A fascicle is a bundle of fibres! The characteristic cell in tendons responsible for the secretion of the ECM, and thus collagen assembly and turnover, is the tenocyte. These cells are a specialized set of fibroblasts that are typically arranged in longitudinal rows, in close proximity to the collagen fibrils Fig. Although there is no unique marker that selectively distinguishes tenocytes at all stages of development, a number of molecules have been considered as markers.
It is particularly interesting to note that there may be an initial period in the training programme of an athlete where collagen turnover in tendons i. The authors suggest that this could enable a tendon to restructure and adapt to the increased loading pattern. They point out that it is not until training progresses that there is a net gain in collagen synthesis. Initially, the mean diameter of the fibrils increases after 1 week of exercise , but later between weeks 3—7 falls to a value less than the controls.
At a cellular level, there seems to be no difference in the response of tenocytes to mechanical load between cells that have been extracted from different tendons, e. However, in a given tendon, different stress patterns provoke different cellular reactions depending on the amount and duration of the tensional stress applied. One of the best lines of evidence that tenocytes can modulate their activity according to changing mechanical load comes from the observation that tendon cells in vitro can upregulate collagen synthesis when subjected to tensional forces.
The modulation of ECM synthesis involves two types of gap junctions — those characterized by the presence of connexin 32 and those containing connexin Tenocytes may have a basal level of synthesis maintained by systems involving connexin 32 signalling, which is enhanced by mechanical stress. The signalling of connexin 43 then becomes active, damps down the response to mechanical stress and maintains control.
Since tendon cells can respond individually to mechanical stimuli, it must be important for their response to be coordinated along the tendon, so that local areas of weaker ECM do not develop. The authors suggest that this acts as a protective mechanism against mechanical overuse of tendon cells during healing. These are among the substances commonly used by clinicians to suppress inflammation in patients with tendon injuries. If such effects also occur in vivo , then this may explain why the integrity of the tendon as a whole may be affected by corticosteroid treatment. In clinical practice, this has encouraged attempts to use pharmaceuticals that are intended to increase nitric oxide levels in the tissue in patients with tendinopathies Murrell, An appreciation of the blood supply of tendons is of special interest to surgeons and thus our current understanding largely stems from studies of certain tendons in particular, viz.
A number of different approaches have been used to visualize the vessels — vascular injections of coloured dyes with and without microdissections , routine histology or immunolabelling for laminin a component of the basal lamina which surrounds all vessels , and Doppler ultrasonography. Unfortunately, results obtained by the use of one technique may be difficult to reconcile with those obtained by another. As a general rule, tendons have a vascular supply that is considerably less than that of the more metabolically active muscles with which they are associated.
This is why fresh tendons are white and muscles are red. Nevertheless, contrary to the view of early anatomists, tendons are still vascularized, and the presence of vessels is important for the normal functioning of tendon cells and the ability of tendons to repair. The blood flow within the tendon itself and in the muscle belly of gastrocnemius remains at a lower level for an extended period of time after tenotomy and this may inhibit repair. It is also commonly argued that reduced tendon blood supply can lead to tendon degeneration, particularly in association with certain tendons that have avascular or poorly vascularized regions, e.
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They are a feature both of the internum of the tendon and of its surface epitenon. In the tendon itself, the vessels run longitudinally, parallel to the fascicles and within the endotenon. In digital tendons at least, most of the vessels are arterioles and venules, with the latter being more numerous Brockis, Anastomoses between parallel vessels are common Edwards, Numerous vessels enter tendons at their myotendinous junctions and some vascular injection studies suggest that this is also the case at entheses. However, Edwards , who used such techniques extensively, was of the opinion that the enthesis is not an important region for the entry of blood vessels, which then supply the rest of the tendon.
According to Edwards it is, however, a site where relatively large lymphatic vessels may be seen on the surface of the tendon. Consequently, such attachment sites can indeed be regarded as vascularized tissues. Furthermore, there is clear histological evidence of vascular continuity between bone and tendon at such sites. However, the latter work was based on macroscopical studies only. Where tendons are surrounded by true synovial sheaths, their supplying vessels enter via a mesotenon.
The location of the blood vessels seems to be dictated by the relation of the tendons either to the phalanges or to the pulleys associated with the flexor sheaths. The vinculae are remnants of the mesotenon and convey blood vessels to the tendons. As certain tendons e. It follows that angiogenesis must be inhibited, either because inhibitory factors are expressed by tendon fibrocartilage cells or because of the inability of such cells to express stimulatory peptides.
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The sensory innervation of tendons is of particular interest in relation to tendinopathies and the repair of ruptured tendons. They are autonomic nerves that immunolabel for neuropeptide Y and noradrenaline vasoconstrictive factors and for vasoactive intestinal peptide VIP — a vasodilatory factor. The authors speculate that the absence of nerve fibres is associated with the heavy loading to which the enthesis is subject.
Many tendons can recoil elastically when a stretching force is removed.
The elastic recoil of tendons has attracted considerable interest from those working in the fields of exercise physiology and biomechanics, and the reader is referred to the comprehensive reviews of Maganaris and Reeves for further details. Thus, only a brief consideration is given to the issue in the current article. The ability of tendons to stretch and recoil enables them to save energy in running by allowing the limb to have shorter muscle fascicles or slower muscle fibres that can generate force more economically Alexander, It is during this last time interval that all of the stored energy is released.
