Distribution of hyaluronan in the epiphysial growth plate: turnover by CD44-expressing osteoprogenitor cells

A sugary solution: Harnessing polysaccharide-based materials for osteoporosis treatment

Osteoporosis, a prevalent skeletal disorder characterized by diminished bone density, compromised microstructure, and heightened fracture susceptibility, poses a growing public health concern exacerbated by aging demographics. Polysaccharides-based materials, derived from a diverse range of sources, exhibit exceptional biocompatibility. They possess a structure similar to the extracellular matrix, which can enhance cell adhesion in vivo, and demonstrate superior biological activity compared to artificial materials. This study delved into an in-depth examination of the various biomaterials and polysaccharide families associated with the treatment of osteoporosis. This article elucidates the benefits and attributes of polysaccharide-based materials in contrast to current clinical treatment modalities, delineating how these materials address prevalent challenges in the clinical management of osteoporosis. An overview of the prospective applications of polysaccharide-based materials in the future is also provided, as well as outlines the challenges that should be addressed prior to the clinical implementation of such materials.

Novel PVA-Hyaluronan-Siloxane Hybrid Nanofiber Mats for Bone Tissue Engineering

Hyaluronan (HA) is a natural biodegradable biopolymer; its biological functions include cell adhesion, cell proliferation, and differentiation as well as decreasing inflammation, angiogenesis, and regeneration of damaged tissue. This makes it a suitable candidate for fabricating nanomaterials with potential use in tissue engineering. However, HA nanofiber production is restricted due to the high viscosity, low evaporation rate, and high surface tension of HA solutions. Here, hybrids in the form of continuous and randomly aligned polyvinyl alcohol (PVA)-(HA)-siloxane nanofibers were obtained using an electrospinning process. PVA-HA fibers were crosslinked by a 3D siloxane organic-inorganic matrix via sol-gel that restricts natural hydrophilicity and stiffens the structure. The hybrid nanofiber mats were characterized by FT-IR, micro-Raman spectroscopy, SEM, and biological properties. The PVA/HA ratio influenced the morphology of the hybrid nanofibers. Nanofibers with high PVA content (10PVA-8 and 10PVA-10) form mats with few beaded nanofibers, while those with high HA content (5PVA-8 and 5PVA-10) exhibit mats with mound patterns formed by "ribbon-like" nanofibers. The hybrid nanofibers were used as mats to support osteoblast growth, and they showed outstanding biological properties supporting cell adhesion, cell proliferation, and cell differentiation. Importantly, the 5PVA-8 mats show 3D spherical osteoblast morphology; this suggests the formation of tissue growth. These novel HA-based nanomaterials represent a relevant advance in designing nanofibers with unique properties for potential tissue regeneration.

Use of hyaluronic acid for regeneration of maxillofacial bones

Hyaluronic acid (HA) has been widely used in medicine and is currently of particular interest to maxillofacial surgeons. Several applications have been introduced, including those in which HA is used as a scaffold for bone regeneration, either alone or in combination with other grafting materials, to enhance bone growth. This review aims to analyze the available literature on the use of HA for maxillofacial bone regenerative procedures including socket preservation, sinus augmentation, and ridge augmentation. Medline and PubMed databases were searched for relevant reports published between January 2000 and April 2021. Nine publications describing the use of HA to augment bone volume were identified. Although further studies are needed, these findings are encouraging as they suggest that HA could be used effectively used, in combination with graft materials, in maxillofacial bone regenerative procedures. HA facilitates manipulation of bone grafts, improves handling characteristics and promotes osteoblast activity that stimulates bone regeneration and repair.

Recent Advances in Synthetic and Natural Biomaterials-Based Therapy for Bone Defects

Synthetic and natural biomaterials are a promising alternative for the treatment of critical-sized bone defects. Several parameters such as their porosity, surface, and mechanical properties are extensively pointed out as key points to recapitulate the bone microenvironment. Many biomaterials with this pursuit are employed to provide a matrix, which can supply the specific environment and architecture for an adequate bone growth. Nevertheless, some queries remain unanswered. This review discusses the recent advances achieved by some synthetic and natural biomaterials to mimic the native structure of bone and the manufacturing technology applied to obtain biomaterial candidates. The focus of this review is placed in the recent advances in the development of biomaterial-based therapy for bone defects in different types of bone. In this context, this review gives an overview of the potentialities of synthetic and natural biomaterials: polyurethanes, polyesters, hyaluronic acid, collagen, titanium, and silica as successful candidates for the treatment of bone defects.

