The effect of intralesional injection of bone marrow derived mesenchymal stem cells and bone marrow supernatant on collagen fibril size in a surgical model of equine superficial digital flexor tendonitis

Reasons for performing study: Collagen fibril size is decreased in repair tissue following tendon injury compared to normal tendon matrix in horses. Mesenchymal stem cells have been suggested to promote regeneration of tendon matrix rather than fibrotic repair following injury, although this concept remains unproven. Objectives: To explore the hypothesis that implantation of autologous mesenchymal stem cells derived from bone marrow into a surgically created central core defect in the superficial digital flexor tendon (SDFT) of horses would induce the formation of a matrix with greater ultrastructural similarities to tendon matrix than the fibrotic scar tissue formed in control defects. Methods: Tissue was collected 16 weeks after induction of injury and 12 weeks after treatment from normal and injured regions of control and treated limbs of 6 horses and examined using transmission electron microscopy. Collagen fibril diameters were measured manually with image analysis software and surface areas calculated. Three parameters assessed for normal and injured tissue were mass average diameter (MAD), collagen fibril index (CFI) and the area dependent diameter (ADD). Results: Normal regions from both treated and control limbs displayed higher MAD and CFI values, as well as a characteristic bimodal distribution in fibril size. Injured regions from both treated and control limbs displayed significantly lower MAD and CFI values, as well as a unimodal distribution in fibril size. There were no significant differences between treated and control limbs for any of the parameters assessed. Conclusions: Intralesional injection of autologous bone marrow derived mesenchymal stem cells had no measurable effect on the fibril diameter of collagen in healing tissue in the SDFT of this experimental model 16 weeks after injury. Potential relevance: Favouring matrix regeneration over fibrotic repair may not be the mechanism by which autologous mesenchymal stem cells assist healing of tendon injury.     

Age-related changes to the molecular and cellular components of equine flexor tendons.

Specific tendons show a high incidence of partial central core rupture which is preceded by degeneration. In the performance horse, the superficial digital flexor tendon (SDFT) is most often affected. We have described previously the molecular changes that are associated with degeneration in the central core region of the equine SDFT. The pathophysiological mechanism leading to change in synthetic activity of central zone cells in degenerated tendons is not known. In this study, we test the hypothesis that ageing results in matrix composition changes within the central zone of the SDFT. Extracellular matrix composition and cellularity were analysed in equine SDFTs collected from Thoroughbred horses and compared with a flexor tendon which rarely shows degenerative change and subsequent injury (deep digital flexor tendon, DDFT). Data were examined for age-related changes to central and peripheral zone tissue of the SDFT and DDFT. Ageing in both tendons (SDFT and DDFT) resulted in a significant increase in collagen-linked fluorescence and a decrease in cellularity in the DDFT but not the SDFT. The central zone tissue from the SDFT had a significantly higher proportion of type III collagen than the peripheral zone of the tendon. The highest level of type III collagen was found in the central zone tissue of the SDFT from the older group of horses and this may represent the early stages of a degenerative change. Collagen content did not differ between the 2 flexor tendons; however, there were differences in collagen type and organisation. The SDFT had a higher type III collagen content, higher levels of the mature trifunctional collagen crosslink hydroxylysylpyridinoline, lower total chondroitin sulphate equivalent glycosaminoglycan content, smaller diameter collagen fibrils and a higher cellularity than the DDFT. In conclusion, differences in macromolecular composition exist between the flexor tendons and ageing contributes to a tendon specific change in composition.     

Microwave thermography: a non-invasive technique for investigation of injury of the superficial digital flexor tendon in the horse.

Microwave thermographs were recorded from 77 normal horses. In 51% the lowest temperature was recorded in the mid-metacarpal region, and in 41% it was in the distal metacarpal region. The mean temperature of the normal limbs ranged from 25.04 to 37.4 degrees C. Maximum temperature differences between symmetrical points in both forelimbs ranged from 0 to 5.33 degrees C and differences in mean limb temperatures between both forelimbs ranged from 0 to 2.91 degrees C. In 48 horses with acute (less than 4 weeks' duration) injury of the superficial digital flexor tendon (SDFT) (36 unilateral, 12 bilateral) and 12 horses with acute injury of the soft tissues of the palmar metacarpal region other than the SDFT (all unilateral) 66% of forelimbs had acute SDFT injury, and 50% of those with other soft tissue injuries, had elevations of the temperature in the mid- or distal metacarpal region. Abnormal values for mean limb temperature, difference in mean limb temperature and maximum temperature difference between locations in opposite forelimbs were detected in 75% of the horses with SDFT injury and in only 16% of the horses with other soft tissue injury. The sensitivity of microwave thermography for the detection of SDFT injury was 81% and the specificity 74%. When 30 horses in National Hunt training were examined weekly for 5 weeks, 2 horses sustained SDFT injury during that period. The microwave thermographs recorded from these 2 horses, at 1 and 2 weeks before the onset of clinical signs, were abnormal. However, 16 horses which did not develop tendon injury also displayed thermographic abnormalities.     

