into other kinds of cell or whether resorption of cells occurs during development. Therefore, the midportion of Meckel's cartilage from the mouse and the rat was subdivided into anterior and posterior portions. The ultimate fates of these tissues were analyzed with a focus on resorption -related cells, death of chondrocytes by apoptosis, and transformation of the chondrocytes themselves. Cellular and extracellular features of mouse Meckel's cartilage were observed after von Kossa's staining and staining for acid phosphatase (APase) activity, as well as by light and electron microscopy. To identify resorbing cells, immunostaining specific for macrophages and staining for tartrate-resistant acid phosphatase (TRAP) were performed. The DNA nick end-labeling (TUNEL) method was used for the detection of death of chondrocytes by apoptosis. The replacement of the extracellular matrix of rat Meckel's cartilage was examined with double immunofluorescence staining for type I and type II collagens. When the anterior midportion from embryonic mice on day 18 was examined after von Kossa's staining, it was clear that the extracellular matrix had already calcified and vascularization had been initiated that reflected the calcified matrix. TRAP staining and immunostaining for macrophages revealed two types of osteoclast and macrophages that were involved in resorption of the matrix. In the posterior midportion, no vascular invasion was evident, and chondrocytes were transformed directly into fibroblastic cells by phenotypic conversion. In such cells we found reaction products specific for APase activity, suggestive of the intracellular degradation of fine collagenous fibrils. Double immunofluorescence staining showed that cartilage-specific type II collagen was replaced by type I collagen with the phenotypic transformation to fibroblastic cells. There were no significant changes in the number of TUNEL-positive apoptotic cells from day 17 of gestation to day 6 after parturition. Death of chondrocytes by apoptosis was not, therefore, involved directly in the disappearance of Meckel's cartilage. These results in the posterior midportion served as an instance of phenotypic switches in differentiated cells from chondrocytes to fibroblast-like cells. The present study indicates that there is a difference between the ultimate fate of cells in the posterior part and that of cells in the anterior part in the midportion of Meckel's cartilage in the mouse and rat.

Immunocytochemical expression of type I and type II collagens by rat Meckel's chondrocytes in culture during phenotypic transformation

In culture, chondrocytes of Meckel's cartilage can differentiate further to become bone-type collagen-synthesizing cells. Here, the replacement of type II collagen by type I collagen, accompanying expression of the osteocytic phenotype, was analysed by double immunofluorescence staining, histochemistry and electron microscopy. After 1 week in culture, formation of a toluidine blue-positive matrix, demonstrating the synthesis of cartilaginous proteoglycans, and the expression of type II collagen were detected. After 2 weeks, immunoreactivity specific for type II collagen was detected along the cartilaginous areas of the nodules, and type I collagen appeared in association with the immunopositive extracellular matrix around spindle-shaped cells. Electron microscopy revealed that the extracellular matrix at this stage was composed of homogeneous fine fibrils of type II collagen and thick cross-banded bundles of type I collagen: there was also continuity between the type I and II collagens. Double immunofluorescence staining of 3 week-old cultures revealed that type II collagen had been replaced by type I which was synthesized by small round cells that appeared at the top of the nodules. With further passage of time in culture, the distribution of type I collagen expanded further towards the peripheral areas from the central areas of the nodules. The present combination of ultrastructural analysis and double immunofluorescence staining shows that the transition from synthesis of cartilage-specific type II collagen to expression of type I collagen occurred sequentially in spindle-shaped cells located at the top of nodules in conjunction with the further differentiation of Meckel's cartilage cells.

Ultrastructural and histochemical changes and apoptosis of inner enamel epithelium in rat enamel-free area

The formation of an enamel-free area (EFA), a region of the dentin without an enamel cap at the cusp tip of rodent molar, is thought to depend on the specific differentiation and function of inner enamel epithelium of EFAs (EFA cells). The authors attempted to clarify both the ultrastructure and alkaline phosphatase (ALPase) activity of EFA cells up until tooth eruption by using rat mandibular first molars. Apoptosis was also examined. The EFA cells differentiated into secretory cells resembling differentiating ameloblasts but without Tomes' processes (postnatal day 1-3). No reactivity for ALPase was observed in the EFA cells. Enamel-like crystals were detected in close vicinity to dentin crystals at this stage. Thereafter, EFA cells became maturative ameloblast-like with ruffled border-like structures (postnatal day 5-8) and exhibited a strong reactivity for ALPase. These findings suggest that EFA cells change from secretory to absorptive cells within a short period and become reduced enamel epithelium at the early stage of tooth development. Apoptosis occurs in EFA cells, as it does in ameloblasts, but its significance seems to differ between the two cell types.

A case of pulmonary ossified adenocarcinoma with marked osteoplastic bone metastasis

We report the case of a 53-year-old man suffering from a pulmonary adenocarcinoma with ossification and diffuse metastatic osteoplastic lesions throughout the skeletal system. This is a rare condition in lung carcinomas. Radiographs of the chest and bones demonstrated mineral densities in the primary tumor and multiple expansive osseous lesions with a diffuse sclerotic pattern resembling multiple bone metastases from prostatic carcinoma.

