Aberrant metabolism of matrix components in neonatal fibular cartilage of brachypod (bpH) mice

GDF-5 deficiency in mice alters the ultrastructure, mechanical properties and composition of the Achilles tendon

Acromesomelic dysplasia of the Hunter-Thompson and Grebe types are rare human disorders based on growth/differentiation factor (GDF)-5/CDMP-1 genetic mutations. Numerous skeletal abnormalities are present in these individuals, including shortened limb bones and severe dislocations of the knee. In the GDF-5 deficient brachypodism mouse, similar, although less severe, phenotypes are observed. It is unknown whether the joint dislocations observed in these disorders are due to a defect in the original formation of joints such as the knee, or to abnormalities in the tendons and ligaments themselves. We hypothesized that tendons from GDF-5 deficient mice would exhibit altered composition, mechanical properties, and ultrastructure when compared with heterozygous control littermates. GDF-5 deficient Achilles tendons were structurally weaker than controls, and structural strength differences appeared to be caused by compromised material properties: after normalizing by collagen per unit length, mutant tendons were still 50% weaker (P < 0.0001) and 50% more compliant (P < 0.001) than controls. Despite comparable levels of skeletal maturity in the two cohorts, the majority of mutant tendon failures occurred in the mid-substance of the tendon (64% of all failures), whereas the majority of control failures occurred via avulsion (92% of all failures). Mutant Achilles tendons contained 40% less collagen per microgram of DNA when compared to controls (P = 0.004). No significant difference in glycosaminoglycan (GAG)/DNA was detected. Ultrastructural analyses indicated a slight trend toward increased frequency of small diameter (30-100 nm) collagen fibrils in the mutant Achilles. Our findings suggest that increased tendon and ligament laxity may be the cause of the joint dislocations seen in patients with Hunter-Thompson and Grebe type dysplasia, rather than developmental abnormalities in the joints themselves.

The membranous skeleton: the role of cell condensations in vertebrate skeletogenesis

Elements of the vertebrate skeleton are initiated as cell condensations, collectively termed the 'membranous skeleton' whether cartilages or bones by Grüneberg (1963). Condensations, which were identified as the basic cellular units in a recent model of morphological change in development and evolution (Atchley and Hall 1991) are reviewed in this paper. Condensations are initiated either by increased mitotic activity or by aggregation of cells towards a centre. Prechondrogenic (limb bud) and preosteogenic (scleral ossicle) condensations are discussed and contrasted. Both types of skeletogenic condensations arise following epithelial-mesenchymal interactions; condensations are identified as the first cellular product of such tissue interactions. Molecular characteristics of condensations are discussed, including peanut agglutinin lectin, which is used to visualize prechondrogenic condensations, and hyaluronan, hyaladherins, heparan sulphate proteoglycan, chondroitin sulphate proteoglycan, versican, tenascin, syndecan, N-CAM, alkaline phosphatase, retinoic acid and homeo-box-containing genes. The importance for the initiation of chondrogenesis or osteogenesis of upper and lower limits to condensation size and the numbers of cells in a condensation are discussed, as illustrated by in vitro studies and by mutant embryos, including Talpid3 in the chick and Brachypod, Congenital hydrocephalus and Phocomelia in the mouse. Evidence that genes specific to the skeletal type are selectively activated at condensation is discussed, as is a recent model involving TGF-beta and fibronectin in condensation formation. Condensations emerge as a pivotal stage in initiation of the vertebrate skeleton in embryonic development and in the modification of skeletal morphology during evolution.

Alterations in cell surface galactosyltransferase activity during limb chondrogenesis in brachypod mutant mouse embryos