The stiffness of tendons varies with age, sex and physical activity.go to link
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It is evident that fatigue may change the elastic properties of the tendon of vastus lateralis. Thus, the elasticity of a fatigued tendon and aponeurosis tends to be greater as evidenced by its ability to lengthen further with the same load. Limb lengthening by distraction osteotomy has become a routine surgical procedure and studies in goats have shown that it is the muscle rather than the tendon that provides the extra length within the muscle—tendon unit necessary for proper limb function.
It is important to note that length changes are more pronounced in younger i. Many tendons attach immediately beyond the joint on which they principally act. This increases the speed with which they can move the joint, albeit at the expense of the most effective moment arm Wood Jones, a. This is well documented in the glenohumeral joint where the rotator cuff tendons blend imperceptibly with the joint capsule, but it is also a feature of the interphalangeal joints in both the fingers and toes, where the extensor tendon replaces the capsule dorsally Fig.
It should be recognized that the capsules of highly mobile joints need a degree of laxity to allow the joint with which they are associated to function throughout its whole range of movement. However, such laxity carries with it the risk that the capsule could get pinched within the joint.
This was well recognized in the older literature, where the consensus was that the deeper fibres of certain muscles e. The common thread in all such examples is that tendon—capsule fusion reduces the risk of capsular entrapment and eliminates the need for an extra muscle purely concerned with tensing the capsule. A sagittal section through the interphalangeal joint of the thumb stained with Masson's trichrome, showing how the tendon of extensor pollicis longus EPL replaces the joint capsule dorsally.
DP, distal phalanx; PP, proximal phalanx. It is worth recognizing that although tendons can pass over joints without fusing with the capsule, they can still press on the capsule, altering its shape and that of the joint cavity when their muscle contracts. There is a close, but somewhat neglected, link between tendons and fasciae, for most tendons attach not only to bone, but also to adjacent dense fibrous connective tissues.
This is a basic strategy for dissipating stress concentration at entheses and thus reducing the risk of failure or local wear and tear. One of the classic examples of a tendon that has both bony and fibrous attachments is the distal tendon of biceps brachii.
This has a bony insertion on the radial tuberosity and a fascial connection to the deep fascia on the medial side of the forearm via the bicipital aponeurosis Fig. By tensing the deep fascia, the aponeurosis increases the effectiveness of the muscle as a supinator. Another example is the quadriceps tendon.
The biceps brachii muscle BM of the forearm has a tendon that attaches to the bicipital tuberosity of the radius R and an aponeurotic expansion A that merges with the deep fascia of the forearm. The bicipital bursa BB has been opened up at the tendon attachment site. The importance of fascia and its functional relationship to muscles and tendons was well understood by Wood Jones b who considered fascia to form what he called an ectoskeleton within the limbs.
An external skeleton commonly referred to as an exoskeleton is typical of arthropods. These animals are covered by a hard shell of chitin to the inner surface of which muscles are attached. It relates closely to the current recognition developed from animal studies that muscles do not simply transmit their load to tendons and then to bone — and that muscles cannot be viewed as structures that are mechanically independent Huijing, There is thus an increasing awareness that muscles can transmit forces beyond their confining epimysial envelope.
Wood Jones b draws attention to the fact that many upper limb muscles have small, precise tendinous insertions on bones, but those in the lower limb often have larger and less discrete skeletal footprints — in line with their more powerful actions. The larger attachment area of the lower limb muscles is often promoted by an initial anchorage of the muscle bellies or their tendons to fascial sheaths. The fasciae envelop the limb musculature and extend between muscles or muscle groups so as to form septa and other fibrous partitions.
Ultimately, of course, the fasciae also attach to bone. As Wood Jones b has highlighted, certain muscles in the gluteal region rely heavily on indirect attachments to bone via fasciae, rather than direct attachments via tendons. Furthermore, the relative contribution of tendon and fascia to the anchorage mechanism varies with age. Early in development, gluteus maximus is attached predominantly to the gluteal tuberosity, but it later develops a more extensive attachment to the fascia lata of the thigh Wood Jones, b.
At the extreme end of the spectrum, tensor fasciae latae has completely abandoned its bony attachment to the gluteal tuberosity in man and instead attaches entirely to the iliotibial tract Fig. In the upper limb, palmaris longus has also largely abandoned a direct bony attachment by attaching instead to the palmar fascia Fig.
Two examples of tendons that have completely abandoned a bony enthesis and are attached to fascia instead. Certain tendons in the lower limb, which are clearly tendinous and relatively distinct in their more proximal regions nearer to the muscle belly e. Indeed, so widespread are the fascial connections of muscles in the lower limb that as Wood Jones b points out, it is difficult to perform clean dissections of muscles in the leg compared with the forearm. In the opinion of Wood Jones b , it is the upright stance of man that largely accounts for the greater prominence of fascial connections of muscles and tendons in the lower compared with the upper limb.
He suggests that it is a response to the demands for a stabilized limb that must not only provide for locomotion, but also support body weight in an upright position. In other words, the lower limb must act as a rigid column capable of providing passive support — and some muscles attach to the limb as a whole column, rather than to its moving parts. The valuable contribution of Wood Jones b has been to show that muscles and tendons that gain widespread insertions to fasciae, use these extensive sheets as a functional homologue of an invertebrate exoskeleton.
Tendon networks are a particular feature of the hand and foot. On the dorsum of the hand, for example, there is a whole array of flattened extensor tendons that splay out from under the extensor retinaculum and head towards the fingers. The tendons are linked to each other by a highly variable collection of fibrous bands known as juncturae tendinum Fig.
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