Cellular hypertrophy and calcification of embryonal carcinoma-derived chondrogenic cell line ATDC5 in vitro

During the process of endochondral bone formation, proliferating chondrocytes give rise to hypertrophic cells, which then deposit a mineralized matrix to form calcified cartilage prior to replacement by bone. Previously, we reported that a clonal cell line, ATDC5, undergoes efficient chondrogenic differentiation through a cellular condensation stage. Here we report that the differentiated ATDC5 cells became hypertrophic at the center of cartilage nodules, when the cells ceased to grow. Formation of hypertrophic chondrocytes took place in association with type X collagen gene expression and a dramatic elevation of alkaline phosphate (ALPase) activity. After 5 weeks of culture, mineralization of the culture could be discerned as Alizarin red-positive spots, which spread throughout the nodules even in the absence of beta-glycerophosphate. Electron microscopy and electron probe microanalysis revealed that calcification was first initiated at matrix vesicles in the territorial matrix and that it advanced progressively along the collagen fibers in a manner similar to that which occurs in vivo. The infrared spectrum of the mineralized nodules indicated two absorption doublets around 1030 cm-1 and 600 cm-1, which are characteristic of apatitic mineral. Calcifying cultures of ATDC5 cells retained responsiveness to parathyroid hormone (PTH): PTH markedly inhibited elevation of ALPase activity and calcification in the culture in a dose-dependent manner. Thus, we demonstrated that ATDC5 cells keep track of the multistep differentiation process encompassing the stages from mesenchymal condensation to calcification in vitro. ATDC5 cells provide an excellent model to study the molecular mechanism underlying regulation of cartilage differentiation during endochondral bone formation.

Split-course accelerated hyperfractionation radiotherapy for advanced head and neck cancer: influence of split time and overall treatment time on local control

We analyzed 52 patients with stage III and IV head and neck cancer who were given split-course accelerated hyperfractionated radiotherapy with curative intent, focusing particularly on the influence of split-time on local control. An initial complete response was achieved in 16 patients (31%), and the rate of persistent local control at 3 years was 23%. The cause specific survival rate at 3 years was 29%. Univariate analysis of local control according to the split-time duration and overall treatment time showed that shorter duration (< or = 14 days or < or = 45 days, respectively) had a significantly positive impact on local control (P < 0.05). Multivariate analysis using local control as an endpoint also demonstrated that gender (women showing a better outcome than men) and split-time (< or = 14 days was better than > 14 days) were statistically significant factors for local control. These results suggest that shortening the split-time during radiotherapy might improve local control in accelerated hyperfractionation.

Sequential synthesis of cartilage and bone marker proteins during transdifferentiation of mouse Meckel's cartilage chondrocytes in vitro

Meckel's cartilage cells cultured in vitro undergo phenotypic transformation toward osteogenic cells. We examined whether these cells synthesize type X collagen and bone morphogenetic protein-2 (BMP-2). We also examined the results of Alcian blue staining and the expression of type I and type II collagen, osteocalcin and chondroitin sulfate proteoglycan (CSPG) during this transdifferentiation. Meckel's chondrocytes, isolated from day-17 mouse embryos, were inoculated at 1 x 10(4)/penicylinder and cultured in alpha-MEM for periods up to 4 weeks. Alcian blue staining and immunostaining of type II collagen and CSPG confirmed that, after cell culture for 2 weeks, the cartilaginous phenotype was expressed most intensely. Later in culture, chondrocytes underwent modification through the synthesis of bone-type proteins; nodule-forming small round cells showed ALPase activity and were immunoreactive for type I collagen and osteocalcin. Immunoreactivity for type X collagen was detected in the small round cells at the top of the nodules prior to calcification of the matrix, as well as in large hypertrophic cells. BMP-2 was also expressed first in similar small round cells after 3 weeks in culture, and it subsequently extended along the extracellular matrix in the calcified nodules. These results indicate that small round cells that are differentiating toward osteocyte-like cells from Meckel's chondrocytes express type X collagen and BMP-2 sequentially.

Clinical experience of radiation therapy for Graves' ophthalmopathy

The effect of radiation therapy for Graves' ophthalmopathy was evaluated. Ten patients with Graves' ophthalmopathy were treated with radiation therapy between 1992 and 1993 in Gunma University Hospital. All patients had a past history of hyperthyroidism and received 2,000 cGy to the retrobulbar tissues in 20 fractions. Nine of ten patients were treated with radiation therapy after the failure of corticosteroids. Six patients (60%) showed good or excellent responses. The exophthalmos type was more responsive to radiation therapy than the double vision type in this series. Two of five patients with the exophthalmos type demonstrated excellent responses, and their symptoms disappeared almost completely. The improvement of symptoms appeared within 3-6 months, and obvious clinical effects were demonstrated after 6 months of radiotherapy. Radiation therapy was well tolerated, and we have not observed any side effects of radiation therapy. In conclusion, radiation therapy is effective treatment for Graves' ophthalmopathy.

In vitro characterization of enamel epithelium and pulp cells in mouse tooth germs

Mixed cell populations consisting of enamel epithelium and pulp cells obtained from 18-day embryonic mouse tooth germs were cultured in vitro. Epithelial cells and pulp cells were also cultured individually and examined. Isolated cells were cultured in alpha-MEM supplemented with 10% fetal bovine serum for up to four weeks and examined morphologically using histological procedures including immunostaining, light and electron microscopy, and electron probe microanalysis. The pulp cells proliferated and differentiated in the absence of epithelium, but the number of epithelial cells showed a strong dependence on the pulp cells. Pulp cells showing fibroblastic morphology in the mixed culture gradually became elliptical, and eventually transformed into spherical cells surrounded by a calcified extracellular matrix. Alkaline phosphatase (ALPase) activity was expressed on the pulp cells prior to calcification of the extracellular matrix, as shown by von Kossa's and alizarin-red staining. Calcification deposition, which is closely associated with thick banded type-I collagen fibrils, was shown to be composed of calcium and phosphorous using electron probe microanalysis. Type-I collagen immunoreactivity was detected on the extracellular matrix after two weeks of culturing. The present results show that the proliferation and differentiation of pulp cells, and subsequent calcification of the extracellular matrix occur in the presence or absence of epithelial elements, but that the proliferation of epithelial cells depends on the presence of pulp cells.

