Sharpey Fibers: Composition, Discovery, Fetal Bone Development and Incidence in Mature Bone

They are known skeletally in dental anchoring and elsewhere provide anchoring for the periosteum.

Immunohistochemistry has transformed its potential importance by identifying its content of type III collagen (CIII) and allowing its mapping in domains as permeable fiber matrices protected from osteoclastic resorption due to their poor mineralization.

As periosteal extensions, they are crucial for early skeletal development and central and intramembranous bone healing, providing unique microanatomic avenues for musculoskeletal exchange.

Composition of Sharpey Fibers

They are made up of type VI collagen, elastin, and tenascin combined with a multiaxial insertion pattern that suggests a more complex role than attachment alone would warrant.

A proportion passes through the cortex to the endosteum, fusing into an osteoid layer that encompasses all resting surfaces and with which they apparently form a spherical structure.

This intraosseous system behaves in favor of bone loss or gain that depends on foreign stimuli.

Therefore, these fibers are sensitive to humoral factors (eg estrogen causes retraction, rat femur model), physical activity (eg running causes expansion, rat model), aging (eg causes fragmentation, pig jaw model) and pathology (eg, atrophied in osteoporosis, hypertrophy in osteoarthritis, proximal human femur).

In this way, the discrete network of periosteal Sharpey Fibers can regulate bone status, perhaps even contributing to predictable hot spots of trabecular disconnection, particularly at sites of stress prone to fatigue, and with the network deteriorating significantly prior to loss. of the bone matrix.

Discovery

The description and discovery of these fibers was by William Sharpey, in 1867 and indicated that they are crossed matrix lamellae and are particularly abundant in the tooth socket.

Also reported at this time were H. Muller who recognized the elastic nature of the fibers and a tendency to “escape calcification.”

Later Weidenreich (1923), confirmed their poor mineralized state, and although they were apparently short and superficial, he believed that they influenced not only the external anatomy but also the internal bone structure.

On the other hand, the reports by Tomes (1876) and Black (1887) indicated that the Sharpey fibers constituted the cementum-alveolar fibers of the periodontal ligament.

Other related reports followed, such as that by Jones and Boyde (1974) detailing its presence in cranial sutures and muscle accessories, as well as in tooth sockets.

However, later literature focused almost exclusively on the Sharpey Fibers that function as the periodontal ligament and how this special tooth structure altered with age both organic and inorganic, weakening its ability to hold teeth.

The detrimental changes observed included fibrosis, increased cellularity, and progressive calcification (Sloan et al., 1993).

At present, sufficient evidence is accumulating to suggest that the relative neglect of the abundant Sharpey Fibers located far from the dentition may be unwarranted.

By correcting the balance in favor of its structural importance elsewhere in the skeleton, and complementing Johnson’s (1987) classification, Al-Qtaitat (2004), 2007 identified two types of Sharpey fibers, one thick (8-25 μm of thickness) and the other fine (<8 μm thick).

Its angle of entry into the subperiosteal bone was multiaxial. It included the nearly horizontal (ie, tangential) fibers especially common with age and often found between inserted muscle bundles, functionally propagating biomechanical exchange through the periosteum.

It also included perpendicular (i.e. vertical) fibers, which frequently cross the cortex into the cancellous region and generally of the thick type in bundles <40 μm thick, adding functional complexity to the muscle-bone interface that can influence atrophy. I mean.

In addition, oblique fibers, which are the most numerous and predominant in the young skeleton, functionally intervene in the exchange between the periosteum and the outer cortex and provide soft tissue anchoring.

While some of these insertions apparently ended abruptly (as rows of short, regular, and parallel stitches), it was the proportion that traversed the medulla, some becoming intertrabecular, others with scattered fan-shaped intrabony ends that were of particular interest.

Added to this was its unusual cross-sectional profile, which was not the expected simple circle, but showed clearly defined surface indentations and configurations ranging from a horseshoe shape to a ‘hollow’ core (Aaron and Skerry, 1994). .

Further examination using an established histochemical test for elastin (Verhoeff’s stain) supported Muller’s earlier observation that (unlike type I collagen, CI) they have elastic properties that can absorb stress.

Also, the elastin staining was not uniform, but suggested the discrete contours of a spiral surrounding some of the individual coarse fibers (Aaron and Skerry, 1994).

Elastin’s mechanical properties are unique. Unlike non-stretchable collagen, it can stretch, recede, branch, and impart flexibility.

However, it has rarely been documented in bone (Johnson and Low, 1981; Keene et al., 1991), except, that is, at tendon and ligament insertion sites, and its presence will alter the biophysical properties of Sharpey Fibers. .

Sharpey fibers in fetal bone development

A continuous Sharpey fiber system with the ectodermal membrane is present from an early embryonic stage. They appear as dorsoventral fibrillar bundles, approximately 1 μm thick, that also contain IC, fibronectin, and tenascin.

They occupy an area that becomes an intracortical domain in the limb bud that is linked to tendon generation and variations from the norm can have pathological consequences.

This is illustrated by comparing intramembranous bone development in normal human femoral anagen with that of dysplastic lesions.

The key structural molecules in the genesis of new trabeculae are not only collagen types III and VI, but the glycoproteins tenascin and fibronectin are also adherent.

Considered as “biological organizer” molecules, they carry the adhesive sequence RGD, fibronectin, which apparently influences fibroblast migration.

However, in relation to the Sharpey fibers, it is tenascin that seems to have a special role, where it can mediate the binding of osteoblasts through its cell recognition signal.

The occasional surface location of alkaline phosphatase in some fibers may be related to this signal and may indicate the expansion of thinner fibers with circumferential apposition in response to brief loading.

Immunostaining for tenascin indicates that it adopts a highly characteristic bead pattern, the linear alignment of which is critical for normal development.

Adjacent to the periosteum surrounding the developing intramembranous bone, there are assemblages of Sharpey Fibers that apparently form a scaffold on which the new trabeculae assemble and the bone shaping event occurs.

The framework is recognized by antibodies and fibronectin, but these affinities disappear as the Sharpey Fibers surround themselves with calcified bone tissue.

However, staying in association is tenacity in a remarkable regular showdown. Intramembranous bone formation can only apparently continue in an orderly fashion towards maturity provided that tenascin specifically associates with Sharpey’s Fibers at this crucial stage.

In its absence, bone tissue is permanently destined to remain disorganized and immature, as is the case with fibrous dysplasia.

The preliminary framework appears to persist to maturity (absent from endochondrally derived bone) as periosteal myotendinous insertions of the Fibers of Sharpey.

By providing this continuous and elastic intermediary between the developing musculature and the developing bone matrix, the CIII fibers can allow the translation of the stresses generated by the contractile tissues into compatible modeling and remodeling of the contiguous trabecular architecture in the anlagen femur.

It can be envisioned that understanding such interactions between organizing proteins (such as tenascin and fibronectin) and extracellular structures such as CIII fibers that are critical for early trabecular development in early life may lead to new strategies for restitution of the atrophied skeleton in stages. later life.

Sharpey Fibers in mature bone with aging

As an intermediary between soft muscle and hard bone, the periosteum is so susceptible to the passage of time.

With age, fibroblasts tend to synthesize more IC and less CIII, so that the overall CIII ratio is significantly lower than in young people.

In addition to this, increased collagenase production has been attributed to fibrocytes in some conditions and can cause fiber degradation over time.

In this way, the periosteum and its Sharpey Fiber appendage can change organically with age, and it can also change inorganically with implications for both musculoskeletal exchange and bone structural ‘quality’, making it more susceptible to calcification.