Collagen fibrils forming in developing tendon show an early and abrupt limitation in diameter at the growing tips

The formation of very long and near-uniform diameter collagen fibrils is fundamental to the assembly of the extracellular matrix of animals. However, how growth in length and diameter is regulated, and how fibrils increase in diameter during development, are poorly understood. The approach in this study was to examine the tips and central shaft regions of fibrils from 12 and 18-day embryonic chick metatarsal tendon using quantitative mass mapping electron microscopy. We found that the fibrils had smoothly tapered C and N-terminal tips, which had linear axial mass distributions and were consequently parabolic in shape. An invariant feature of all tips (N and C) was an abrupt stop in lateral growth leading to a local plateau in diameter. The distance from the end of the fibril to the abrupt stop occurred at multiples of five D-periods (where D=67 nm). This implies that D-periods at the ends of fibrils are not equivalent sites for accretion, and that diameter regulation relies on surface structural features, which repeat every 5D. Mass mapping of entire fibrils at day 12 showed that, on average, the coarseness of the fibril tips was independent of fibril length, consistent with individual fibrils growing at constant tip shape. Comparison of diameters in the plateau (close to the tips) and shaft regions of the fibril showed that fibrils in day 12 tendons grow in length at constant diameter. Analysis of tendons from day 18 embryos showed that the increase in diameter at this stage of development was the result of both increases in the coarseness of the tips and continued lateral accretion of mass onto the central shafts at distances away from the growing tips. Regulated tip growth provides an attractive explanation for how cells are able to synthesise very long fibrils during the organisation of the extracellular matrix.

Muscle and tendon morphogenesis in the avian hind limb

The proper development of the musculoskeletal system in the tetrapod limb requires the coordinated development of muscle, tendon and cartilage. This paper examines the morphogenesis of muscle and tendon in the developing avian hind limb. Based on a developmental series of embryos labeled with myosin and tenascin antibodies in whole mount, an integrative description of the temporal sequence and spatial pattern of muscle and tendon morphogenesis and their relationship to cartilage throughout the chick hind limb is presented for the first time. Anatomically distinct muscles arise by the progressive segregation of muscle: differentiated myotubes first appear as a pair of dorsal and ventral muscle masses; these masses subdivide into dorsal and ventral thigh, shank and foot muscle masses; and finally these six masses segregate into individual muscles. From their initial appearance, most myotubes are precisely oriented and their pattern presages the pattern of future, individual muscles. Anatomically distinct tendons emerge from three tendon primordia associated with the major joints of the limb. Contrary to previous reports, comparison of muscle and tendon reveals that much of their morphogenesis is temporally and spatially closely associated. To test whether reciprocal muscle-tendon interactions are necessary for correct muscle-tendon patterning or whether morphogenesis of each of these tissues is autonomous, two sets of experiments were conducted: (1) tendon development was examined in muscleless limbs produced by coelomic grafting of early limb buds and (2) muscle development was analyzed in limbs where tendon had been surgically altered. These experiments demonstrate that in the avian hind limb the initial morphogenetic events, formation of tendon primordia and initial differentiation of myogenic precursors, occur autonomously with respect to one another. However, later morphogenetic events, such as subdivision of muscle masses and segregation of tendon primordia into individual tendons, do require to various degrees reciprocal interactions between muscle and tendon. The dependence of these later morphogenetic events on tissue interactions differs between different proximodistal regions of the limb.

Differential in situ expression of alpha2(XI) collagen mRNA isoforms in the developing mouse

Type XI collagen is an essential structural component of the extracellular matrix of cartilage and plays a role in collagen fibril formation and skeletal morphogenesis. The expression of all three type XI collagen genes is not restricted to cartilage. In addition, alternative exon usage seems to increase the structural diversity and functional potential of type XI collagen during development. In order to investigate type XI collagen expression during development, we have examined alpha2(XI) and alpha1(XI) collagen genes by in situ hybridization in mice. Transcripts of the alpha2(XI) collagen gene were first detected in the notochord of mouse embryos after 11.5 days of gestation. Subsequently, alpha2(XI) mRNA was mainly found in the cartilaginous tissues of the developing limbs and axial skeleton together with transcripts of the alpha1(XI) gene. The alpha2(XI) transcripts seemed to be alternatively spliced isoforms lacking exons 6-8, which code for an acidic domain. Expression of alpha2(XI) outside the cartilage was relatively restricted, whereas expression of the alpha1(XI) gene was widespread. However, expression of alpha2(XI) transcripts containing exons 6-8 was found in non-chondrogenic tissues, including the calvarium and periosteum where intramembranous ossification occurs. These results indicate that alpha2(XI) mRNA isoforms are differentially expressed in various tissues during development. In addition, alpha2(XI) mRNA isoforms containing alternative exons are present in osteogenic cells, and their expression may be closely related to the formation of bone or cartilage.