Mouse Meckel's cartilage chondrocytes evoke bone-like matrix and further transform into osteocyte-like cells in culture

BACKGROUND

We reported that when Meckel's cartilage was transplanted ectopically, chondrocytes transformed into osteocyte-like cells accompanying the extracellular calcified matrix. However, we could not determine whether the osteocyte-like cells were derived from host tissues or from Meckel's cartilage itself. Therefore, we examined whether the Meckel's cartilage chondrocytes, which have a retrogressive ultimate fate, are capable of inducing the observed calcification and further transform into osteocyte-like cells in culture.

METHODS

Meckelian chondrocytes isolated enzymatically were plated at a low density and grown in alpha-MEM containing 10% FBS at 37 degrees C under 5% CO2 in air for up to 4 weeks.

RESULTS

Chondrocytes were fibroblast-like cells early in culture, but gradually transformed from polygonal cells into typical chondrocytes showing metachromasia with toluidine blue staining. After an additional week of culture, the chondrocytes transformed from large to small round cells accompanying nodule formations. Small round cells multiple-layered actively, and showed more intense alkaline phosphatase (ALPase) activity. Immunostaining identified type II collagen in the extracellular matrix at 2 weeks of culture, and type I collagen and osteocalcin were later synthesized by round cells. von Kossa's reaction showed extensive precipitation of calcification throughout the flocculent materials. Ultrastructural analysis showed that the cells surrounded by calcified matrix strongly resembled osteocytes.

CONCLUSIONS

The present study suggested that the Meckel's cartilage chondrocytes can express the osteocyte-like phenotype in vitro during synthesis of bone-type marker proteins such as osteocalcin or type I collagen.

Chondrogenic differentiation of clonal mouse embryonic cell line ATDC5 in vitro: differentiation-dependent gene expression of parathyroid hormone (PTH)/PTH-related peptide receptor

The regulatory role of parathyroid hormone (PTH)/PTH-related peptide (PTHrP) signaling has been implicated in embryonic skeletal development. Here, we studied chondrogenic differentiation of the mouse embryonal carcinoma-derived clonal cell line ATDC5 as a model of chondrogenesis in the early stages of endochondral bone development. ATDC5 cells retain the properties of chondroprogenitor cells, and rapidly proliferate in the presence of 5% FBS. Insulin (10 micrograms/ml) induced chondrogenic differentiation of the cells in a postconfluent phase through a cellular condensation process, resulting in the formation of cartilage nodules, as evidenced by expression of type II collagen and aggrecan genes. We found that differentiated cultures of ATDC5 cells abundantly expressed the high affinity receptor for PTH (Mr approximately 80 kD; Kd = 3.9 nM; 3.2 x 10(5) sites/cell). The receptors on differentiated cells were functionally active, as evidenced by a PTH-dependent activation of adenylate cyclase. Specific binding of PTH to cells markedly increased with the formation of cartilage nodules, while undifferentiated cells failed to show specific binding of PTH. Northern blot analysis indicated that expression of the PTH/PTHrP receptor gene became detectable at the early stage of chondrogenesis of ATDC5 cells, preceding induction of aggrecan gene expression. Expression of the PTH/PTHrP receptor gene was undetectable in undifferentiated cells. The level of PTH/PTHrP receptor mRNA was markedly elevated parallel to that of type II collagen mRNA. These lines of evidence suggest that the expression of functional PTH/PTHrP receptor is associated with the onset of chondrogenesis. In addition, activation of the receptor by exogenous PTH or PTHrP significantly interfered with cellular condensation and the subsequent formation of cartilage nodules, suggesting a novel site of PTHrP action.

Light and electron microscopy of stage-specific features of the transdifferentiation of mouse Meckel's cartilage chondrocytes in vitro

The changes in the morphological characteristics of Meckel's cartilage cells in culture can be divided into five stages during their transdifferentiation to cells with an osteocyte-like phenotype, as demonstrated in this study by light and electron microscopy. Chondrocytes were isolated from day-17 embryonic mice by enzymatic digestion and inoculated at a density of 1 x 10(4) cells. The cultures were incubated at 37 degrees C and cells were harvested every week for 4 weeks and processed for light and electron microscopy, as well as an immunohistochemical analysis of type I and II collagens. The primary cultures were characterized by fibroblastic cells (stage 1) around days 2-4. Polygonal cells (stage 2) expanded on a poorly formed extracellular matrix on day 7, and then they differentiated into large, round cells (stage 3) which began to form cellular nodules. The large, round cells contained many vacuoles, which were rapidly released into the pericellular space with a significant resultant decrease in cell size. Immunostaining showed that the extracellular matrix was occupied predominantly by type II collagen after 2 weeks in culture. However, the type II collagen was gradually replaced by type 1 collagen. Cells that formed multiple layers gradually changed into small, round cells (stage 4) at the tops of nodules and ultimately expressed an osteocytic phenotype (stage 5) after further transdifferentiation.