The application of hyaluronic acid in bone regeneration

Hyaluronic acid (HA) exists naturally as an important component of the extracellular matrix (ECM) in the human body. In recent decades, HA has been widely used in bone regeneration, and is currently a popular topic, particularly in the craniofacial and dental fields. From maxilla augmentation to craniofacial bone trauma, there is now a large demand for bone regenerative therapy. Serving as a cell-seeding scaffold or a carrier for bioactive components, hyaluronic acid-incorporated scaffolds and carriers in bone regeneration can be fabricated into either rigid or colloidal forms. Since the type of material used is a critical factor in the biological properties of a scaffold, HA derivatives or HA-incorporated composite scaffolds have shown excellent potential for improving osteogenesis and mineralization. Furthermore, in order to better enhance osteogenesis, local delivery carriers based on hyaluronic acid derivatives, rather than specifically serving as scaffolds, can be established by loading different osteoinductive or osteogenetic components and acquiring different release patterns. Such osteoinductive carriers immobilized on implant surfaces are also effective in improving osseointegration. Thus, as such a competent biomaterial, hyaluronic acid should be considered a promising tool in bone regeneration.

Evaluation of Proliferation and Osteogenic Differentiation of Human Umbilical Cord-Derived Mesenchymal Stem Cells in Porous Scaffolds

INTRODUCTION

Human umbilical cord-derived mesenchymal stem cells (UCMSCs) are multiple potential stem cells that can differentiate into various kinds of functional cells, including adipocytes, osteoblasts, and chondroblasts. Thus, UCMSCs have recently been used in both stem cell therapy and tissue engineering applications to produce various functional tissues. This study aimed to evaluate the proliferation and differentiation of UCMSCs on porous scaffolds.

METHODS

UCMSCs were established in a previous study and kept in liquid nitrogen. They were thawed and expanded in vitro to yield enough cells for further experiments. The cells were characterized as having MSC phenotype. They were seeded onto culture medium-treated porous scaffolds or on non-treated porous scaffolds at different densities of UCMSCs (10⁵, 2.1 × 10⁵, and 5 × 10⁵ cells/0.005 g scaffold). The existence of UCMSCs on the scaffold was evaluated by nucleic staining using Hoechst 33342 dye, while cell proliferation on the scaffold was determined by MTT assay. Osteogenic differentiation was evaluated by changes in cellular morphology, accumulation of extracellular calcium, and expression of osteoblast-specific genes (including runx2, osteopontin (OPN), and osteocalcin (OCN)).

RESULTS

The data showed that UCMSCs could attach, proliferate, and differentiate on both treated and non-treated scaffolds but were better on the treated scaffold. At a cell density of 10⁵ cells/0.005 g scaffold, the adherent and proliferative abilities of UCMSCs were higher than that of the other densities after 14 days of culture (p < 0.05). Adherent UCMSCs on the scaffold could be induced into osteoblasts in the osteogenic medium after 21 days of induction. These cells accumulated calcium in the extracellular matrix that was positive with Alizarin Red staining. They also expressed some genes related to osteoblasts, including runx2, OPN, and OCN.

CONCLUSION

UCMSCs could adhere, proliferate, and differentiate into osteoblasts on porous scaffolds. Therefore, porous scaffolds (such as Variotis) may be suitable scaffolds for producing bone tissue in combination with UCMSCs.

ECM signaling in cartilage development and endochondral ossification

During cartilage development chondrocytes undergo a multi-step process characterized by consecutive changes in cell morphology and gene expression. Cell proliferation, polarity, differentiation, and migration are influenced by chemical and mechanical signaling between the extracellular matrix (ECM) and the cell. Several structurally diverse transmembrane receptors such as integrins, discoidin domain receptor 2 (DDR 2), and CD44 mediate the crosstalk between cells and their ECM. However, the contribution of cell-matrix interactions during early chondrogenesis and further cartilage development through cell receptors and their signal transduction pathways is still not fully understood. Determination of receptor signaling pathways and the function of downstream targets will aid in a better understanding of musculoskeletal pathologies such as chondrodysplasia, and the development of new approaches for the treatment of cartilage disorders. We will summarize recent findings, linking cell receptors and their potential signaling pathways to the control of chondrocyte behavior during early chondrogenesis and endochondral ossification.