Absence of PS integrins or laminin A affects extracellular adhesion, but not intracellular assembly, of hemiadherens and neuromuscular junctions in Drosophila embryos

We have examined the role of integrins in the formation of the cell junctions that connect muscles to epidermis (muscle attachments) and muscles to neurons (neuromuscular junctions). To this end we have analyzed muscle attachments and neuromuscular junctions ultrastructurally in single or double mutant Drosophila embryos lacking PS1 integrin (alphaPS1betaPS), PS2 integrin (alphaPS2betaPS), and/or their potential extracellular ligand laminin A. At the muscle attachments PS integrins are essential for the adhesion of hemiadherens junctions (HAJs) to extracellular matrix, but not for their intracellular link to the cytoskeleton. The PS2 integrin is only expressed in the muscles, but it is essential for the adhesion of muscle and epidermal HAJs to electron dense extracellular matrix. It is also required for adhesion of muscle HAJs to a less electron dense form of extracellular matrix, the basement membrane. The PS1 integrin is expressed in epidermal cells and can mediate adhesion of the epidermal HAJs to the basement membrane. The ligands involved in adhesion mediated by both PS integrins seem distinct because adhesion mediated by PS1 appears to require the extracellular matrix component laminin A, while adhesion mediated by PS2 integrin does not. At neuromuscular junctions the formation of functional synapses occurs normally in embryos lacking PS integrins and/or laminin A, but the extent of contact between neuronal and muscle surfaces is altered significantly. We suggest that neuromuscular contact in part requires basement membrane adhesion to the general muscle surface, and this form of adhesion is completely abolished in the absence of laminin A.

The muscleblind gene participates in the organization of Z-bands and epidermal attachments of Drosophila muscles and is regulated by Dmef2

We report the embryonic phenotype of muscleblind (mbl), a recently described Drosophila gene involved in terminal differentiation of adult ommatidia. mbl is a nuclear protein expressed late in the embryo in pharyngeal, visceral, and somatic muscles, the ventral nerve cord, and the larval photoreceptor system. All three mbl alleles studied exhibit a lethal phenotype and die as stage 17 embryos or first instar larvae. These larvae are partially paralyzed, show a characteristically contracted abdomen, and lack striation of muscles. Our analysis of the somatic musculature shows that the pattern of muscles is established correctly, and they form morphologically normal synapses. Ultrastructural analysis, however, reveals two defects in the terminal differentiation of the muscles: inability to differentiate Z-bands in the sarcomeric apparatus and reduction of extracellular tendon matrix at attachment sites to the epidermis. Failure to differentiate both structures could explain the partial paralysis and contracted abdomen phenotype. Analysis of mbl expression in embryos that are either mutant for Dmef2 or ectopically express Dmef2 places mbl downstream of Dmef2 function in the myogenic differentiation program. mbl, therefore, may act as a critical element in the execution of two Dmef2-dependent processes in the terminal differentiation of muscles.

Molecular structural changes in human fetal tissue during the early stages of embryogenesis

Low-angle synchrotron X-ray diffraction studies of human fetal extensor tendon and skin collagen, centred in time about the period of first major movement by the fetus, indicate that alignment of the tendon collagen fibrils occurs about this time. At this stage there appears to be no detectable structural difference between tendon and dermis. By three weeks post-partum, marked differences between these tissues can be detected. The distribution of the intermediate filament diameters for all fetal tendons investigated was unimodal (mean 41.2 +/- 0.4 nm) in contrast with that for post-partum tendon which is multimodal. Equatorial periodicities of 353 +/- 3 nm and 32.1 +/- 0.1 nm, consistent with the presence of type IV collagen, were obtained from all fetal samples examined. Neither of these periodicities were observed in post-partum normal tendon and only the larger were observed in post-partum normal skin. The consistency of the results suggest that low-angle X-ray diffraction could be used for the identification of fetal-like tissues found in pathological tissues.