Meckel's cartilage chondrocytes in organ culture synthesize bone-type proteins accompanying osteocytic phenotype expression

We examined whether Meckel's cartilage of embryonic mice, 17 days in utero, undergo the cellular transformation into the osteocyte-like phenotype under organ culture conditions. Explants were grown by our original pithole method modified Trowell-type cultures for up to 4 weeks at 37 degrees C under 5% CO2 in air. Specimens were examined using histological procedures including immunostaining and electron microscopy. In addition, the effects of beta-glycerophosphate on matrix calcification were also examined in cultures with or without beta-glycerophosphate. Addition of beta-glycerophosphate induced calcification at a higher level, but calcium mineral deposition occurred regardless of the addition of beta-glycerophosphate to the culture medium. Light and electron microscopic analyses showed that freshly isolated chondrocytes prior to cell culture had typical hypertrophic morphology, but shortly after commencement of culture, they showed morphological modifications. The cells showing chondrocytic phenotypes became basophilic elliptical cells, and eventually transformed into flattened osteocyte-like cells. Bone-like features for cellular elements were characterized by spindle-shaped cells with elongated processes accompanying bone-specific thick-banded collagen fibrils. Immunostaining showed that at 2 weeks in culture, type I and type II collagens coexisted in the matrix, but subsequently type II collagen synthesis ceased and was replaced by type I collagen synthesis. Immunofluorescent labeling for osteocalcin was noted first in the peripheral cells by 1 week, but at 3 weeks this reaction spread to the central zone in explants. Alkaline phosphatase activity (ALPase) was expressed on the cells in the central zone prior to calcium mineral deposition as shown by von Kossa's reaction at 3 weeks in culture. These results showed that Meckel's cartilage chondrocytes in organ culture synthesize bone-type proteins accompanying osteocytic phenotype expression.

Morphological changes during survival, cellular transformation, and calcification of the embryonic mouse: Meckel's cartilage transplanted into heterotopic sites

Meckel's cartilages obtained from 18-day-old embryonic mice were transplanted into heterotopic sites to examine their site-specific effects on cellular modification, calcification, and long-term survival. The explants were isografted into the liver, subcutaneous tissue, anterior chamber of the eye, kidney, peritoneal cavity, and intrafemoral muscle of mouse for up to 2 weeks. In addition, Meckel's cartilages were wrapped in a Nucleopore filter and transplanted into the spleen. Grafted Meckel's cartilages were observed by light and electron microscopy, including application of von Kossa's reaction and osmium-potassium ferrocyanide fixation. It was demonstrated by von Kossa's reaction that when Meckel's cartilage was grafted in liver parenchyma, initial calcification appeared on the territorial matrix as spotted deposits. The chondrocytes gradually transformed into small ovoidal cells and showed a morphology closely resembling that of osteocytes. Transplants in the kidney subcapsular site contained active surviving chondrocytes, and subcutaneously grafted Meckel's cartilage revealed three types of chondrocytes: nonhypertrophic and calcification-inducing hypertrophic chondrocytes, as well as cells showing osteocytelike phenotypes. Intraocular and Nucleopore filter-wrapped explants showed degenerative changes, and peritoneal-cavity transplants showed cellular hypertrophy, but calcified precipites were not observed. The present investigation demonstrated that in blood-rich tissues Meckel's cartilage showed long-term survival and could modulate calcified precipitation. Thus, this adaptation of Meckel's cartilage to the environment of various heterotopic sites suggests that it has the ability to induce calcification.

Morphological modifications during long-term survival of Meckel's cartilage hypertrophic chondrocytes transplanted in the mouse spleen

To examine the ultimate fate of hypertrophic chondrocytes, Meckel's cartilage bars from 18-day-old mouse embryos were transplanted into isogenic mouse spleen for 3, 7, 14 and 21 days and observed at the light and electron microscopic levels. The midportions of these Meckel's cartilage bars were used as explants; they were characterized by many hypertrophic chondrocytes containing euchromatic round nuclei, a large amount of glycogen particles, and some vacuoles. Grafted cartilage adapted well to the splenic tissue, showing intense metachromasia around the territorial matrix. Ultrastructural observations indicated that the number of large vacuoles and glycogen aggregates in the hypertrophic cells became markedly reduced with grafting time, whereas the Golgi apparatus and rough endoplasmic reticulum were well-developed. Needle-like crystals showing initial apatite deposition appeared in association with matrix vesicles; these proliferated as time elapsed after transplantation. On day 14 after transplantation, cells displaying such various structural features as pyknotic nuclei, large vacuoles, and cytoplasmic shrinkage were noted in addition to intact hypertrophic chondrocytes. Following resorption of the calcified cartilage by multinucleated giant cells, many osteoblasts appeared along the border of the calcified matrix. Some remaining hypertrophic cells in the calcified matrix had transformed into osteocyte-like cells. On day 21, the resorbed area of the calcified cartilage was invaded by many blood vessels. Hypertrophic chondrocytes, now exposed from cellular lacunae, and the osteocyte-like cells in the calcified matrix displayed involutional changes. The present study showed that, although the hypertrophic chondrocytes in Meckel's cartilage essentially underwent regressive changes, they retained the ability to stimulate endochondral ossification within the microenvironment of the spleen. In addition, some of these cells were transformed into osteocyte-like cells.