Expression and activity of Runx2 mediated by hyaluronan during chondrocyte differentiation

During endochondral ossification, the production of hyaluronan (HA) is strictly and selectively regulated by chondrocytes, with a temporal peak at the hypertrophic stage. This study was conducted to clarify the effects of HA on expression and activity of runt-related gene 2 (Runx2), a potent transcription factor for chondrocyte differentiation in hypertrophic chondrocytes. Immature chondrocytes from an ATDC5 cell line were cultured and differentiated in DMEM/Ham's F12 with pre-defined supplements. Using real-time PCR, the gene expressions of type II collagen, MMP-13, HAS2, and Runx2 in cultured chondrocytes were analysed from days 0 to 18 of cell differentiation. The activity and expression of Runx2 in hypertrophic chondrocytes were analysed after the treatment with HA oligosaccharide (HAoligo) using AML-3/Runx2 binding, real-time PCR and Western blot analysis. The effects of pre-incubation of anti-CD44 antibody on Runx2 expression were also examined. Expression of type X collagen and Runx2 mRNAs reached a maximum at the terminal differentiation of chondrocytes. The activity and expression of Runx2 was significantly inhibited in hypertrophic chondrocytes treated with HAoligo compared to the untreated controls. High molecular weight-HA did not affect the expression or activity of Runx2. The expression of Runx2 mRNA was significantly decreased in hypertrophic chondrocytes treated with anti-CD44 antibody. These results suggest that HAoligo may affect the terminal differentiation of chondrocytes during the endochondral ossification by inhibiting the expression and activity of Runx2.

The metabolism of hyaluronan in cultured rabbit growth plate chondrocytes during differentiation

Hyaluronan (HA) is one of the major extracellular matrix components in cartilage. In addition to the biomechanical functions, HA has various important roles in the differentiation of chondrocytes. The purpose of this study was to clarify the nature of HA synthesis during chondrocyte differentiation. Growth plate chondrocytes were isolated from rabbit ribs and cultured in chondrocyte differentiation medium. The amount of HA and HA synthase (HAS) mRNA levels were analyzed for each stage of chondrocyte differentiation by means of high-performance liquid chromatography (HPLC) and real-time PCR, respectively. The distribution of HA in cultured chondrocytes was observed by histochemical staining. The amount of HA, ranging widely in size, was increased substantially during the hypertrophic stage. The expression levels of HAS2 and HAS3 mRNAs were low during the matrix-forming stage. HAS2 mRNA level was substantially enhanced at the pre-hypertrophic stage, whereas HAS3 mRNA level exhibited a slight increase. HAS1 mRNA was not detected. The intensity of HA staining was high around the hypertrophic chondrocytes. These results suggest that HA metabolism in chondrocyte differentiation is regulated by the selective expression of HASs, and HAS2 and the related large size-HA may have a certain association with the hypertrophic changes of chondrocytes.

Hyaluronidase expression in cultured growth plate chondrocytes during differentiation

Hyaluronan (HA) is a major component of the extracellular matrix of cartilage, contributes to its structural and functional integrity, and has various important roles in the differentiation of chondrocytes. HA metabolism is regulated by both anabolic and catabolic processes; however, the details have not yet been clarified. The purpose of this study was to clarify the expression patterns of hyaluronidase (HAase) mRNAs (from the relevant HAase genes: the HYALs) and HAase activity during chondrocyte differentiation. Cartilage tissue and growth plate chondrocytes were isolated from the ribs of 4-week-old male Japanese rabbits. The expression of HYAL mRNAs in cartilage was analyzed by in situ hybridization. The expression levels of HYAL mRNAs in the culture were analyzed for each of the chondrocyte differentiation stages by means of quantitative real-time polymerase chain reaction analysis. Enzymatic activity in the conditioned medium from the cultures was examined by using HA zymography and an enzyme-linked immunosorbent-like assay. The expression levels of HYAL1 and HYAL2 mRNAs were enhanced about 2.8-fold and 3.2-fold at the maximum during the early matrix forming stage, respectively, and by about 3.2-fold and 2.0-fold at the maximum in the hypertrophic stage, respectively. HYAL3 mRNA was not detected throughout the experimental period. HAase activity was enhanced at the early matrix forming and hypertrophic stages. These results suggest that selective expression of HYALs is essential for extracellular HA metabolism during chondrocyte differentiation.