Differential expression of genes associated with collagen fibril growth in the chicken tendon: identification of structural and regulatory genes by subtractive hybridization

Collagen fibril growth is a very rapid and abrupt process, resulting in a 4- to 5-fold increase in fibril length between 16 and 18 days of chicken metatarsal tendon development. This fibril growth is due to a postdepositional fusion/association of preformed intermediates, termed fibril segments. We propose that the regulated assembly of collagen fibrils from the segment intermediates is mediated by interactions of structural macromolecules. The cells could modulate this process by responding to cytokines and altering cell-matrix signaling, transcription, and translation. To identify the genes involved in this process a subtractive hybridization procedure was utilized. Genes of cell proliferation were excluded as major contributors to differential gene expression in avian tendon on days 14 and 19 of development after analysis of BrdUr incorporation. The BrdUr incorporation studies revealed little, if any, tendon fibroblast proliferation at both stages. This suggested that observed alterations in gene expression would be related to the pre- and postfibril growth phases in developing tendons. A total of 80 unique up- and down-regulated cDNA fragments were isolated and 26 of these were identified. There was an up-regulation of structural proteins (for example, collagen types I, VI, and XI and fibromodulin), a number of regulatory proteins (including TGF-beta 2 and IGF-1), as well as other enzymes/proteins. Northern analysis confirmed the up-regulation of mRNAs for all the structural proteins. The observed 20-fold increase of mRNA for the isolated clone corresponding to the 3' UTR of alpha 1(VI) collagen makes it a possible marker for the postfibril growth stage of collagen fibrillogenesis. The large number of isolated genes differentially expressed during the rapid phase of fibril growth reveals a fine and possibly tissue-specific control of fibrillogenesis.

Association of type VI collagen with D-periodic collagen fibrils in developing tail tendons of mice

The process of the arrangement of D-periodic collagen fibrils and their growth in maturing tail tendon of mice were studied with the association of type VI collagen, from fetal day 10 to 10 weeks after birth. In tail tendons, the amount of collagen fibers gradually increased along with the diameters of D-periodic collagen fibrils during maturation. Type VI collagens first appeared on fetal day 10, when D-periodic collagen fibrils were not recognizable. Type VI collagens were observed around the fibroblastic cells in early stages of development, but were among thick collagen fibrils in the adult tendon. While the periodic distances of type VI collagen fibrils were over 100 nm at fetal days, they were packed to 80-90 nm after birth. The periodic bands were stained well with ruthenium red in adult but not in young tendons, indicating the close association of proteoglycans or glycosaminoglycans (PGs/GAGs) with maturing type VI collagens. Since type VI collagen in native form is known to associate with D-periodic collagen fibrils via PGs/ GAGs, ruthenium red-stainability on the surface of D-periodic collagen fibrils was also examined; results showed that ruthenium red-stainable elements were D-periodically associated. When the surface morphology of D-periodic collagen fibrils in adult animals was examined by atomic force microscopy, a large depth of the groove between elevated and depressed surfaces became prominent when the fibril surface was digested with hyaluronidase. Thus, it is possible to observe topologically the association of PGs/GAGs and probably that of type VI collagens with D-periodic collagen fibrils.

Accumulation of muscle ankyrin repeat protein transcript reveals local activation of primary myotube endcompartments during muscle morphogenesis

The characteristic shapes and positions of each individual body muscle are established during the process of muscle morphogenesis in response to patterning information from the surrounding mesenchyme. Throughout muscle morphogenesis, primary myotubes are arranged in small parallel bundles, each myotube spanning the forming muscles from end to end. This unique arrangement potentially assigns a crucial role to primary myotube end regions for muscle morphogenesis. We have cloned muscle ankyrin repeat protein (MARP) as a gene induced in adult rat skeletal muscle by denervation. MARP is the rodent homologue of human C-193 (Chu, W., D.K. Burns, R.A. Swerick, and D.H. Presky. 1995. J. Biol. Chem. 270:10236-10245) and is identical to rat cardiac ankyrin repeat protein. (Zou, Y., S. Evans, J. Chen, H.-C. Kuo, R.P. Harvey, and K.R. Chien. 1997. Development. 124:793-804). In denervated muscle fibers, MARP transcript accumulated in a unique perisynaptic pattern. MARP was also expressed in large blood vessels and in cardiac muscle, where it was further induced by cardiac hypertrophy. During embryonic development, MARP was expressed in forming skeletal muscle. In situ hybridization analysis in mouse embryos revealed that MARP transcript exclusively accumulates at the end regions of primary myotubes during muscle morphogenesis. This closely coincided with the expression of thrombospondin-4 in adjacent prospective tendon mesenchyme, suggesting that these two compartments may constitute a functional unit involved in muscle morphogenesis. Transfection experiments established that MARP protein accumulates in the nucleus and that the levels of both MARP mRNA and protein are controlled by rapid degradation mechanisms characteristic of regulatory early response genes. The results establish the existence of novel regulatory muscle fiber subcompartments associated with muscle morphogenesis and denervation and suggest that MARP may be a crucial nuclear cofactor in local signaling pathways from prospective tendon mesenchyme to forming muscle and from activated muscle interstitial cells to denervated muscle fibers.

