It was not until the middle of the 19th century that microscopes had developed to the point where someone could see this phenomenon. It happened to be the physiologist William Sharpey, a friend of pioneering biologist Charles Darwin. Sharpey, a Scottish academician and member of the prestigious Royal Society of London for Improving Natural Knowledge, came across the mineral deposits in , noticing their thread-like appearance in various parts of the body.
The discovery added another piece to the puzzle of human scientific understanding. Within the mouth, at the ends of the gum's periodontal ligaments , is the first group of Sharpey's fibres. Username Please enter your Username. Password Please enter your Password. Forgot password? Don't have an account? Sign in via your Institution. You could not be signed in, please check and try again. Sign in with your library card Please enter your library card number. Sharpey's fibres bone fibres , or perforating fibres are a matrix of connective tissue consisting of bundles of strong collagenous fibres connecting periosteum to bone.
They are part of the outer fibrous layer of periosteum, entering into the outer circumferential and interstitial lamellae of bone tissue. In the teeth , Sharpey's fibres are the ends of principal fibres that insert into the cementum. A study on rats suggests that the three-dimensional structure of Sharpey's fibres intensifies the continuity between the periodontal ligament fibre and the alveolar bone tooth socket , and acts as a buffer medium against stress.
While the unstressed periosteum seems biochemically quiescent, short bursts of loading stimulates the rapid induction of enzyme activity within discrete periosteal and bone matrix domains Skerry et al. Also reporting them at this time was H. Other related reports followed, such as that by Jones and Boyde outlining further their presence in the cranial sutures and muscle attachments as well as in tooth sockets.
The detrimental changes observed included fibrosis, increased cellularity, and progressive calcification Sloan et al. Human proximal femur. En bloc gentian violet stain.
Their entry angle into the subperiosteal bone was multiaxial. It included the almost horizontal i. It also included the perpendicular i. In addition were the oblique fibers, these being the most numerous and predominant in the young skeleton, functionally mediating exchange between the periosteum and outer cortex and providing soft tissue anchorage.
While some of these insertions apparently ended abruptly like rows of short, regular parallel stitches , it was the proportion that traversed to the medulla, some becoming intertrabecular, others with dispersed intra-osseous fan-like termini that were of special interest.
Moreover, the elastin staining was not uniform but suggested the discrete contours of a spiral encircling some of the individual coarse fibers Aaron and Skerry, The mechanical properties of elastin are unique.
Unlike non-extensible collagen it can be stretched, recoils, branches, and imparts flexibility. However, it has been rarely documented in bone Johnson and Low, ; Keene et al. It required a technological advance to demonstrate the otherwise hidden scale of Sharpey fiber permeation and to establish their biochemical composition more extensively see Aaron and Shore, b for technical details.
Polarized light showed a highly birefringent nature consistent with collagen, but little else could be deduced by simple staining Smith, , with for example picro-sirius red stain, or by the Goldner tetrachrome method Aaron and Shore, b.
The prospect was transformed by the introduction of heavy duty cryomicrotomy see for example, Aaron and Carter, ; Carter et al. Prior to this, the organic matrix biochemistry was based on tissue homogenates and extracts. The new method enabled a structural face to be applied. This identified previously unsuspected matrix sub-divisions, showing a mosaic of biochemically distinct domains, defined by boundaries and with differential aptitudes for signal trafficking through, for example, endochondrally derived versus intramembranous regions.
As with the more prominent CI, so CIII is also found in all interstitial connective tissues but in contrast there was little evidence for its occurrence in bone, with the exception of the earliest mesenchymal condensations Pratt, ; Page et al. After Carter et al. It is recognized that different collagens, e. Like CI, the structure of CIII consists of long nm uninterrupted triple helices, chemically distinguished from CI by an increased level of 4-Hyp and the occurrence of cysteine, facilitating disulphide bond formation.
As well as being argyrophilic, above, these fibers were known histologically as reticulin fibers, and were especially associated with epithelial basement membrane stability, where their contribution to organ containment cannot be overestimated.
It was reported by Bailey et al. CVI is microfibrillary, composed of a short triple helical axis and globular termini, creating its typical dumbell shape. It has been suggested that the removal of CVI is a factor that may permit remodeling Sloan et al.
They are the only continuous anatomical structure to i integrate directly with the muscles, ligaments, and tendons, ii traverse the periosteum from which they arise, and iii permeate the extracellular matrix multiaxially and to varying degrees.
