Formation of chondrous and osseous tissues in micromass cultures of rat frontonasal and mandibular ectomesenchyme

Immunolocalization of markers for bone formation during guided bone regeneration in osteopenic rats

Objective

The aim of this paper was to evaluate the repair of onlay autogenous bone grafts covered or not covered by an expanded polytetrafluoroethylene (e-PTFE) membrane using immunohistochemistry in rats with induced estrogen deficiency.

Material and Methods

Eighty female rats were randomly divided into two groups: ovariectomized (OVX) and with a simulation of the surgical procedure (SHAM). Each of these groups was again divided into groups with either placement of an autogenous bone graft alone (BG) or an autogenous bone graft associated with an e-PTFE membrane (BGM). Animals were euthanized on days 0, 7, 21, 45, and 60. The specimens were subjected to immunohistochemistry for bone sialoprotein (BSP), osteonectin (ONC), and osteocalcin (OCC).

Results

All groups (OVX+BG, OVX+BMG, SHAM+BG, and SHAM+BMG) showed greater bone formation, observed between 7 and 21 days, when BSP and ONC staining were more intense. At the 45-day, the bone graft showed direct bonding to the recipient bed in all specimens. The ONC and OCC showed more expressed in granulation tissue, in the membrane groups, independently of estrogen deficiency.

Conclusions

The expression of bone forming markers was not negatively influenced by estrogen deficiency. However, the markers could be influenced by the presence of the e-PTFE membrane.

Regulation of the chondrogenic phenotype in culture

In recent years, there has been a great deal of interest in the development of regenerative approaches to produce hyaline cartilage ex vivo that can be utilized for the repair or replacement of damaged or diseased tissue. It is clinically imperative that cartilage engineered in vitro mimics the molecular composition and organization of and exhibits biomechanical properties similar to persistent hyaline cartilage in vivo. Experimentally, much of our current knowledge pertaining to the regulation of cartilage formation, or chondrogenesis, has been acquired in vitro utilizing high-density cultures of undifferentiated chondroprogenitor cells stimulated to differentiate into chondrocytes. In this review, we describe the extracellular matrix molecules, nuclear transcription factors, cytoplasmic protein kinases, cytoskeletal components, and plasma membrane receptors that characterize cells undergoing chondrogenesis in vitro and regulate the progression of these cells through the chondrogenic differentiation program. We also provide an extensive list of growth factors and other extracellular signaling molecules, as well as chromatin remodeling proteins such as histone deacetylases, known to regulate chondrogenic differentiation in culture. In addition, we selectively highlight experiments that demonstrate how an understanding of normal hyaline cartilage formation can lead to the development of novel cartilage tissue engineering strategies. Finally, we present directions for future studies that may yield information applicable to the in vitro generation of hyaline cartilage that more closely resembles native tissue.

Biodegradable polymers in chondrogenesis of human articular chondrocytes

The aim of this study was to evaluate the potential role of polyglycolic acid (PGA), poly(glycolic acid-epsilon-caprolactone) (PGCL), poly(L-lactic acid-glycolic acid) (PLGA), poly(L-lactic acid-epsilon-caprolactone, 75:25 (w/w)) [P(LA-CL)25], poly-epsilon-caprolactone (tetrabutoxy titanium) [PCL(Ti)], and fullerene C-60 dimalonic acid (DMA) in cartilage transplants. After 4 weeks of culture of human articular cartilage, the levels of cell proliferation and differentiation and the expression of cartilage-specific matrix genes were estimated. The relationship between cell differentiation and gap junction protein connexin 43 (Cx43) was also evaluated. All materials except PCL(Ti) retained cell proliferation activities similar to the controls. Cell differentiation levels from the highest to the lowest were in the following order: PGA >> PLGA > PGCL > Control = DMSO > P(LA-CL)25 = PCL(Ti) >> fullerene C-60 DMA. Expression of the collagen type II gene was selectively upregulated for PGA, PGCL, and PLGA and slightly increased for P(LA-CL)25 polymers but was downregulated for fullerene C-60 DMA. Aggrecan gene expression was strongest with PGA and was consistently expressed with other matrices, especially with PGCL and PLGA. However, the expression patterns of the connexin 43 gene were different from the former two genes. Multiple regression analysis revealed a high correlation between cartilage proteoglycans production and expression levels of these three genes.