The Drosophila neuregulin homolog Vein mediates inductive interactions between myotubes and their epidermal attachment cells

Inductive interactions between cells of distinct fates underlie the basis for morphogenesis and organogenesis across species. In the Drosophila embryo, somatic myotubes form specific interactions with their epidermal muscle attachment (EMA) cells. The establishment of these interactions is a first step toward further differentiation of the EMA cells into elongated tendon cells containing an organized array of microtubules and microfilaments. Here we show that the molecular signal for terminal differentiation of tendon cells is the secreted Drosophila neuregulin-like growth factor Vein produced by the myotubes. Although vein mRNA is produced by all of the myotubes, Vein protein is secreted and accumulates specifically at the muscle-tendon cell junctional site. In loss-of-function vein mutant embryos, muscle-dependent differentiation of tendon cells, measured by the level of expression of specific markers (Delilah and beta1 tubulin) is blocked. When Vein is expressed in ectopic ectodermal cells, it induces the ectopic expression of these genes. Our results favor the possibility that the Drosophila EGF receptor DER/Egfr expressed by the EMA cells functions as a receptor for Vein. We show that Vein/Egfr binding activates the Ras pathway in the EMA cells leading to the transcription of the tendon-specific genes, stripe, delilah, and beta1 tubulin. In Egfr1F26 mutant embryos that lack functional Egfr expression, the levels of Delilah and beta1 tubulin are very low. In addition, the ability of ectopic Vein to induce the expression of Delilah and beta1 tubulin depends on the presence of functional Egfrs. Finally, activation of the Egfr signaling pathway by either ectopically secreted Spitz, or activated Ras, leads to the ectopic expression of Delilah. These results suggest that inductive interactions between myotubes and their epidermal muscle attachment cells are initiated by the binding of Vein, to the Egfr on the surface of EMA cells.

Torsion of the tendon of peroneus brevis

The authors present a previously undescribed torsion located within the tendon of peroneus brevis. The musculotendinous unit of peroneus brevis was isolated from 46 lower extremities of cadavers. A goniometer was constructed and utilized to quantify the degree of torsion located within each peroneus brevis tendon. Torsion was present in all 46 cadaver specimens, with a mean of 38.5 degrees and a range of 26 degrees to 56 degrees. The regional anatomy and biomechanical functions of peroneus brevis are discussed, and proposed bases for the embryologic origins and functional significance of the torsion are presented.

Reciprocal signaling between Drosophila epidermal muscle attachment cells and their corresponding muscles

Directed intercellular interactions between distinct cell types underlie the basis for organogenesis during embryonic development. This paper focuses on the establishment of the final somatic muscle pattern in Drosophila, and on the possible cross-talk between the myotubes and the epidermal muscle attachment cells, occurring while both cell types undergo distinct developmental programs. Our findings suggest that the stripe gene is necessary and sufficient to initiate the developmental program of epidermal muscle attachment cells. In stripe mutant embryos, these cells do not differentiate correctly. Ectopic expression of Stripe in various epidermal cells transforms these cells into muscle-attachment cells expressing an array of epidermal muscle attachment cell-specific markers. Moreover, these ectopic epidermal muscle attachment cells are capable of attracting somatic myotubes from a limited distance, providing that the myotube has not yet been attached to or been influenced by a closer wild-type attachment cell. Analysis of the relationships between muscle binding and differentiation of the epidermal muscle attachment cell was performed in mutant embryos in which loss of muscles, or ectopic muscles were induced. This analysis indicated that, although the initial expression of epidermal muscle-attachment cell-specific genes including stripe and groovin is muscle independent, their continuous expression is maintained only in epidermal muscle attachment cells that are connected to muscles. These results suggest that the binding of a somatic muscle to an epidermal muscle attachment cell triggers a signal affecting gene expression in the attachment cell. Taken together, our results suggest the presence of a reciprocal signaling mechanism between the approaching muscles and the epidermal muscle attachment cells. First the epidermal muscle attachment cells signal the myotubes and induce myotube attraction and adhesion to their target cells. Following this binding, the muscle cells send a reciprocal signal to the epidermal muscle attachment cells inducing their terminal differentiation into tendon-like cells.