Morphological characteristics of the life cycle of resting cartilage cells in mouse rib investigated in intrasplenic isografts

Resting cartilages taken from 2-day-old mouse ribs were transplanted into spleens in order to carry out morphological investigations of the life cycles of their chondrocytes. The explants were isografted for periods of up to 60 days and examined at light and electron microscopic levels, using von Kossa's reaction or osmium-potassium ferrocyanide (OPF) fixation. By day 3 after transplantation, resting cartilage containing immature chondrocytes was well adapted to splenic tissue and by 7 days after transplantation these chondrocytes had changed into early hypertrophic chondrocytes containing large vacuoles, glycogen aggregates and abundant secretory organelles. It was also demonstrated by von Kossa's reaction that the initial calcification occurred in the territorial matrix during this period. In spite of the hypertrophic chondrocytes in the central zone being surrounded by an extensively calcified matrix during days 14-21 after transplantation, these cells had well-preserved organized organelles, except that Golgi-associated elements and endoplasmic reticulum revealed a tendency toward degenerative changes. With increased duration of the grafting period, from 30-60 days, the calcification zone progressed gradually, and the number of hypertrophic chondrocytes embedded in the calcified matrix decreased considerably. By day 60, degenerating hypertrophic chondrocytes of two types were distinguished:flattened cells containing large vacuoles, poorly developed Golgi apparatuses, and rough endoplasmic reticulum; and shrunken dark cells displaying terminal hypertrophy. During the present study, we observed no vascular invasion into the calcified matrix, or appearance of bone-related cells, and the morphological changes from the resting chondrocytes to cellular hypertrophy accompanied by the formation of a calcified matrix were observed at day 60. These findings indicate that resting cartilage cells of the mouse have the capacity for terminal differentiation when transplanted into the spleen.

Features of the aperiodic microfibrils associated with mouse dental basement membrane demonstrated by ultrastructural histochemistry

Histochemical features of aperiodic microfibrils (AMF) in mouse tooth germs were examined at the electron microscopic level. Intact and EDTA-isolated materials obtained from one day old first molars were used for ruthenium red (RR) staining, ferritin permeability, periodic acid-silver methenamine (PAM) impregnation, fibronectin localisation, negative staining on cryo-sections and tannic acid fixation. Electron microscopy and negative staining demonstrated that AMF traverse the basal lamina and penetrate below the inner enamel epithelium. In addition to RR staining, PAM impregnation and tannic acid fixation showed deposition on the AMF which was associated with basal laminae. RR staining and tannic acid fixation also indicated the presence of glycoprotein-rich materials in the lamina lucida. The AMF were derived from the lamina lucida which was closely associated with tannic acid-positive granular materials. The precipitation of silver particles by PAM impregnation was seen on banded collagen fibrils, basal lamina and AMF, but the staining features of AMF differed distinctly from those of collagen fibrils. The distribution of ferritin particles revealed that the basal lamina covering EDTA-isolated papilla tissue is a continuous structure. Immuno-reactions for fibronectin were detected on the basal lamina and AMF. Our results suggest that AMF are derived from the glycoprotein-rich lamina lucida and that their histochemical characteristics resemble those of basal lamina.

Endochondral calcification by hypertrophic chondrocytes in the Meckel's cartilage grafted into the isogenic mouse spleen

We found that when the midsection of Meckel's cartilage bars obtained from mice on the eighteenth day of gestation were grafted into isogenic mouse spleen, chondrocytes induced an endochondral calcification. Concurrent with the onset of calcification throughout Meckel's cartilage matrix, periodic banded thick collagen fibrils and matrix vesicles were observed around the chondrocytes. Although most of the chondrocytes prior to grafting were hypertrophic cells, they survived for seven days in the splenic tissue and had well-developed secretory organelles. The cells which were surrounded by calcified matrix were relatively small, spherical, and showed a morphology closely resembling that of osteocytes. These findings suggest that the life span of hypertrophic chondrocytes is influenced by the microenvironment of the spleen.

Calcification capacity of dental papilla mesenchymal cells transplanted in the isogenic mouse spleen

The capacity of the dental pulp to form calcified tissue was examined in papilla cells dissociated from first molar tooth germs of the neonatal mouse and isografted in the spleen for up to 7 days. To obtain papilla cell populations without odontoblasts, pulpal mesenchyme was isolated mechanically from the enamel organ after 0.1% trypsin treatment and rolled on a membrane filter. On day 3 after transplantation, the grafted papilla cells had changed into large, spindle-shaped cells, and initial calcification with needle-like crystals began in association with the collagenous matrix surrounding those cells. On day 7 after transplantation, the spindle cells transformed into odontoblast-like cells containing well-developed secretory organelles, and irregular, but nontubular, calcified tissues were commonly observed surrounding the extracellular collagenous matrix. The calcified tissue matrix with cellular inclusions displayed a structure similar to that of osteodentin. During this period, an intense positive reaction for alkaline phosphatase (ALPase) activity was demonstrated along the cell membranes of the odontoblast-like cells aligned at the periphery of forming calcified tissue. Enzymatic activity could not be detected on the cells incorporated completely into osteodentin-like matrix. The present results show that the papilla cell population transplanted into the spleen formed osteodentin-like material, thus demonstrating the capacity of papilla cells to produce calcified tissue.

Ultrastructural features of the developing eosinophils in bone marrow and spleen of the musk shrew, Suncus murinus

Eosinophilopoiesis in the musk shrew, Suncus murinus, a representative of the order Insectivora, was studied by light and electron microscopy. To examine biochemical features of cytoplasmic granules, extraction with proteolytic enzymes was carried out on ultrathin sections of bone marrow. In this species, eosinophils are produced in the same manner in both spleen and bone marrow. Developing eosinophils were distinguished as belonging to four stages, recognized by ultrastructural changes in cytoplasmic organelles as well as the eosinophilic granules during maturation. Granulogenesis began by budding of vacuoles containing flocculent material from the concave face of the Golgi apparatus, in the promyelocyte to myelocyte stage. The matrix of developing granules transformed into a finely granular structure, and the large spherical granules of mature eosinophils were homogeneous without crystalline cores. It was shown by proteolytic enzyme extraction that the proteinaceous cores of mature granules were uniformly removed; there was no evidence that they contained crystalloid inclusions. These results indicate that shrew eosinophils can be regarded as cells that retain a prototype of eosinophil granules, probably like those of ancestral mammals rather than those of higher living Mammalia.