Stepwise mechanical stretching inhibits chondrogenesis through cell-matrix adhesion mediated by integrins in embryonic rat limb-bud mesenchymal cells

Biomechanical forces are major epigenetic factors that determine the form and differentiation of skeletal tissues, and may be transduced through cell adhesion to the intracellular biochemical signaling pathway. To test the hypothesis that stepwise stretching is translated to molecular signals during early chondrogenesis, we developed a culture system to study the proliferation and differentiation of chondrocytes. Rat embryonic day-12 limb buds were microdissected and dissociated into cells, which were then micromass cultured on a silicone membrane and maintained for up to 7 days. Stepwise-increased stretching was applied to the silicone membrane, which exerted shearing stress on the cultures on day 4 after the initiation of chondrogenesis. Under stretched conditions, type II collagen expression was significantly inhibited by 44% on day 1 and by 67% on day 2, and this difference in type II collagen reached 80% after 3 days of culture. Accumulation of type II collagen protein and the size of the chondrogenic nodules had decreased by 50% on day 3. On the other hand, expression of the non-chondrogenic marker fibronectin was significantly upregulated by 1.8-fold on day 3, while the up-regulation of type I collagen was minimal, even by day 3. The downregulation in the expression of chondrogenic markers was completely recovered when cell-extracellular matrix attachment was inhibited by Gly-Arg-Gly-Asp-Ser-Pro-Lys peptide or by the application of blocking antibodies for alpha2, alpha5 or beta1 integrins. We conclude that shearing stress generated by stepwise stretching inhibits chondrogenesis through integrins, and propose that signal transduction from biomechanical stimuli may be mediated by cell-extracellular matrix adhesion.

Multilineage differentiation of ectomesenchymal cells isolated from the first branchial arch

Cranial neural crest-derived ectomesenchymal cells may be pluripotent stem cells that are capable of generating a range of phenotypes. The fate of these cells appears to be determined in part by intrinsic genetic programs and also by the influence of extracellular signals in the local environment. The extent of lineage determination once neural crest cells have migrated to the first branchial arch is not clear, although branchial arch pattern is not thought to be the result of crest predetermination. The aim of the present study was to test the hypothesis that ectomesenchymal cells of the first branchial arch show properties of pluripotent stem cells, the lineage of which may be directed by specific molecular signaling. Ectomesenchymal cells were enzymatically isolated from the mandibular processes of BALB/c mice and maintained in an undifferentiated state while cultured with leukemia inhibitory factor or induced to differentiate by lineage-specific induction factors or growth conditions, including transforming growth factor beta, forskolin, and a mineralization-promoting medium. Morphological observations and immunocytochemistry demonstrated that cells could be induced to differentiate into smooth muscle cells, glial cells, and osteoblasts, respectively. In the presence of the mineralization-promoting medium, alkaline phosphatase activity increased significantly and mineralization nodules formed. The data reported support the concept that many, although not all, first branchial arch-derived ectomesenchymal cells show properties of multipotent stem cells, the subsequent fate of which can be influenced by induction factors and growth conditions. Some cells, however, showed a degree of commitment with respect to their fate. The possible application of first branchial arch-derived stem cells to tissue engineering of the orofacial tissues should involve consideration of the developmental stage of cell harvesting and the desired cell fate.

Association of enhanced expression of gap junctions with in vitro chondrogenic differentiation of rat nasal septal cartilage-released cells following their dedifferentiation and redifferentiation

The nasal septum is an important centre of endochondral ossification during the development of the facial region. Previous studies have shown that it is possible to recapitulate the differentiation programme of 21-day-old rat nasal chondrocytes in vitro. The purpose now was to investigate, in vitro, the cell condensation phase that represents the earliest morphological event associated with cartilage differentiation in skeletal development. The study focuses on the ability of the cells to form condensations before overt differentiation, with special emphasis on gap-junction expression. The gap-junction protein connexin 43 was localized by indirect immunofluorescence as primarily intracellular and, on day 5, at the condensation stage, as spot-like contacts between cells. Intracellular injection of the permeable dye Lucifer yellow led to the staining of up to 20 neighbouring cells, indicating functional gap junctions and coupling. In contrast, treatment of cultures with the gap-junction blocker glycyrrhetinic acid inhibited dye coupling and reduced cartilage differentiation. Northern blotting of connexin 43 mRNA showed a faint band during the first days of culture, with a striking increase after day 4. In addition, the mRNA of the homeodomain-containing gene Cart-1 began to be expressed in prechondrogenic condensations and corresponded to the expression of type II collagen mRNA. These data indicate that the early stage of in vitro chondrocyte differentiation is the formation of cell condensations and the ability to establish cell-to-cell communication. Connexin 43, together with other molecular mechanisms, mediates the condensation phase of chondrogenesis and sets up the optimal environment in which nasal septal cells may terminally differentiate into chondrocytes.