Matrix metalloproteinase-9 expression, tartrate-resistant acid phosphatase activity, and DNA fragmentation in vascular and cellular invasion into cartilage preceding primary endochondral ossification in long bones

Vascular and cellular invasion into cartilage are essential for endochondral ossification. Recently it has been shown that matrix metalloproteinase-9 (MMP-9)/gelatinase B is a key regulator of growth plate angiogenesis and apoptosis of hypertrophic chondrocytes. To study vascular and cellular invasion into cartilage preceding primary endochondral ossification in long bones, precursor femurs from 13- to 16-day-old murine embryos were sectioned. Tartrate-resistant acid phosphatase (TRAP) activity, in situ hybridization for matrix metalloproteinase-9 (MMP-9), immunostaining for CD31, and in situ detection of apoptosis (TUNEL) were studied. TRAP activity, MMP-9 mRNA, and CD31 expression were initially detected in the intertrabecular spaces of the perichondral collar, and then in cells migrating into the cartilage. The first cells involved in the primary invasion into cartilage were CD31-positive vascular endothelial cells and MMP-9-positive cells, followed by TRAP-positive cells. At the cartilage-marrow interface, CD31-positive vascular endothelial cells and MMP-9-positive cells were predominant. These results suggest that MMP-9-positive cells cooperate with vascular endothelial cells in cartilage angiogenesis. TUNEL-positive staining was detected on chondrocytes attached to the inner surface of the perichondral collar, and also detected in the area where cartilage was removed. These results suggest that chondrocytes separated from the cartilage matrix may undergo apoptosis.

Hyaluronan in the bovine oviduct--modulation of synthases and receptors during the estrous cycle

The extracellular matrix (ECM) component hyaluronan (HA) is considered to contribute to the optimal development of the gametes and the embryo. In this study, the regulation and localization of HA, its synthases (HAS 1-3) and the receptors CD44, RHAMM, and putative HARE were investigated in bovine oviducts during the estrous cycle. HA could be demonstrated in the entire lamina propria of the oviduct. Immunoreactive CD44 was found in the muscle layer additionally to the lamina propria. As for HA the luminal epithelium was devoid of CD44. Introducing quantitative real-time RT-PCR, a remarkable cyclic change of CD44 occurred. In addition, CD44 expression was much higher in the isthmus than in the ampulla. RHAMM transcripts were elevated particularly in the ampulla during estrus. A major part of the bovine putative HARE coding sequence was characterized. An analysis of the tissue distribution revealed transcripts not restricted to liver, spleen, and lymph node, providing evidence for a local putative HARE-mediated turnover of HA. These results suggest that in the bovine oviduct the cell surface receptor CD44 in particular might inversely regulate HA during the estrous cycle. The local turnover of HA through synthases and receptors may indicate its participation in the bovine oviduct supporting gamete maturation and early embryonic development.

Parathyroid Hormone Rapidly Stimulates Hyaluronan Synthesis by Periosteal Osteoblasts in the Tibial Diaphysis of the Growing Rat

Short term treatment (3-24 h) with parathyroid hormone (PTH) stimulated the synthesis and accumulation of hyaluronan (HyA) in explant cultures of tibial diaphyses from young rats. PTH increased the overall HyA content of periosteum 5-fold, with the basal cambium layer exhibiting the greatest enhancement ( approximately 8-fold). PTH increased the HyA content of cortical bone by 2-fold while not affecting the HyA content of bone marrow. PTH treatment greatly enhanced HyA staining throughout all layers of the periosteum, although its most dramatic effect occurred in the basal cambium layer. Here, unlike in the control tissue sections, nearly all cambium-lining osteoblasts stained intensely positive for HyA. PTH treatment enhanced the HyA staining of osteocytes in cortical bone tissue sections to the extent that the lacunocanalicular system became visualized. Three significant findings were revealed in this study. First, mature periosteal osteoblasts, under natural conditions, do not contain much HyA in their surrounding extracellular matrix but dramatically enhance their matrix HyA content when treated with PTH. Second, pre-osteocytes and osteocytes contain more HyA in their natural matrix than mature lining osteoblasts, and they appear to have functional PTH receptors because they responded to PTH treatment with an enhancement of HyA content. Finally, it was observed that the lining cells along the endosteal surface of the diaphysis did not stain strongly positive for HyA either naturally or when exposed to PTH treatment. This indicates that periosteal and endosteal osteoblastic cell populations exhibit metabolic differences in their extracellular matrix responses to PTH.