Calretinin is transiently expressed in tendon fibroblasts of the paravertebral region of the chick embryo

Although calretinin is an intracellular calcium acceptor protein essentially located in neurons, we have previously shown that calretinin was also expressed in the mesenchymal cells forming the intervertebral disc. Here, we describe, using immunohistochemistry, the transient expression of calretinin in fibroblasts responsible for the tendon formation in the paravertebral region. We have looked at chick embryos from embryonic day 4 to day 18. At embryonic day 6, calretinin immunoreactivity was detected in chick embryo cells located laterally to the spinal cord between two groups of developing muscular cells. At embryonic day 8, calretinin immunoreactivity intensity was the highest in cells showing a fibroblast morphology. After embryonic day 8, when fibroblast proliferation decreased and differentiation increased, calretinin immunoreactivity progressively disappeared. Interestingly, calretinin could not be detected in fibroblasts of the anterior and posterior limbs at any investigated age. Because calretinin expression appeared selectively and transiently in the fibroblasts of the paravertebral region, we may conclude that the phenotype of those fibroblasts is different from the limbs fibroblasts.

Mechanical loading and TGF-beta regulate proteoglycan synthesis in tendon

Fibrocartilage is found in tendon at sites where the tissue is subjected to transverse compressive loading in vivo. A significant characteristic of the tissue transition from tendon to fibrocartilage in bovine deep flexor tendon is increased gene expression, synthesis, and accumulation of both a large proteoglycan, aggrecan, and a small proteoglyoan, biglycan. In order to investigate the cellular events involved in this response, segments of fetal bovine deep flexor tendon were subjected in vitro to cyclic compressive load for 72 h. Following loading, the level of aggrecan mRNA in cells from loaded tissue was increased 200-450% compared to matched nonloaded tissue segments, as determined by slot-blot analysis. The level of biglycan mRNA increased 100%, and the level of versican mRNA increased 130% in the loaded tissue. The level of decorin mRNA remained virtually unchanged, while expression of alpha 1(I) collagen increased only 40%. When tissue segments were cultured in the presence of transforming growth factor (TGF)-beta 1 (1 ng/ml), the synthesis and expression of mRNA for both aggrecan and biglycan increased, whereas decorin expression was not affected. Similarity in both the direction and the pattern of the cellular response to mechanical load and TGF-beta suggested a causal relationship. Both loading of tendon segments and TGF-beta treatment increased expression of mRNA for TGF-beta by approximately 40% compared to control tissue. In addition, the amount of newly synthesized TGF-beta immunoprecipitated from extracts of loaded tissue was several-fold greater than that from nonloaded tissue. The experiments of this study support a hypothesis suggesting that one aspect of the response of cells in fetal tendon to compressive load is increased TGF-beta synthesis which, in turn, stimulates synthesis of extracellular matrix proteoglycans and leads toward fibrocartilage formation.

Mechanical loading and TGF-beta regulate proteoglycan synthesis in tendon

Fibrocartilage is found in tendon at sites where the tissue is subjected to transverse compressive loading in vivo. A significant characteristic of the tissue transition from tendon to fibrocartilage in bovine deep flexor tendon is increased gene expression, synthesis, and accumulation of both a large proteoglycan, aggrecan, and a small proteoglyoan, biglycan. In order to investigate the cellular events involved in this response, segments of fetal bovine deep flexor tendon were subjected in vitro to cyclic compressive load for 72 h. Following loading, the level of aggrecan mRNA in cells from loaded tissue was increased 200-450% compared to matched nonloaded tissue segments, as determined by slot-blot analysis. The level of biglycan mRNA increased 100%, and the level of versican mRNA increased 130% in the loaded tissue. The level of decorin mRNA remained virtually unchanged, while expression of alpha 1(I) collagen increased only 40%. When tissue segments were cultured in the presence of transforming growth factor (TGF)-beta 1 (1 ng/ml), the synthesis and expression of mRNA for both aggrecan and biglycan increased, whereas decorin expression was not affected. Similarity in both the direction and the pattern of the cellular response to mechanical load and TGF-beta suggested a causal relationship. Both loading of tendon segments and TGF-beta treatment increased expression of mRNA for TGF-beta by approximately 40% compared to control tissue. In addition, the amount of newly synthesized TGF-beta immunoprecipitated from extracts of loaded tissue was several-fold greater than that from nonloaded tissue. The experiments of this study support a hypothesis suggesting that one aspect of the response of cells in fetal tendon to compressive load is increased TGF-beta synthesis which, in turn, stimulates synthesis of extracellular matrix proteoglycans and leads toward fibrocartilage formation.