Effects of exercise on the diameter of collagen fibrils in the central core and periphery of the superficial digital flexor tendon in foals

OBJECTIVE: To determine the effects of exercise on collagen fibril diameter distribution in the superficial digital flexor tendon (SDFT) of foals. ANIMALS: 43 Dutch Warmblood foals. PROCEDURE: From 1 week until 5 months of age, group-1 foals (n = 14) were housed in stalls and not exercised, group-2 foals (14) were housed in stalls but were exercised, and group-3 foals (15) were maintained at pasture. Biopsy specimens were collected from the SDFT at 2 months, and 8 foals in each group were euthanatized at 5 months. Remaining foals were housed together in a loose stall and paddock until euthanatized at 11 months. After euthanasia, specimens were collected from the SDFT; all specimens were analyzed by use of electron microscopy. Collagen fibrillar index (CFI), mass average diameter (MAvD), and area dependent diameter (ADD) were compared among groups. RESULTS: Exercise-related differences in fibril distribution were not detected among groups at 2 months. At 5 months, ADD in peripheral specimens was significantly greater in group 1 than group 3. At 11 months, MAvD in core specimens was significantly less in group 3, compared with the other groups. However, in peripheral specimens, MAvD was significantly less in groups 2 and 3. CONCLUSIONS AND CLINICAL RELEVANCE: Collagen fibril restructuring in the SDFT of foals is in part an exercise-driven process. Withholding exercise may cause a delay in fibril development that can be partially overcome by increasing exercise at a later age. Exercise type may also affect remodeling of the SDFT in foals.     

Exercise modifies the age-related change in crimp pattern in the core region of the equine superficial digital flexor tendon

One of the current concepts with regard to equine superficial digital flexor tendonitis is that cumulative subclinical microscopic damage weakens the structure, predisposing the tendon to partial or complete rupture. This microtrauma is likely to affect the waveform or crimp of the collagen fibrils, which are the units of tensile strength. Collagen fibril crimp morphology characteristics were determined in superficial digital flexor tendons of 18 horses presented for necropsy. Horses were separated into exercised (n = 9) and non-exercised groups (n = 9), based on recent function. Five of the eight exercised horses below the age of 10 years showed a significantly lower crimp angle in the central region of the tendon in comparison with the periphery. Three of those five animals also showed a significantly lower crimp period length in the centre. No non-exercised horses in this age group showed such regional differences. Horses classified as non-exercised may have Horses classified as non-exercised may have undergone competitive galloping activity at an earlier age, implying that changes in central region crimp morphology did not occur at that time, or had reverted to normal values in the intervening period. A lower crimp angle in the core region in comparison with the periphery is abnormal in animals younger than 10 years, on the basis of previous data obtained from wild horses. It is hypothesised that imposed exercise regimens involving galloping modify the normal age-related reduction of crimp angle in the tendon core, probably as a result of the increased number of rapid high-strain cycles experienced by the collagen fibrils.     

Observations on the ultrastructure of equatorial scleral collagen fibrils in sheep eyes