Expression of CD44 isoforms correlates with the metastatic potential of osteosarcoma

CD44 is a family of cell surface glycoproteins that not only function as receptors to hyaluronan and other extracellular matrix ligands but also is implicated in the invasiveness and metastasis of malignancies. The expression of CD44 and its variant isoforms was evaluated by immunohistochemistry in primary osteosarcoma, and the metastatic significance and prognostic relevance were assessed. Fifty formalin-fixed, paraffin-embedded conventional osteosarcoma samples were immunostained with monoclonal antibodies to CD44s, CD44v5, and CD44v6. When more than 50% of the tumor cells were stained, the result was regarded as ++ (overexpression). Staining patterns of CD44 mainly were membranous with a minor cytoplasmic pattern contribution. CD44s was overexpressed in 16% (eight of 50), CD44v6 in 20% (10 of 50), and CD44v5 in 46% (23 of 50) of the osteosarcomas. Of 26 patients with metastasis, CD44s was overexpressed in six (23%), CD44v6 in four (15%), and CD44v5 in 17 (65%). Overexpression of CD44v5 correlated significantly with metastasis. The survival rate of the patients in the CD44v5-positive (overexpressed) group was significantly lower than that of the patients in the CD44v5-negative group. Overexpression of CD44v5 may play a role in the metastatic behavior of osteosarcoma and may be a prognostic parameter of osteosarcoma.

A dominant interference collagen X mutation disrupts hypertrophic chondrocyte pericellular matrix and glycosaminoglycan and proteoglycan distribution in transgenic mice

Collagen X transgenic (Tg) mice displayed skeleto-hematopoietic defects in tissues derived by endochondral skeletogenesis.(1) Here we demonstrate that co-expression of the transgene product containing truncated chicken collagen X with full-length mouse collagen X in a cell-free translation system yielded chicken-mouse hybrid trimers and truncated chicken homotrimers; this indicated that the mutant could assemble with endogenous collagen X and thus had potential for dominant interference. Moreover, species-specific collagen X antibodies co-localized the transgene product with endogenous collagen X to hypertrophic cartilage in growth plates and ossification centers; proliferative chondrocytes also stained diffusely. Electron microscopy revealed a disrupted hexagonal lattice network in the hypertrophic chondrocyte pericellular matrix in Tg growth plates, as well as altered mineral deposition. Ruthenium hexamine trichloride-positive aggregates, likely glycosaminoglycans (GAGs)/proteoglycans (PGs), were also dispersed throughout the chondro-osseous junction. These defects likely resulted from transgene co-localization and dominant interference with endogenous collagen X. Moreover, altered GAG/PG distribution in growth plates of both collagen X Tg and null mice was confirmed by a paucity of staining for hyaluronan and heparan sulfate PG. A provocative hypothesis links the disruption of the collagen X pericellular network and GAG/PG decompartmentalization to the potential locus for hematopoietic failure in the collagen X mice.

The distribution pattern of the hyaluronan receptor CD44 during human tooth development

The aim was to investigate the expression pattern of the major cell-surface hyaluronan receptor CD44, as there are no existing data on its presence or absence in human dental structures at different developmental stages. Immunohistochemical localization of CD44 was studied using a monoclonal antibody, H3, that specifically recognizes an epitope in the common backbone of all CD44 isoforms. The dental lamina displayed a strong CD44 signal; the external enamel epithelium was negative. In the coronal region of the tooth germ the presecretory ameloblasts showed an intense reaction whereas the less differentiated inner enamel epithelial cells showed no signal at the cervical loop where they meet the external enamel epithelium. In the stellate reticulum a moderate reaction was detected. The secretory ameloblasts and the stratum intermedium showed a strong cell-surface CD44 signal. A strong signal was also observed on the odontoblasts and their processes. In the pulp, close to the odontoblastic layer, weak labelling was seen in the walls of capillary vessels. The distribution of CD44 in the human tooth germ corresponds to that of hyaluronan in most locations, suggesting that during tooth development this transmembrane protein plays an important part in hyaluronan-mediated events.