Fetal and postnatal development of the patella, patellar tendon and suprapatella in the rabbit; changes in the distribution of the fibrillar collagens

The development of the patella, its associated tendons, and suprapatella of the rabbit knee joint is described from the 17 d fetus to the mature adult. The patellar tendon (ligament) with the patella on its posterior surface is seen in the 17 d fetus and is fully developed by 1 postnatal wk. It is composed of bundles of types I and V collagens separated by endotenons of types III and V collagens. Anteriorly there is an epitenon of types III and V collagens while synovium and a fat pad cover its posterior surface. In the 25 d fetus, the patella is cartilaginous and is separated from the femoral condyles. The cartilage contains type II collagen, but types I, III and V collagens are found along the articular surface. Ossification starts 1 postnatal wk and at 6 wk only the articular cartilage remains. In addition to type II, types III and V collagens are located around the chondrocyte lacunae. The long anterior junction between the patella and its tendon is fibrocartilaginous at 1 wk, but as ossification proceeds this is replaced by bone. Types I and V collagens are found in this region. The suprapatella on the posterior surface of the quadriceps tendon is first seen 1 wk postnatally as an area of irregularly organised fibres and chondrocyte-like cells. Types I, II, III and V collagens are present from 3 wk onwards. It is compared with the fibrocartilage of other tendons that are under compression. The arrangement of the collagens in the patellar tendon is discussed in relation to its use as a replacement for injured anterior cruciate ligaments. It is suggested that the structural differences between the patellar tendon and anterior cruciate ligament preclude the translocated tendon acquiring mechanical strength similar to that of a normal cruciate ligament. The designation 'patellar ligament' as opposed to 'patellar tendon' is questioned. It is argued that the term patellar tendon reflects its structure more accurately than patellar ligament.

Localization of collagen types I, III and V during tendon development. Changes in collagen types I and III are correlated with changes in fibril diameter

Collagen types I, III and V were localized at different stages of tendon development: a stage when tendon architecture is established, but immature (14-day), a mature tendon (hatchling) and an intermediate point where there is a rapid growth of tendon fibrils (17-day). The tendon fascicles and their connective tissue investments (endotendenium) were studied. The data show a changing pattern of type III collagen expression in the developing metatarsal tendon. In the immature tendon at 14 days of development, collagen types I and III are codistributed throughout the fascicles and their connective tissue investments. At this stage all of the fibrils regardless of the site are small. With development the regions segregate and become easily recognizable. As this occurs, the fibril diameter distributions diverge; those in the fascicle become larger while those in the endotendenium remain small. During this period, the fascicle loses type III collagen expression, while the endotendenium becomes type III collagen rich. At the same time, the connective tissue investments develop, and the fibrils of the endotendenium remain small during this period, but then become larger in the mature tendon. The increases in diameter are associated with a decrease in type III collagen reactivity. At hatching, both significant collagen type III reactivity and small diameter fibrils are restricted to the outer sheaths. During all stages of tendon development there is a constant small, but detectable amount of type V collagen. However, no correlation between type V reactivity and fibril diameter was observed at any stage of development. These results indicate an inverse relationship between type III collagen reactivity and fibril diameter in the developing tendon.