Type I collagen influences cartilage calcification: an immunoblocking study in differentiating chick limb-bud mesenchymal cell cultures

Chick limb-bud mesenchymal cells, plated in high-density micro-mass culture, differentiate and form a matrix resembling chick epiphyseal cartilage. In the presence of 4 mM inorganic phosphate or 2.5 mM beta-glycerophosphate mineral deposits upon this matrix forming a mineralized tissue that, based on electron microscopy, x-ray diffraction and Fourier Transform Infrared microspectoscopy, is like that of chick calcified cartilage. In this culture system the initial mineral deposits are found on the periphery of the chondrocyte nodules. During differentiation of the cells in the high-density micro-mass cultures there is a switch from expression of type I collagen to type II, and then to type X collagen. However, type I collagen persists in the matrix. Because there is some debate about whether type I collagen influences cartilage calcification, an immunoblocking technique was used to determine the importance of type I collagen on the mineralization process in this system. Studies using nonspecific goat anti-chick IgG demonstrated that 1-100 ng/ml antibody added with the media after the cartilage nodules had developed (day 7) had no effect on the accumulation of mineral in the cultures. Nonspecific antibody added before day 7 blocked development of the cultures. Parallel solution based cell-free studies showed that IgG did not have a strong affinity for apatite crystals, and had no significant effect on apatite crystal growth. Type I collagen antibodies (1-200 ng/ml) added to cultures one time on day 9 (before mineralization started), or on day 11 (at the start of mineralization), slightly inhibited the accumulation of mineral. There was a statistically significant decrease in mineral accretion with 100 or 200 ng/ml collagen antibody addition continuously after these times. Fab' fragments of nonspecific and type I collagen antibodies had effects parallel to those of the intact antibodies, indicating that the decreased mineralization was not attributable to the presence of the larger, bulkier antibodies. The altered accumulation of mineral was not associated with cell death in the presence of antibody (demonstrated by fluorescent labeling of DNA) or with increased apoptosis (TUNEL-stain). In the immunoblocked cultures, EM analysis demonstrated that mineral continued to deposit on collagen fibrils, but there appeared to be fewer deposits. The data demonstrate that type I collagen is important for the mineralization of these cultures.

Bisphosphonate modulates proliferation and differentiation of rat periodontal ligament cells during wound healing

BACKGROUND

Periodontal ligament (PL) width is precisely maintained throughout the lifetime of adult mammals, but the biological mechanisms that regulate the spatial locations of the cell populations for bone, cementum, and PL are unknown.

METHODS

As bisphosphonates induce ankylosis in mouse molar teeth, we used ethane-1-hydroxy-1, 1-bisphosphonate-(HEBP, Etidronate; Didronel) in combination with a periodontal window wound model to identify those cell populations involved in the regulation of PL width during the reformation of cellular domains after wounding. Four groups of Wistar rats were wounded by drilling through the alveolar bone and extirpation of the PL. Rats were administered HEBP for 1 week and then sacrificed or allowed to recover for an additional week prior to sacrifice. Control rats were sacrificed after 1 or 2 weeks. One hour prior to sacrifice, rats were injected with 3H-thymidine to label proliferating cells. Tissue sections were immunohistochemically stained for osteopontin (OPN) or bone sialoprotein (BSP) or were prepared for in situ hybridization (BSP) to identify extra- and intracellular expression of these non-collagenous bone proteins associated with periodontal healing.

RESULTS

HEBP treatment for 1 week induced a twofold increase in the thickness of the alveolar bone matrix in which weak immuno-staining for OPN and BSP mRNA signal was seen. During the recovery phase the increased bone width was reduced but was still considerably thicker than in control (P < 0.001). OPN staining as well as the BSP mRNA signal were much more intense than at 1 week. HEBP induced a > 40% reduction of PL width which returned to normal dimensions following the recovery phase. HEBP also modulated PL cell proliferation and differentiation: PL cell counts and labelling indices were reduced fivefold after 1 week of HEBP but returned to control values after the recovery phase. In controls, PL cells did not express OPN and BSP, but after HEBP treatment, and particularly after the recovery phase, PL cells expressed both of these markers intensely. In contrast, gingival and pulp connective tissues that were contiguous with the PL were not stained for OPN and did not express BSP mRNA after HEBP treatment.