Insight into their most basic structural modulation may be found in tooth movement where orthodontic forces strengthen the CIII periodontal attachment Wang et al. After Aaron and Skerry Its candidature has been a lengthy one. The precise nature of the musculoskeletal exchange mechanism instigated remains to be established, for example, a piezoelectric phenomenon the piezoelectric modulus of tendon is apparently fold that of bone; Marino and Becker, or one involving stress-regulated excitatory amino acids analogous to neural pathways Mason et al.
They occupy an area that becomes an intracortical CIII-rich domain in the limb bud that is linked to tendon generation Hurle et al.
This is illustrated by comparing intramembranous bone development in the normal human femoral anlagen with that of dysplastic lesions Carter et al. Key structural molecules in the genesis of new trabeculae are not only collagen types III and VI, but also adherent are the glycoproteins tenascin and fibronectin. The occasional surface location of alkaline phosphatase on some fibers may relate to this signal and may indicate the expansion of thinner fibers with circumferential apposition in response to brief loading Aaron, b.
Immunostaining for tenascin indicates that it adopts a highly characteristic beaded pattern Figure 3 the linear alignment of which is critical for normal development, as follows. Remaining in association, however, is tenascin in a remarkable regular beaded arrangement.
In its absence the bony tissue is permanently destined to remain disorganized and immature, as is the case in fibrous dysplasia Sloan et al. By providing this continuous, elastic Keene et al. It may be envisaged that an understanding of such interactions between organizing proteins like tenascin and fibronectin and extracellular structures like CIII fibers which are fundamental to early trabecular development in the first stages of life may direct novel strategies for restitution of the atrophied skeleton in later life.
It is upon this assembly that the replacement primary trabeculae gain support, and in regions where the scaffold is absent, so also absent is trabecular genesis. This endosteally derived fibrous framework remains unmineralized and therefore apparently protected from osteoclastic resorption Aaron, a , aided by other inhibitory intrinsic factors such as CVI.
It apparently survives, even when the thickening primary bars are significantly opened up by resorption channels into a typical network of mature secondary trabeculae. The outcome of this endosteal activity is the guaranteed presence of a persistent fibrillar assembly that crosses domain boundaries without interruption, bonding soft to hard tissues and new bone to old, and which seems central to a self-repair process of admirable efficacy.
The ovariectomized rat is an established model of bone loss precipitated by declining estrogen levels. The configuration of this domain is such that three-dimensional maps are essential in its recognition Luther, ; Luther et al.
Resembling in transverse section a fluorescent subperiosteal collar after immunostaining for CIII, the domain commences about one-third along the shaft from the knee where two opposing subperiosteal stained sectors progressively expand and merge into a single circumferential band, which in turn broadens to occupy the entire width of the cortex at the level of the third trochanter and above. The arrangement is so striking that the domain resembles a bony encapsulated tendon, well placed to maximize direct functional adaptation Figure 4A.
However, if estrogen levels fall, as is the case after ovariectomy OVX , this stability apparently soon diminishes Luther et al. After Luther et al. Insight into a remarkable example of form-and-function adaptation by the periosteum and its appendages came about unexpectedly in mapping the perimeter of the proximal domain above. Along the part of its inner boundary that occurs approximately mid-cortex were distributed at intervals, like small buttons, islands of cartilage, looking at first sight like remnants of endochondral ossification Figure 4.
This explains their staining for CVI with its concentrated adhesive groups, as well as the more usual CII of cartilage. An association of these unusual objects with tendinous insertions is now recognized by a number of other authors Benjamin and Ralphs, ; Clarke and Stechschulte, ; Benjamin et al. The rodent femoral proximal domain above is responsive to levels of physical activity and this has been illustrated by comparing its features in animals with voluntary access to a running wheel and those confined to their cage.
Running increased subperiosteal apposition to a peak adaptation after which there was no more augmentation induced by prolonging the exercise period further. The CIII fibers were densest near the proximal i. This structural dichotomy relative to a possible proactive role in bone status of the fibers is supported by the independent observation of Newhall et al. It also follows that the highest levels of tension and compression occur proximally and decline distally Rybicki et al.
From another quarter was a second independent piece of supporting evidence using young rabbits. Hert et al. Inexplicably, however, there was again a dichotomy in the response of the tissue.
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