Objective To investigate collagen fibrils of the equatorial sclera in relation to the age‐related changes in eye size in sheep. Animals studied Lambs and outbred ewes. Procedures Sheep eyes (three lamb and three from adult outbred ewes), presumed disease‐free, were processed for transmission electron microscopy (TEM) immediately postmortem. Tissue blocks from the equatorial region were sectioned across fibril bundles orientated along the equator. Micrographs including at least 500 fibrils were projected at 22 000× magnification for measures of fibril diameters (FDs). Results Lamb eyes were smaller than those of adult ewes but equatorial scleral thickness was only marginally less at 0.232 ± 0.013 vs. 0.254 ± 0.012 mm (P value not significant). Scleral tissue was composed of compacted bundles of collagen fibers that tended to be rounder in outer compared to being flatter in inner regions. In typical (normal) appearing regions, FDs were distinctly larger (68–410 nm) in outer sclera compared to inner sclera (63–281 nm). Outer sclera FDs were bimodal averaging 192 ± 58 nm, compared to unimodal distributions at inner locations averaging 156 ± 48 nm (P < 0.001). Some atypical regions, especially at outer‐mid sclera locations, were also noted where the FD distribution was bimodal but also included numerous microfibrils (<50 nm diameter), with similar appearances being found for both lamb and adult ewe eyes. Conclusions The equatorial sclera is a mixture of rounder versus flatter collagen fiber bundles, the former being more likely to be made up of a mixture of both smaller and larger fibrils, as compared to slightly smaller fibrils.     

Functional adaptation through changes in regional biochemical characteristics during maturation of equine superficial digital flexor tendons

OBJECTIVE: To quantify and compare biochemical characteristics of the extracellular matrix (ECM) of specimens harvested from tensional and compressive regions of the superficial digital flexor tendon (SDFT) of horses in age classes that include neonates to mature horses. SAMPLE POPULATION: Tendon specimens were collected on postmortem examination from 40 juvenile horses (0, 5, 12, and 36 months old) without macroscopically visible signs of tendonitis. PROCEDURE: Central core specimens of the SDFT were obtained with a 4-mm-diameter biopsy punch from 2 loaded sites, the central part of the mid-metacarpal region and the central part of the mid-sesamoid region. Biochemical characteristics of the collagenous ECM content (ie, collagen, hydroxylysylpyridinoline crosslink, and pentosidine crosslink concentrations and percentage of degraded collagen) and noncollagenous ECM content (percentage of water and glycosaminoglycans, DNA, and hyaluronic acid concentrations) were measured. RESULTS: The biochemical composition of equine SDFT was not homogeneous at birth with respect to DNA, glycosaminoglycans, and pentosidine concentrations. For most biochemical variables, the amounts present at birth were dissimilar to those found in mature horses. Fast and substantial changes in all components of the matrix occurred in the period of growth and development after birth. CONCLUSIONS AND CLINICAL RELEVANCE: Unlike cartilage, tendon tissue is not biochemically blank (ie, homogeneous) at birth. However, a process of functional adaptation occurs during maturation that changes the composition of equine SDFT from birth to maturity. Understanding of the maturation process of the juvenile equine SDFT may be useful in developing exercise programs that minimize tendon injuries later in life that result from overuse.     

Morphometric Analysis of Collagen: a Comparative Study in Cow and Pig Skins

Light and electron microscopic studies were conducted on dermis samples taken from different regions of adult cows and pigs to determine whether there is a correlation between subfibrillar architecture and collagen fibril diameter in dermis tissue. Dermis thicknesses, collagen fibril diameters, subfibrillar architecture diameters, and angles of the helical arrangement of subfibrillar architectures were measured in all dermis samples. The results showed that the differences between subfibrillar architecture diameters in dermis samples from different regions of the same species were not significant and that the diameters of subfibrillar architectures in dermis samples were similar (22-28 nm) in the two species (cow and pig) examined in this study. These findings therefore suggest that the collagen fibril diameter depends on the number of subfibrillar architectures in each fibril.     

Histology and ultrastructure of the developing superficial digital flexor tendon in rabbits

This study was designed to investigate the developmental changes in the superficial digital flexor tendon (SDFT) of the white New Zealand rabbit, from 5 to 7 days pre-natal to 112 days post-natal (PN), at histological and ultrastructural levels. The tendons changed from being highly cellular with 13079 ± 2538 cell/mm² and little directional organization to a longitudinally oriented, predominantly connective tissue structure with relatively few, mature tenocytes (2384 ± 365 cell/mm²). Fibrillogenesis was seen in isolated vacuoles of fibroblast cytoplasm during fetal life as well as after birth, but less frequently with increasing age. At the ultrastructural level, there was a progressive increase in the mean diameter of collagen fibrils with age throughout the population , until PN day 28. A bimodal distribution of collagen fibril size was first observed on PN day 56 while at day 112, the fibrils were fully differentiated and showed a multimodal size distribution. The development of elastic fibres preceded that of collagen fibres and accompanied progressively more marked sinusoidal crimping in collagen fibres on PN days 7 and 14. The tendons of older animals became less tightly crimped. The cells of the epi- and endotenon were less mature and relatively undifferentiated compared with the cells in the body of the tendon of the same age, which might explain their ability to initiate healing of tendon injuries in older animals. In conclusion, extensive changes were observed in tendons during growth and maturation, with diameters of collagen fibrils, tissue orientation and cellularity being the parameters affected, probably due largely to increased physical activity.     