Hyaluronidase expression in human skin fibroblasts

Hyaluronidase activity has been detected for the first time in normal human dermal fibroblasts (HS27), as well as in fetal fibroblasts (FF24) and fibrosarcoma cells (HT1080). Enzymatic activity was secreted predominantly into the culture media, with minor amounts of activity associated with the cell layer. In both classes of fibroblasts, hyaluronidase expression was confluence-dependent, with highest levels of activity occurring in quiescent, post-confluent cells. However, in the fibrosarcoma cell cultures, expression was independent of cell density. The enzyme had a pH optimum of 3.7 and on hyaluronan substrate gel zymography, activity occurred as a single band corresponding to an approximate molecular size of 57 kDa. The enzyme could be immunoprecipitated in its entirety using monoclonal antibodies raised against Hyal-1, human plasma hyaluronidase. PCR confirmed that fibroblast hyaluronidase was identical to Hyal-1. The conclusion by previous investigators using earlier technologies that fibroblasts do not contain hyaluronidase activity should be reevaluated.

Inhibition of tumor growth in vivo by hyaluronan oligomers

One of the critical events in tumor growth and metastasis is the interaction between tumor cells and host tissue stroma, mediated by different adhesion receptor repertoires in different tumor cell types. Several lines of evidence indicate that interaction between the hyaluronan receptor CD44, expressed on tumor cells, and host tissue stromal hyaluronan can enhance growth and invasiveness of certain tumors. Disruption of CD44-hyaluronan interaction by soluble recombinant CD44 has been shown to inhibit tumor formation by lymphoma and melanoma cells transfected with CD44. Since hyaluronan is a ubiquitous glycosaminoglycan polymer from which oligosaccharides of defined size can be readily purified, we tested the ability of hyaluronan oligomers to inhibit tumor formation by subcutaneously (s.c.) injected B16F10 melanoma cells. Our results indicate that hyaluronan oligomers injected at concentrations as low as 1 mg/ml can markedly inhibit B16F10 melanoma growth, providing a potentially attractive reagent for the control of local tumor development.

In vitro characterization of chondrogenic cells isolated from chick embryonic muscle using peanut agglutinin affinity chromatography

Specific binding to the lectin, peanut agglutinin (PNA), has been reported in embryonic precartilage tissues, including the condensing limb bud blastema and the caudal half of the developing somite. The present study aimed to test the hypothesis that PNA-binding may be a surface characteristic of chondroprogenitor cells residing within noncartilage tissues, such as muscle, which have the potential of being induced to form cartilage, e.g., in the presence of bone matrix-derived factors. Day-14 chick embryonic pectoral muscle, which contained histochemically detectable PNA-binding cells, was dissociated into single cells (TM cells) and fractionated by PNA affinity chromatography into PNA-binding (PNA+) and nonbinding (PNA-) cells by PNA-Sepharose 6 MB affinity chromatography. The differentiation potential of the PNA-affinity fractionated cells in vitro was analyzed as a function of culture plating cell density. Immunohistochemistry of a number of cell-type-specific differentiation markers, including sarcomeric actin, collagen type II, and aggrecan core protein, demonstrated that PNA+ cells, when cultured as a micromass at high density (20 x 10(6) cells/ml), exhibited a chondrocyte-like phenotype, whereas the PNA-cells remained myogenic; however, both PNA+ and PNA- monolayer cultures (4 x 10(4) cells/ml) behaved as myoblastic cells. The expression of collagen type II mRNA was also confirmed by coupled reverse transcription/polymerase chain reaction analysis. These observations suggest that PNA binding, i.e., the presence of specific galactose-containing cell surface moieties, is likely to be one of the characteristics of chondrogenic cells residing in mesenchymally derived embryonic tissues.