Morphological evidence of the formation of intracellular collagen fibrils in the embryonic mouse molar odontoblasts induced by colchicine administration

The effects of colchicine on collagen formation were examined ultrastructurally using secretory odontoblasts in mouse molar tooth germs isografted to the spleen for 1 week. Colchicine in concentrations of 0.025 or 0.05 mg/0.1 ml was injected intravenously 12-24 h prior to harvesting. Colchicine induced the disruption of the Golgi apparatus and caused the accumulation of various types of Golgi-associated vacuoles containing collagenous fibrillar structures. Many vacuoles containing fine particles, nonstriated parallel filaments, banding patterns with a periodicity of approximately 63-nm intervals, and occasionally segment-long-spacing-like assemblies were aggregated in the cytoplasm during the experimental period. These morphological changes in vacuole contents may reflect the initial steps for polymerization of the intracellular collagen fibrils. The majority of the aggregated vacuoles were degraded by fusion with lysosomes but banded filamentous material in some vacuoles appeared to polymerize into the collagen fibrils with native structures. These results suggested that in unsecreted vacuoles accumulated in the odontoblasts as a result of colchicine administration the polymerization of collagen fibrils with native structures can occur.

Immunohistochemical and cytochemical evidences for a possible localization of leucine aminopeptidase in the Merkel cell granule

Localization of leucine aminopeptidase (LAP) in the Merkel cell-axon complex was studied immunohistochemically and cytochemically in the labial tissues of the mouse, rat, dog, and monkey. Anti-LAP was obtained in rabbits by the injection of commercially supplied swine LAP which was confirmed as electrophoretically pure. The Merkel cells of the mouse, rat, and monkey were positively stained by treatment with anti-LAP but the Merkel cells of the dog were negative. When ultrathin sections of the hair follicle from the rat whisker pad, which contain an abundance of Merkel cells, were processed by immuno-peroxidase or by the immuno-gold method, the reaction products were predominantly deposited on the Merkel cells granules. Furthermore, an immuno-blot assay revealed that an extract of the hair follicles from murine whisker pads contained a molecule of relative molecular mass Mr = 60,000 which is similar in size to a subunit of swine LAP. Thus, it appears that Merkel cell granules of rodents and the monkey contain a protein which resembles lucine aminopeptidase.

Morphogenesis of mineralized tissues induced by neonatal mouse molar pulp isografts in the spleen

Tooth pulps dissociated intact with EDTA were isografted for up to 40 days, and examined by light and electron microscopy for hard tissue morphodifferentiation. Grafts formed tubular dentine and osteodentine. Tubular dentine, penetrated regularly by elongated odontoblast processes, resembled normal dentine and was formed when the original odontoblasts continued normal matrix secretion. Osteodentine was formed by spindle-shaped cells with large round nuclei which presumably were transformed pulp cells, and incorporated the same elements as found in cells of non-tubular dentine. Occasionally, odontoblasts were contiguous with both the regular dentine and the osteodentine. Thus in EDTA-dissociated pulps transplanted to the spleen, the original odontoblasts produce tubular dentine and other pulp cell differentiate to form osteodentine.

Influence of 1,25-dihydroxyvitamin D3 on cell proliferation during odontogenesis of the mouse embryonic molars in vitro

Mandibular first molars from 17-day-old mouse embryos were cultured for 2 and 4 days in alpha-MEM supplemented with 10% newborn bovine serum containing 0.02% ethanol or 0.1, 1.0 or 10 ng/ml 1,25-(OH)2D3. After embedding, every 6th section was stained and mitotic features of inner dental epithelium (IDE) and dental papilla cells (DP) were counted under a light microscope. On the 2nd day, no significant differences were observed in the IDE and DP mitotic indexes among the control, ethanol, and the three vitamin D groups. On the 4th day, the indexes for all groups decreased. But the IDE indexes for all vitamin D groups were significantly different from those for the control (P less than 0.01) and ethanol (P less than 0.05) groups, whereas the DP index was significant (P less than 0.05) only in the 10 ng/ml vitamin D group. The present results suggest that vitamin D3 may have an influence on cell proliferation of IDE and DP in mouse embryonic molars in vitro.

Nerve endings in the vermilion border and mucosal areas of the rat lip

Organized mechanoreceptors in mucosae and vermilion borders of rat lower lips were studied by light and electron microscopy. Cholinesterase histochemistry was applied to whole-mount preparations of mucosae for the light and electron microscopic identification of mechanoreceptors. Three types of lamellated corpuscles (a simple corpuscle; a coiled, simple corpuscle; and a Meissner-like corpuscle), and a unique, organized, bush-like assembly of free nerve terminals were identified. The simple corpuscles were found exclusively in the vestibular mucosa facing the incisor teeth. In contrast, bush-like endings were confined to the vestibular mucosa and to the lateral, eminent mucosa that faced the diastema. Furthermore, coiled simple corpuscles and Meissner-like corpuscles were localized in the boundary zone between the vestibular and lateral eminent mucosae and in the vermilion border. From a functional viewpoint, it is of interest that different areas of the rat lip contain different morphological patterns of mechanoreceptors.