Extracellular matrix composition of the equine superficial digital flexor tendon: relationship with age and anatomical site

The objectives of the present study were to test the hypotheses that: (1) the composition of the extracellular matrix of the equine superficial digital flexor tendon (SDFT) shows great functional similarities with articular cartilage, i.e. that significant differences exist in biochemical composition of differently loaded areas (which in the case of tendons may be more apparent as tendon shows more obvious differences than cartilage); and (2) that, as in articular cartilage, no substantial alterations in biochemical composition take place during ageing once adulthood has been attained. Tendon samples were taken from 60 adult slaughter horses from a central area at cross-section in the mid-metacarpal region and at the height of the proximal sesamoid bones (sesamoid region) of the SDFT. Contents of collagenous and non-collagenous components were determined. None of the parameters were correlated with age in either region, except for a significant increase in pentosidine cross-links with age in the sesamoid region. Between the two anatomical regions, there were significant differences in all variables, except for hydroxylysylpyridinoline cross-links. It was concluded that in the equine SDFT, similar to articular cartilage, most molecular parameters are not influenced by age in mature horses, indicating a low remodelling rate. Tendon composition is clearly different between regions, apparently reflecting different specific modes of biomechanical loading at the points sampled.     

Comparison of mean bone densities of three preparations of the distal portion of the equine third metacarpal bone measured by use of quantitative computed tomography

OBJECTIVE: To evaluate whether cutting equine subchondral bone to demarcate specific regions of interest (ROIs) influences the mean density for that bone as measured via quantitative computed tomography (QCT). Sample population-2 metacarpophalangeal joints from equine cadavers. PROCEDURES: The distal portion of the third metacarpal bone of each intact metacarpophalangeal joint was scanned via CT to simulate in vivo conditions. Each joint was subsequently disarticulated and dissected, and the distal portion of the dissected third metacarpal bone in air was scanned. Then, six 1-cm(2) areas representing ROIs were cut into the distal condylar surfaces to depths of approximately 1 cm, and the bone was scanned again. Three-dimensional CT models of the 3 bone preparations were generated for each third metacarpal bone on the basis of data from each set of scan images, and densities of the 6 ROIs were measured. Mean bone densities for the 6 ROIs were compared among models of intact, dissected, and cut third metacarpal bone scans. RESULTS: Mean bone density was significantly lower in cut bone preparations, compared with that in intact or dissected bone. Differences between mean bone densities for intact and dissected bone preparations were not significant. CONCLUSIONS AND CLINICAL RELEVANCE: Cutting subchondral bone to demarcate specific ROIs prior to CT imaging significantly lowered mean bone density as measured via QCT and thus introduced substantial artifacts. These findings have direct implications on techniques for CT modeling of equine subchondral bone in the characterization of joint diseases in horses.     

Structural variation of the distal condyles of the third metacarpal and third metatarsal bones in the horse

This study examined 3-dimensional (3D) distribution of sectors with contrasting density in the equine third metacarpal (McIII) and third metatarsal (MtIII) bones with a view to explaining the aetiology of distal condylar fractures. Macroradiography and computed tomographic (CT) imaging were used in the nondestructive study of bones obtained from horses, most of which were Thoroughbreds in race training. Distal condylar regions of McIII and MtIII were also studied in microradiographs of 100 microm thick mediolateral sections cut perpendicular to the dorsal and palmar/plantar articular surfaces. Qualitative and quantitative results from all methods used (radiography, CT and microradiographic stereology) demonstrated a densification (sclerosis) of subchondral bone located in the palmar/plantar regions of the medial and lateral condyles of both McIII and MtIII. Substantial density gradients between the denser condyles and the subchondral bone of the sagittal groove were shown to equate with anatomical differences in loading intensity during locomotion. It is hypothesised that such differences in bone density results in stress concentration at the palmar/plantar aspect of the condylar grooves, which may predispose to fracture.     