Hyaluronan synthesis by epiphysial chondrocytes is regulated by growth hormone, insulin-like growth factor-1, parathyroid hormone and transforming growth factor-beta 1

In a previous study, we presented evidence that the synthesis of hyaluronan by hypertrophic chondrocytes is one of the principal factors driving the interstitial expansion of the growth plate (Pavasant et al., J. Cell Sci. 109: 327-334, 1996). To test this possibility further, we used two different approaches to examine the effects of hormones on the production of hyaluronan in the growth plate. In the first approach, we examined the growth plate of the lit/lit mouse that lacks growth hormone and found that its hypertrophic lacunae were smaller and contained less hyaluronan than those of wild type mice. Moreover, the ratios of hyaluronan staining density to total area of the lacunae were similar for the lit/lit and the wt/wt mice, indicating that the amount of hyaluronan is directly related to lacuna size. In the second approach, we examined the effects of hormones on segments of the epiphysial growth plate placed in organ culture. Under normal culture conditions, a band of hyaluronan staining progressed across the length of the growth plate, reflecting the maturation of chondrocytes into the hypertrophic stage. When insulin-like growth factor-1, a factor known to promote chondrocyte maturation, was added to the culture medium, the production of hyaluronan and the enlargement of the lacunae were stimulated. In contrast, when either parathyroid hormone or transforming growth factor-beta 1, both of which inhibit chondrocyte differentiation, was added to the medium of cultured segments, new pericellular hyaluronan was not detected and the lacunae did not enlarge. Taken together, these results indicate that factors that either stimulate or inhibit the maturation of epiphysial chondrocytes have a corresponding effect on the production of hyaluronan. This, in turn, further supports the importance of hyaluronan in the process of lacuna enlargement.

Expression of multiple CD44 isoforms in the apical ectodermal ridge of the embryonic mouse limb

Previous immunohistochemical studies have shown that CD44 is highly enriched within the apical ectodermal ridge of the developing limb (Wheatley et al. [1993] Development 119: 295-306), but the particular isoforms of CD44 were not identified. We show here that CD44s (standard or "hemopoietic" isoform) and several CD44 variants, especially V3-V10, V4-V10, and V6-V10, are concentrated in the apical ectodermal ridge in the early mouse limb. Since CD44s is a major cell surface receptor for hyaluronan, we compared its localization with that of hyaluronan. In the early limb bud, hyaluronan is distributed throughout the mesoderm but is absent from all regions of the ectoderm. Hyaluronan is especially enriched in the basement membrane separating ectoderm and mesoderm, except beneath the apical ectodermal ridge where it is absent. Since CD44s is a known endocytic receptor for hyaluronan, its presence in ridge ectoderm could lead to degradation of hyaluronan destined for the neighboring region of basement membrane, thus facilitating interaction of the ridge with underlying mesoderm. The CD44 (V3-V10) isoform found in the ridge is expressed elsewhere as a proteoglycan with heparan sulfate chains that bind fibroblast growth factors. Since fibroblast growth factors are present in the ridge and are essential for limb morphogenesis, CD44 (V3-V10) is likely to act as a cofactor or modulator in the growth-promoting action or maintenance of the ridge.

Ultrastructural, enzyme-, lectin, and immunohistochemical studies of the erosion zone in rat tibiae

To clarify the process of endochondral ossification, we used ultrastructural, enzyme-, lectin-, and immunohistochemical techniques to study perivascular cells located in the erosion zones of rat tibiae. In growth plate erosion zones, perivascular cells directly connected to blood capillaries were seen invading cartilage. These cells contained a well-developed rough endoplasmic reticulum and Golgi apparatus in their cytoplasm and formed finger-like cytoplasmic processes toward uncalcified transverse cartilage walls. These processes were seen to stretch as far as the degenerated chondrocytes located in the calcified layer of the growth plate. Interestingly, these perivascular cells showed neither alkaline phosphatase activity nor tartrate-resistant acid phosphatase activity. Lectin histochemistry revealed specific staining by Dolichos Biflorus agglutinin (DBA) on the perivascular cells. No reactivity for DBA was detected on either endothelial cells, osteoblasts, chondroclasts, or osteoclasts. In addition, immuno-histochemical studies showed that the perivascular cells neither expressed CD44, which was localized on the plasma membrane of chondroclasts, osteoclasts, and osteocytes, nor were surrounded by laminin. These results suggest that the perivascular cells in the erosion zone are distinct from endothelial cells, osteoblasts, chondroclasts, and osteoclasts; that they may resorb uncalcified cartilage matrix and degenerated chondrocytes; and that perivascular cells may play an important role in the capillary invasion during the process of endochondral ossification.