Distinct types of encapsulated sensory corpuscles in the oral mucosa of the dog: immunohistochemical and electron microscopic studies

The types, structure, and distribution of encapsulated sensory endings that have lamellar investments in the oral mucosa and vermilion border of the lip of adult dogs were studied by light and electron microscopy. Immunohistochemistry for cholinesterase was used to identify the corpuscules by light microscopy. Two different types of corpuscular end-organs containing definite inner cores were distinguished. One was a typical, simple corpuscle, which contains only one, but sometimes two, inner cores composed of densely piled cytoplasmic lamellae surrounding a central axon terminal. The other type was characterized by the coexistence of convoluted inner cores, arborized free endings, and thin nerve bundles within a perineural capsule; we term this type "compound corpuscle." The ultrastructure of the inner cores in compound corpuscles was similar to that of mature, simple corpuscles. The arborized free endings in the compound corpuscles usually contained an accumulation of mitochondria and small clear vesicles. The compound corpuscles were frequently encountered in the vermilion border of the lip and in the labial and buccal mucosae but were rare in the masticatory mucosa of the gingiva and hard palate. From the results, it was concluded that the compound corpuscle is a distinct type of the sensory end-organ containing inner cores.

The development of mandibular molar tooth germs isografted in the mouse spleen

Tooth germs taken from 13-day-old embryonic mice and isografted in the spleen of adult mice were examined by light and electron microscopy. Well-organized tooth structures from the early cap stage to fully developed and mineralized mature teeth were obtained up to day 60 after transplantation. Germs on day 2 were similar to those prior to the onset of grafting but reached the late cap stage of development on day 4. On day 6, enamel and dentine formation were initiated and inner enamel epithelium and dental papilla cells were polarized. On days 10-15, enamel-matrix secretion was completed and almost all ameloblasts had become resorptive enamel epithelium. India ink injected from the recipient caudal vein accumulated to the capillaries within the pulp throughout the newly-formed vessels. On day 20, defined root formation had begun but occasionally irregular and cellular osteodentine was formed in root areas. On day 30, transplants were covered with reduced enamel epithelium and acellular cementum was formed at the root areas together with rudimentary periodontal ligament fibres. Cellular cementum became thicker up to day 40. There was little evidence of cellular infiltration from recipient tissue up to day 60. The spleen seems to be a suitable site for transplantation of tooth germs.

Ultrastructural observations on the intraodontoblastic collagen fibrils of the mouse tooth germs

We examined electron-microscopically and histochemically the ultrastructural features of the intraodontoblastic collagen fibrils of the mouse. These collagen fibrils were most common in secreting odontoblasts (pre-odontoblasts) of the maturating stages. In such cells they were most numerous at the peripheral zone of the Golgi apparatus, and were sometimes seen in odontoblastic processes. Intraodontoblastic collagen fibrils also had morphological variations including a banded structure enclosed by limiting membranes of vacuoles, fusion with primary lysosomes, and an electron-dense material covering with a structure that was not banded. Study of acid phosphatase activity showed that these structural changes were caused by the degradation of intraodontoblastic collagen fibrils by lysosomes. The results of studies of the permeation of lanthanum nitrate and the alkaline phosphatase reaction showed that these collagen fibrils were separate from the extracellular matrix and that there was no phagocytosis of the odontoblasts.

Ultrastructural evidence for a possible secretory function of Merkel cells in the barbels of a teleost fish, Cyprinus carpio

Examination of barbels of the carp (Cyprinus carpio) revealed cells showing the characteristics of Merkel cells. Some ultrastructural features of these cells suggest a secretory function.

Hemopoietic sites and development of eosinophil granulocytes in the loach, Misgurnus anguillicaudatus

Unique eosinophils, each of which contained only one eosinophilic granule, have been found in the peripheral blood of the loach (Misgurnus anguillicaudatus). Several loach organs have been studied by light and electron microscopy to determine the hemopoietic site of this cell type. Eosinophils are produced mainly in the spleen and to a small extent in the kidney, but not in other organs. Presumed myeloblasts are identified as large lymphoid cells containing a number of small-dense granules (diameter, 0.12-0.16 micron) in the cytoplasm. These granules have been observed throughout eosinophilopoiesis but they are most abundant in the promyelocyte stage. The largest cells have been identified as myelocytes which contain a number of large granules (diameter, 0.7-1.4 micron) with electron-dense crystalline cores. These large granules are present from the myelocyte to metamyelocyte stage. Metamyelocytes differ from myelocytes in having more large granules. Mature eosinophils are morphologically similar to metamyelocytes but are characterized by the presence of only one very large electron-dense granule (diameter, 2.5-2.8 microns) with a crystalline core. The nature of these granules has been studied by enzyme digestion using pepsin and trypsin. The results indicate that the crystalline cores are almost pure protein.

Ultrastructure of the effects of calcitonin on the development of mouse tooth germs in vitro

Mandibular first molars, from 17-day-old embryos, were cultivated in control medium or medium containing 0.1, 0.01 or 0.001 unit/ml of calcitonin (CT) for periods up to 10 days. In untreated tooth germs, cells of the dental papilla differentiated into pre-odontoblasts up to 4 days and predentine was seen on day 6. Cells treated with 0.1 unit/ml of CT differentiated into pre-odontoblasts up to 4 days, but no predentine was formed even after 10 days in culture. With 0.01 unit/ml, cells differentiated into odontoblasts, and had already secreted predentine a few days earlier than the untreated group. With 0.001 unit/ml, the developing germs were similar to the control explants during the entire 10-day cultivation period. The proportional area of rough endoplasmic reticulum to cytoplasm of the odontoblasts was low at 0.1 unit/ml of CT and high at 0.01 unit/ml compared to the untreated explants.