Unilateral basement membrane zone alteration of the regenerated laminar region in equine chronic laminitis

Between the laminar epidermis and the laminar dermis of laminar region (LR) in equine foot, it can be observed the basement membrane zone (BMZ), which is composed of a basement membrane and its accompaniments like the hemidesmosome and anchoring fibril. Alteration in the BMZ in equine laminitis is possibly related with not only development but also recovery outcome and recurrence of this disease. However, there is little known about the structure of the BMZ during the recovery phase of this disease. To assess the condition of the BMZ of LR affected by chronic laminitis, the tissue was examined in three cases at two weeks, four weeks and three months after the onset of laminitis, using pathological, immunohistochemical and electron microscopic techniques. Histologically in all laminitis cases, there was a regenerated laminar epidermis with proliferating keratinocytes between the Stratum medium and the dermis, but it included the undeveloped secondary epidermal laminae (ud-SELs) structure in one side of the primary epidermal laminae, especially in the part of the deep area of LR. Immunohistochemical results were positive for the anti-type IV collagen, anti-type VII collagen and anti-laminin 5 antibodies in the most BMZs. However, partial BMZs adjacent to the ud-SELs were negative for the anti-type VII collagen and anti-laminin 5 antibodies. Ultrastructurally, in the BMZ of the ud-SEL, the lamina densa and the lamina lucida were present. In contrast, the anchoring fibrils and the hemidesmosomes were either absent, or present at lower than normal levels. In conclusion, the present study indicated that the part of regenerated LR in chronic laminitis was not able to fully restore to construct the BMZ for a long time, especially in the unilateral side of laminar epidermis. It might be related with recurrence of this disease.     

Effect of skeletal muscle decorin on collagen fibrillogenesis in vitro

The effect of skeletal muscle decorin on collagen fibrillogenesis was investigated, in order to provide background for understanding the functions of decorin in skeletal muscle. The self‐assembly of type I and III collagen with the addition of decorin or the core protein of decorin from bovine neonatal skeletal muscle was monitored using a spectrophotmeter. Time course changes in the absorbance of collagen solutions showed typical sigmoidal curves composed of three phases. The time of the initial phase was not different between the collagen solution with decorin and that without decorin. The increase rate of the absorbance in the second phase decreased with concentration of decorin added in collagen solutions. Similar effects on fibrillogenesis of type I and III collagens were observed when the core protein of decorin was added in collagen solutions. These results suggest that regulation of collagen fibrillogenesis by decorin depends on its core protein. The networks of reconstructed collagen fibrils with decorin were looser than those without decorin. Bovine skeletal muscle decorin could participate in the regulation of collagen fibrillogenesis and in the arrangement of collagen fibrils in the intramuscular connective tissue.     

Tendonitis of branches of the superficial digital flexor tendon in Standardbred racehorses: 15 cases

This case report describes the clinical and ultrasonographic findings and performance outcome of distal tendonitis of the superficial digital flexor tendon (SDFT) in Standardbred racehorses. Cases with distal tendonitis of the SDFT in isolation (not associated with a metacarpal lesion) were reviewed retrospectively. The clinical and ultrasonographic findings and performance outcome were reviewed. The injury was located on the left forelimb in 67% (10/15) of horses and on the right forelimb in 33% (5/15). None of the horses were affected bilaterally. The lateral branch was involved in 80% of the cases (12/15) and medial branch in 20% (3/15). None of the horses were affected biaxially. Mean time between injury and first start ± s.d. was 302 ± 142 days. Returning to racing was observed in 93% of the horses (14/15), with a reinjury rate of 22% (2/9). For horses which raced after the injury, the number of starts, shows and wins was not significantly different before and after injury, regardless of the location of the lesion. Tendonitis of branches of the SDFT in isolation carries a good prognosis in Standardbred racehorses. A higher return to racing and a lower reinjury rate may be expected compared to lesions in the metacarpal region.     