Collagen fibrillogenesis in situ: fibril segments become long fibrils as the developing tendon matures

Tissue architecture, stability, and mechanical attributes are all determined by the structure and organization of collagen fibrils. Therefore, the characterization of fibril growth steps and determination of how this growth is regulated is essential to the elucidation of how tissues are assembled. We have proposed that fibril segments are intermediates in the formation of mature fibrils. The purpose of this study was to determine the length and structure of fibrils within a relatively mature tendon. The in situ determination of length performed here was only the second direct determination of fibril length in a vertebrate connective tissue and the first for a relatively mature tissue. The data demonstrate that the fibrils were discontinuous at 18 days of tendon development. However, both ends were not present in any of the analyzed fibrils within the 18-day tendon. Because the data set was 50-60 microm, this indicates a mean fibril length greater than 60 microm. These data are in contrast to data from the 14-day tendon, in which 80% of the fibrils had both ends in a 26-microm data set and the mean segment length was shown to be 10-30 microm. There were equal numbers of alpha and beta ends in the 18-day tendon. The structure of the ends was comparable to that in the less mature tendon. The data also indicate that fibril asymmetry and structure were maintained. The increase in fibril length is interpreted as being the result of a post-depositional, regulated assembly of segments via a lateral association/fusion to form mature fibrils. This hypothesis predicts an increase in diameter at this stage of development. The diameter increases have been documented, but this is the first demonstration of increases in length and maintenance of segment structure during this important stage of tendon development.

Is there such a thing as the "tendon of the infundibulum" in the heart?

The fibrous skeleton of the heart has featured prominently in anatomical and surgical descriptions, although all its purported components are difficult to demonstrate. In descriptions of the skeleton, there have been repeated references to the presence of a tendon (or ligament) between the aortic and pulmonary roots. Such a tendon is rarely, if ever, discussed in the context of surgical procedures being carried out on the ventricular outflow tracts. Our study was undertaken, therefore, to investigate the existence and nature of such a tendon or ligament. Serial transverse sections were made through roots of aorta and pulmonary trunk in an intact fetal heart. In addition, ten normal adult hearts were dissected to display the components of the fibrous skeleton of the heart. No discrete fibrous or elastic structure could be detected in the tissue plane between the aortic sinuses and the subpulmonary muscular infundibulum, although a fascial strand was observed in one heart. Apart from this specimen, the space between the free-standing muscular subpulmonary infundibulum and the sinuses of aorta hearing the coronary arteries was occupied only by loose fibroareolar tissue. The initial presence of the ligament was described following studies of animal and macerated human hearts. Subsequently, it would seem its existence has been passed down through generations of morphologists and surgeons without its presence being reconfirmed. We have been unable to demonstrate any structure approximating to the initial illustrations.

Modeling tendon morphogenesis in vivo based on cell density signaling in cell culture

A mathematical model of tendon morphogenesis is presented that is consistent with the dramatic transitions seen in this tissue as it progresses from rapid growth early in development to no growth in the adult. To accomplish this change, the embryonic chick tendon is hypercellular with each cell dedicating half of its protein production to procollagen but over time, as growth subsides, the tissue gradually becomes hypocellular with each cell producing only about 1% procollagen. Making this transition from the embryonic to the adult state, forming a roughly cylindrical tissue composed of approximately 90% collagen, and linking the correct muscle to the right bone, is a complex task. The proposed solution requires only two factors: an activator of growth and an inhibitor complex, composed of the activator and another molecule that modifies the activity of the activator. From a diverse set of cell culture observations, these two factors were deduced as the primary components of the mechanism that allows cells to signal their presence to their neighbors. Since cell density signaling is the principal regulator of both collagen synthesis and cell proliferation, its components should play the key role in tendon development. A mathematical model based on the changes in the concentrations of these factors with cell density correlates well with the transitions observed in vivo. Furthermore, the model predicts that in the maturing chicken there should be a high cell density region at the muscle tendon interface. Experimental observations of frozen sections of tendon from a 4 month old chicken confirm this prediction.

Differential expression of fibromodulin mRNA associated with tendon fibril growth: isolation and characterization of a chicken fibromodulin cDNA