The autosomal mutation brachypod (bpH/bpH) in the mouse affects the development of precartilage mesenchymal condensation in the limb-bud. We have previously shown that this defect is localized to the expression of terminal N-acetylglucosamine (GlcNAc) glycoproteins in the plasma membrane (Elmer and Wright, '83). The present study is focused on cell surface galactosyltransferase (GalTase), an ectoenzyme that transfers galactose to its GlcNAc substrate. Purified plasma membrane preparations derived from wild-type (+/+), heterozygote (+/bpH) and brachypod (bpH/bpH) embryonic mouse limb cells were assayed for GalTase activity during in vitro and in utero chondrogenesis using High-Performance Liquid Chromatography (HPLC). On embryonic day E12, prior to overt expression of the mutant gene, no significant difference in GalTase activity was observed. By the third day in culture, all major chondrogenic elements of the autopod were present in +/+ and +/bpH embryos, whereas the mutant autopods were markedly deficient in staining and appeared consistently shorter. The accumulation of alcianophilic cartilage matrix in the wild-type was accompanied by a 29% increase in GalTase activity, which reflected the net change (29%) observed during development from days E12 to E13 in utero. The GalTase activity for the in utero E13 mutant (13%) was significantly different from control. In culture, day E12 mutant autopods actually decreased in their GalTase level by 3 days so that the activity was reduced to only 57% of the wild-type. Though GalTase activity in the heterozygote showed an intermediate expression, optical image analysis did not reveal consistent differences in cartilage development when compared to +/+, arguing against a gene-dosage effect at the gross anatomical level. These data indicate that an increase in plasma membrane GalTase activity is a natural developmental event that occurs during limb-bud chondrogenesis and a decrease in GalTase activity contributes to the dysmorphogenesis in brachypod limb-buds.

Correlative roentgenography and morphology of the longitudinal epiphyseal bracket

Detailed examination of a complete chondro-osseous specimen from a patient with duplication of the first ray of the foot revealed the involved metatarsal had a trapezoid-shaped, diaphyseal-metaphyseal osseous unit that was longitudinally bracketed along the lateral side by a functioning physis, epiphysis, and secondary (epiphyseal) ossification center. The physis extended as an arc from the medial proximal side toward and along the lateral side and then back to the medial side distally. The medial side of the diaphysis had a normal periosteum. The longitudinal epiphyseal ossification bracket was a composite of initially separate proximal and distal secondary ossification centers that had progressively extended toward each other and finally coalesced along the laterally placed epiphyseal cartilage. We have termed this deformity the "longitudinal epiphyseal bracket" (LEB). The macroscopic and microscopic anatomy relevant to initial diagnosis and evaluation of sequential roentgenographic changes will be considered.

The histochemical specificity of Streptomyces hyaluronidase and chondroitinase ABC

Treatment of tissue sections with enzymes wich degrade specific types of glycosaminoglycans should provide a means for localizing glycosaminoglycans in tissue sections. The feasibility of this technique was examined by utilizing endogenously labelled glycosaminoglycans in chick and quail embryos. Less than 8% of the total glycosaminoglycans appear to be lost non-specifically during fixation and dehydration. Both Streptomyces hyaluronidase and chondroitinase ABC degraded more than 90% of their respective substrates and demonstrated minimal non-specific extraction of other glycosaminoglycans. The selectivity of chondroitinase ABC for sulphated glycosaminoglycans was substantially increased by raising the pH of the incubation buffer to 8.6. At this pH, chondroitinase ABC degraded negligible amounts of hyaluronic acid. Use of both Streptomyces hyaluronidase and chondroitinase ABC confirmed that embryonic hyaluronic acid binds Alcian Blue under conditions that were previously believed specific for sulphated glycosaminoglycans. We suggest that this may be due to the increased molecular weight of embryonic hyaluronic acid compared to the hyaluronic acid in adult tissues. The results presented suggest that treatment of adjacent sections with buffer, chondroitinase ABC at pH 8.6, and Streptomyces hyaluronidase and subsequent staining with Alcian Blue provides a method for localizing and quantitating glycosaminoglycans in tissue sections.

Biochemical characteristics and biological significance of the genetically-distinct collagens

In recent years it has become evident that genetic polymorphism is dramatically expressed in the structural protein, collagen. Current information on the biochemical properties, biosynthesis, and tissue distribution of Type I, II, and III collagens is summarized with special reference to possible unique functional roles fulfilled by each of these collagens.

Changes in the pattern of RNA synthesis in mouse limb cartilage during early development

Patterns of RNA synthesis using tritiated uridine were studied in organ cultures of mouse embryonic limb cartilage and epiphyseal cartilage from mice 5 days of age. The cold phenol-sodium dodecyl sulfate extracted RNA was characterized by methylated albumin kieselguhr chromatography and polyacrylamide gel electrophoresis. The general pattern of synthesis indicates the persistence of primarily low molecular weight tRNA and a diminishing synthesis of rRNA in the older cartilage. Data show that the increased radioactivity in the low molecular weight fraction is not due to the accumulation of rRNA breakdown products. These findings are discussed in relation to other age-related studies on RNA biosynthesis.