An experimental study of the influence of sensory nerve fibers on Merkel cell differentiation in the labial mucosa of the rabbits

The influence of the sensory nerve fibers on the differentiation of the Merkel cell was examined in the denervated labial mucosa of adult rabbits. Part of the lower labial mucosa was excised following mental nerve resection. Twenty-one and 50 days later, the regenerated mucosa was examined by electron microscopy with special reference to the distribution and maturation of Merkel cells, and compared with the normal and the denervated intact mucosa. A number of immature Merkel cells appeared in the denervated and regenerated epithelium by 21st day after the operation. The distribution density of the Merkel cells in a unit area of the mucosa did not change much but the percentage of the mature Merkel cells increased significantly until 50 days after the operation. The mature Merkel cells in the denervated and regenerated mucosa showed a normal ultrastructure, though their orientation and location were not uniform, shifting more superficially than the cells in the normal mucosa. In the controlateral intact epithelium of the denervated labial mucosa, no substantial decrease in the population density of Merkel cells was recognized, though the desquamation of Merkel cells was observed. It was conspicuous that the percentage of immature Merkel cells to the total number of the Merkel cells was significantly increased after the denervation in the intact labial mucosa. This study suggests that Merkel cells differentiate independently of sensory nerve fibers, but the latter are requisite to maintain the former in a uniform, basal disposition in the epithelium.

Migration of Merkel cells in the labial mucous epithelium of adult rabbits following mental nerve resection

Merkel cells in the lower labial mucosa of adult rabbits were studied electron microscopically, 9, 21, 28, and 50 days after resection of the mental nerves. By day 9, nerve fibers were completely retracted from the epithelial layer of the mucosa. On and after day 21, Merkel cells were located not only in the basal layer but also in the prickle or more superficial cell layers. The ultrastructure of the migrating Merkel cells was unchanged, both as to the amount and location of the specific cored granules in the cytoplasm, until the cells reached the granular cell layer. The position of the migrating Merkel cells differed from cell to cell, and migration continued for at least 50 days. A remarkably large number of immature Merkel cells was observed in the basal and suprabasal cell layers of the denervated epithelium even by day 50. Therefore, the possibility of the reproduction of Merkel cells exists. The migrating Merkel cells, as well as the keratinocytes in the same cell layer, had degenerated drastically in the parakeratinized cell layer. This seems to indicate that the Merkel cells belong to the line of keratinocytes.

Intranuclear inclusions in the bone marrow cells of miniature pigs

Intranuclear inclusions were found in bone marrow cells of miniature pigs, whose ultrastructure and nature were examined by electron microscope and by cytochemical techniques. The inclusions were found predominantly in the granulocyte series and in a megakaryocyte and plasma cell. The inclusions consisted of closely packed, wavy filaments, 20-40 in number. They varied in size from 0.5-1.0 mum in length and 0.2-0.5 mum in width. The largest inclusion extended across the entire length of the nucleus. Occasionally, an inclusion extending toward the nuclear pore was observed. the intranuclear inclusions showed a tendency to appear frequently in immature granulocytes. The experiment on enzyme digestion for the epoxy sections showed that the inclusions were insoluble in RNase, DNase, pepsin, trypsin and protease solutions. These results seem to suggest that the intranuclear inclusions of blood cells differ from filamentous inclusions which have been noted previously in some neurons.

Merkel cell development in the wound healing in the labial mucosa of adult rabbits

Merkel cell development was studied in the regenerative labial mucous membrane of adult rabbits. Fullthick wounds were made on the inferior labial mucous membrane of the rabbits, then the regenerative mucous membrane was examined by electron and light microscopy at time intervals of 2, 7, 10, 14, 21 and 30 days after the injury. By 7 days, the regenerative area of the mucous membrane was replaced with a dense lamellar connective tissue, which was mainly composed of alternately arranged collagen fiber layers and fibroblasts, and an overlying stratified squamous epithelium. No Merkel cell was found 7 days after the injury. Ten days after the injury, a few very immature Merkel cells were identified in the regenerative epithelium. The Merkel cells increased in number and matured in structure during the course of the following regenerative period. Thirty days after the injury, the wounded area healed without forming a scar tissue, and the regenerated epithelial ridges usually included regular type Merkel cells. As adult animals were used in this experiment, the reproduction of the Merkel cell in the regenerative epithelium seemed to be independent of the undifferentiated neuroectodermal tissue.

Scanning electron microscopic observations of tooth germ in tissue culture with special reference to the migration of epithelial cells

Tooth germ derived from the mandible of newborn mice was used in this study. The enamel organs were dissected and attached directly to coverslips without using the plasma clot. By this method it was possible, for the first time as far as is known, to attain preparations in which the major part of the outgrowth from the explant consisted purely of epithelial-like cells which probably were aneloblasts. Abundant desmosomes and tonofilaments were confirmed by transmission electron microscopy. Under the scanning electron microscope this outgrowth of epithelial cells revealed characteristic intercellular connections which were divided into four types. 1)In the part nearest to the explant, cell boundaries were indicated with a large number of short microvilli. 2)In the zone beyond the first type, the intercellular connection was represented by bridge-like processes firmly combined with each other. 3)In a still further zone, the tips of bridge-like processes were free from the cell connection and extended to the adjacent cells with the advance of cell migration. 4)In the most peripheral part of the explant, the cytoplasmic processes were capable of further expansion. These cells were soon separated from the adjacent cells to migrate as free cells.