A 450 bp cDNA fragment similar to that encoding bovine fibromodulin was isolated using a screening procedure to isolate genes differentially expressed between the pre- and post-growth phases of fibril growth in the developing chicken embryo metatarsal tendon. Using this fragment, a 2.4 kb cDNA clone for chicken fibromodulin was isolated from a lambda ZAP library, and the 5' rapid amplification of cDNA ends technique was employed to clone the 5'end of the fibromodulin cDNA. The full-length cDNA contained an open reading frame coding for a 380-amino-acid protein. There was approximately 80% similarity with human, rat and bovine fibromodulins, which confirmed its identity as fibromodulin. Structural features of the deduced sequence include an 18-amino-acid signal peptide, cysteine residues in conserved positions in the N- and C-terminal regions, and a central leucine-rich domain containing eleven repeats of the sequence LXXLXLXXNXL/I. Features unique to chicken fibromodulin include an additional glycosylation site as well as a decreased number of tyrosine residues that could be sulphated, and therefore potential changes in the charge of the molecule. In addition, there was little similarity among the untranslated regions. When compared with chicken decorin and lumican, fibromodulin showed greater similarity to the other keratan sulphate-containing proteoglycan, lumican. Northern blot analysis revealed a 6-8-fold increase in the fibromodulin mRNA level from day 14 to day 19 of development. In the chicken tendon, collagen fibril growth is a process characterized by a precipitous increase in length during a short developmental period. The necessary changes would require the expression of different genes regulating fibril formation and growth, and interactions between fibromodulin and collagen fibrils may participate in the regulation of collagen fibril growth and matrix assembly.

The mouse Engrailed-1 gene and ventral limb patterning

During vertebrate limb development, positional information must be specified along three distinct axes. Although much progress has been made in our understanding of the molecular interactions involved in anterior-posterior and proximal-distal limb patterning, less is known about dorsal-ventral patterning. The genes Wnt-7a and Lmx-1, which are expressed in dorsal limb ectoderm and mesoderm, respectively, are thought to be important regulators of dorsal limb differentiation. Whether a complementary set of molecules controls ventral limb development has not been clear. Here we report that Engrailed-1, a homeodomain-containing transcription factor expressed in embryonic ventral limb ectoderm, is essential for ventral limb patterning. Loss of Engrailed-1 function in mice results in dorsal transformations of ventral paw structures, and in subtle alterations along the proximal-distal limb axis. Engrailed-1 seems to act in part by repressing dorsal differentiation induced by Wnt-7a, and is essential for proper formation of the apical ectodermal ridge.

Expression and regulation of Cek-8, a cell to cell signalling receptor in developing chick limb buds

The Eph-related receptor tyrosine kinase gene, Cek-8, is expressed in mesenchyme at the tip of chick limb buds, with high levels of transcripts posteriorly and apically but fading out anteriorly. Expression of Cek-8 in distal mesenchyme is regulated by apical ridge- and FGF-polarising signals and retinoic acid, and is uniform across the anteroposterior axis in talpid3 mutants. These data indicate that Cek-8 expression responds to regulatory signals during limb patterning and suggest that this receptor tyrosine kinase may have a role in coordinating responses to signals in the progress zone of early buds. Later on in limb development, Cek-8 expression is associated with cell condensations that form tendons and their attachments to cartilage rudiments and then in developing feather buds.

The effect of compressive loading on proteoglycan turnover in cultured fetal tendon

A fibrocartilaginous tissue develops in tendon at the point where the tendon wraps under bone and is subjected to transverse compressive loading in addition to tension. This tissue is characterized by a high level of large proteoglycan (aggrecan), which could accumulate because of increased synthesis, diminished turnover, or both. To examine the effect of loading on proteoglycan turnover segments of fetal tendon in sterile culture were subjected to cyclic, uniaxial compression loading to 30% of initial thickness once every 6 sec. for 72 h, and then allowed to incorporate 35S-sulfate for 12 h. The rate of loss of newly-synthesized 35S-proteoglycans from tissue was determined during subsequent culture for up to 12 days, with or without continued loading. Proteoglycan was lost from fetal tendon segments rapidly during the first 3 days of culture and slowly thereafter. Loss of newly-synthesized proteoglycan from adult tendon fibrocartilage was linear, with a half life of 12 d. Segments of fetal tendon subjected to cyclic compression before labeling synthesized more proteoglycan. These segments lost a greater percent of labeled proteoglycan to medium during a subsequent 12-day culture period than matched segments that had not experienced loading. Analysis of medium and tissue proteoglycans by SDS polyacrylamide gel electrophoresis and sieve chromatography indicated that small proteoglycans (decorin and biglycan) were retained in both loaded and non-loaded tissue whereas large proteoglycans (migrating in the Vo of a Sepharose CL-4B column) were readily lost. It is concluded that the 3-day loading regimen did not diminish turnover of large proteoglycan. To the contrary, although synthesis of large proteoglycan was enhanced by the loading regimen, these proteoglycans were still rapidly lost from the fetal tissue.