Spatial distribution of CD44 and hyaluronan in the proximal tibia of the growing rat

CD44 has been described as a cell surface hyaluronan receptor present on a variety of different cells, and it is generally assumed to be prevalent in most connective tissues that contain hyaluronan. A major aim of this study was to test that presumption by localizing CD44 and hyaluronan within several tissues of the proximal tibia of the growing rat. Comparison of these profiles would reveal whether CD44 and hyaluronan co-localize with high fidelity, as would be expected if CD44 were a major hyaluronan binding protein. Using in situ hybridization and immunohistochemistry, CD44 was identified on osteoclasts, chondroclasts, osteocytes, hematopoietic marrow cells, synovial cells, and connective tissue fibroblasts (ligaments, tendons, and fascia). Although the majority of osteocytes expressed CD44, reduced expression was observed for osteoblasts and ostcoprogenitor cells. Additionally. CD44 was not detected on chondrocytes from epiphyseal and metaphyseal growth cartilages or in meniscal fibrocartilage. Using biotinylated G1 domain from aggrecan and link protein, hyaluronan was observed in the maturational and hypertrophic zones of all growth cartilages, the synovium and other fibroblastic connective tissues, regional areas of the periosteum and endosteum (around osteoblasts, osteoprogenitor cells, and osteoclasts), osteocyte lacunae, and surrounding blood vessels. In regions of co-localization for CD44 and hyaluronan, it seems that CD44 is a likely hyaluronan binding protein in several tissues of the proximal tibia. However, it does not appear to be the predominant hyaluronan binding protein in growing cartilages of the weanling rat.

Influence of vitamin D status on hyaluronan localization in bone

The distribution of hyaluronan was investigated in the proximal tibiotarsal bones of normal (vitamin D-treated) chicks, and chicks with rachitic lesions induced by vitamin D deficiency. Localization studies using a biotinylated hyaluronan-binding probe revealed that in vitamin D-treated chicks, a high level of hyaluronan staining was present in upper proliferative zone cartilage and upper hypertrophic zone cartilage. Hyaluronan staining was greatly reduced in the zone of provisional calcification. In the metaphyses and diaphyses of normal chicks hyaluronan was predominantly localized to the non-bone-forming surfaces of osteoblasts but was also present on the basolateral surfaces of lining cells and osteoclasts. Marked changes in hyaluronan distribution were observed in vitamin D-deficient chicks. The amount of hyaluronan present in proliferative zone growth cartilage was similar to control chicks, although with a more widespread distribution, extending into lower proliferative zone cartilage. In the zone of hypertrophy/calcification, biochemical analyses revealed that hyaluronan levels in rachitic chicks were about 3.6 times greater than in vitamin D-treated chicks; localization studies demonstrated that this increase was associated with the presence of hyaluronan-positive spindle-shaped cells in the metaphyseal vascular spaces. Intense hyaluronan staining was also associated with abundant spindle-shaped cells occupying the marrow spaces of rachitic diaphyseal bone. The distribution of hyaluronan in vitamin D-treated chick bone, and the alterations observed in rachitic tissue suggests a role for hyaluronan in endochondral bone formation.

Hyaluronan contributes to the enlargement of hypertrophic lacunae in the growth plate

Histochemical staining of the epiphysial growth plate revealed that free hyaluronan (i.e. available to the staining probe) was restricted to the zone of hypertrophy, where it was located in the pericellular space between the chondrocytes and the edge of the lacunae. Furthermore, the amount of hyaluronan staining was directly proportional to the size of the lacunae. Autoradiographic analysis of growth plates cultured with isotopically labeled glucosamine indicated that at least a portion of this hyaluronan was newly synthesized by the hypertrophic chondrocytes. Since hyaluronan can adsorb large amounts of water, it is possible that it exerted a hydrostatic pressure on the surrounding territorial matrix and thereby caused the expansion of hypertrophic lacunae. To assess this possibility, segments of the growth plate were placed in organ culture under different conditions. Under normal culture conditions, a band of hyaluronan staining migrated across the segments coinciding with the enlargement of lacunae in these regions, and the segments, as a whole, increased in size. In contrast, when the segments were cultured in the presence of hyaluronidase, which degraded the pericellular hyaluronan, the lacunae did not undergo enlargement and the overall size of the segments did not increase. These results suggest that the production of hyaluronan contributes to the enlargement of hypertrophic lacunae which is important for determining both the body's stature and proportions.