CONCLUSIONS

While wounding induced transient increases of proliferation which were followed by repopulation of the extirpated tissue, the effects of HEBP on cell differentiation were independent of wounding. HEBP modulates the differentiation of PL cells and recruits cells that contribute to alveolar bone formation and loss of PL width homeostasis. Conceivably, bisphosphonates could be used therapeutically to selectively alter the differentiation of PL cells and promote the formation of alveolar bone and cementum.

Chondrogenic differentiation in cultures of embryonic rat mesenchyme

The morphology and fine structure of day 12 rat embryonic mesenchyme from forelimb bud, mandibular arches, and frontonasal prominence is described as the cells undergo chondrogenesis in high density, micromass culture. The cultures began as a multilayered "pavement" of flattened mesenchymal cells, 3-4 deep, with moderate intercellular space but little identifiable electron-dense extracellular matrix. Pre-cartilage condensations, which consisted of aggregates of cells which had rounded up, displaying little or no intercellular space, formed within the first 24 h in limb mesenchyme and after an additional 24 h in mandibular and frontonasal cultures. Gap junctions occur between these cells, indicating a phase of direct cell-cell communication. Chondrogenesis within these aggregates began within the next 24 h in limb cultures but was delayed an additional 24-48 h in the frontonasal and especially in mandibular cultures. The aggregates in both limb and mandibular mesenchyme form discrete nodules bordered by a perichondrium consisting of 2-3 layers of flattened cells. Evidence from late stage mandibular cultures suggests that chondroblasts are added to the nodules from the perichondrium, as occurs in vivo. By contrast, the frontonasal cartilage is initially unbordered and forms anastomosing trabecular arrays. Some of these arrays fuse into larger structures with time, but others become surrounded by a single, flattened perichondrium, resulting in the stacking of these trabeculae as chondrification proceeds. The sequence of cartilage matrix production, as revealed in long-term facial cultures, appears to occur in three stages, an early phase in which the extracellular matrix consists primarily of proteoglycans, followed by a phase of homogeneous collagen-proteoglycan matrix and a mature, territorial matrix. In all three cultures the cartilage ultimately produced resembles mature rat hyaline cartilage with chondroblasts surrounded by a territorial matrix of type II collagen and proteoglycan granules.

Mesoderm and jaw development in vertebrates: the role of growth factors

The head and neck arise during development as the result of a complex series of cellular and molecular interactions that begin in the fertilized egg. In this article, the role of an important class of molecules, growth factors, is examined in two main steps of the developmental sequence: the initial induction of mesoderm and the later induction of jaw cartilage and bone. The article focuses particularly on the roles of members of the transforming growth factor-beta (TGF-beta), fibroblast growth factor (FGF), and epithelial growth factor (EGF) families in these processes and current models of growth factor involvement. Possible experiments for the future are discussed.

Chondrogenesis and myogenesis in micromass cultures of mesenchyme from mouse facial primordia

The face develops from small buds of tissue positioned around the primitive mouth. The chondrogenic and myogenic cell populations contained within these facial primordia in mouse embryos have been investigated in short-term micromass culture. Chondrogenesis occurred in frontonasal mass mesenchyme from E11-E13 embryos, in maxillary mesenchyme from E12.5 embryos and was absent in mandibular mesenchyme. Myogenesis was greatest in mandibular mesenchyme, moderate in maxillary mesenchyme and low in the frontonasal mass. When compared with chick embryos the mouse facial primordia have lower chondrogenic potential, which in the case of the frontonasal mass may be related to the relative outgrowth of the primordia in the two species. Chondrogenesis in the mouse mandibular mesenchyme may be affected by the presence of a large population of odontogenic mesenchyme. The behavior of myogenic cell populations is related to the pattern of the musculature of the face, as the mandible contains the most muscle, the maxilla some, and the frontonasal mass none. However, the presence of myoblasts in early mesenchyme of all primordia may indicate that, as with chick, facial primordia are initially seeded with muscle cells and that the size of the cell population is subsequently controlled according to the development of the musculature within the primordia.