Three dimensional structure of the distal condyles of the third metacarpal bone of the horse

This study examined the three-dimensional (3D) microarchitecture of regions of the equine third metacarpal bone (McIII) commonly involved in distal condylar fractures. Limbs were obtained from Thoroughbred horses (neonates to age 24 years) destroyed for inoperable fractures and a variety of other conditions. Beams, blocks and sections were cut in the principal axes, some embedded in PMMA and others examined unembedded. Several methods were used to study the 3D structure, including conventional and confocal optical microscopy, scanning electron microscopy (SEM) and radiography. The mineralised articular cartilage tends to cleave in the sagittal plane. Proximal to the subchondral bone, the main trabeculae are robust plates running in the sagittal direction with less significant mediolateral connections. Small blood vessel canals lie inside the sagittal plates. This structure gives maximum strength and protection in the sagittal plane in which the bone rotates, but offers minimal resistance to fracture propagation in this plane. The anatomical course of the common distal condylar fractures of the third metacarpal bones can be explained by underlying anisotropic structural features of the mineralised tissues.     

Effects of platelet-derived growth factor-BB on the metabolic function and morphologic features of equine tendon in explant culture

OBJECTIVE: To evaluate the effects of recombinant human platelet-derived growth factor-BB (rhPDGF-BB) on the metabolic function and morphologic features of equine superficial digital flexor tendon (SDFT) in explant culture. Animals-6 euthanized horses (2 to 5 years old). METHODS: Forelimb SDFT explants were cultured for 6 days as untreated control specimens or treated with rhPDGF-BB (1, 10, 50, or 100 ng/mL of medium). Treatment effects on explant gene expression were evaluated via real-time PCR analysis of collagen type I, collagen type III, PDGF-A, and PDGF-B mRNA. Explants were assayed for total collagen, glycosaminoglycan, and DNA content; histologic changes were assessed via H and E staining and immunohistochemical localization of collagen types I and III. RESULTS: No morphologic or proliferative changes were detected in tendon explant sections. After high-dose rhPDGF-BB treatment, gene expression of collagen types I and III was increased and decreased, respectively. Expression of PDGF-A and PDGF-B mRNA was significantly increased at 24 hours, but later decreased to have few or negative autoinductive effects. Although PDGF gene expression waned after 48 hours of culture, collagen type I gene expression was significantly increased at 48 hours and reached peak value on day 6. Glycosaminoglycan and DNA content of explants were unchanged with rhPDGF-BB treatment. CONCLUSIONS AND CLINICAL RELEVANCE: Results suggest that rhPDGF-BB use may be of benefit in the repair of equine tendon, particularly through induction of collagen type I mRNA. Positive autoinductive effects of PDGF-BB in equine SDFT explants were detected early following culture medium supplementation, but these diminished with time.     

The collagen fibrils in fowl medullary bone

Medullary bone from the femurs of laying fowls was examined by electron microscopy, with particular reference to the nature and environment of its collagen fibrils. The collagen fibrils appeared to have a preferred orientation along the long axis of the bone. There were no clear‐cut cyclic changes in the fibril diameters during egg calcification. The tissue was rich in mucopolysaccharides, visualised as strands or sheets connecting the fibrils. Lysis of the fibrils appeared to be associated with osteoclasts rather than osteocytes.     

Effects of exercise on tenocyte cellularity and tenocyte nuclear morphology in immature and mature equine digital tendons

REASON FOR PERFORMING STUDY: The injury-prone, energy-storing equine superficial digital flexor tendon (SDFT) of the mature performance horse has a limited ability to respond to exercise in contrast with the noninjury-prone, anatomically opposing common digital extensor tendon (CDET). Previous studies have indicated low levels of cellular activity in the mature SDFT, but in foal tendons the tenocytes may still have the ability to adapt positively to increased exercise. OBJECTIVES: To measure tenocyte densities and types in histological sections from the SDFT and CDET of horses from controlled long-term, short-term and foal exercise studies. METHODS: Specimens were collected from mid-metacarpal segments of the CDET and SDFT for each horse and processed for histology; central and peripheral regions of the SDFT cross-section were analysed separately (SDFTc, SDFTp). Tenocyte nuclei were counted in a total area of 1.59 mm(2) for each tendon region in each horse. Each nucleus was classified as type 1 (elongate and thin), type 2 (ovoid and plump) or type 3 (chondrocyte-like); type 1 cells are proposed to be less synthetically active than type 2 cells. RESULTS: No significant differences were noted between exercise and control groups in any of the studies, with the exception of an exercise-related reduction in the proportion of type 1 tenocytes for all tendons combined in the long-term study. There were tendon- and site-specific differences in tenocyte densities and proportions of type 1 and 2 cells in all 3 studies. CONCLUSIONS AND POTENTIAL RELEVANCE: There was no indication that exercise increased tenocyte density or proportions of the (theoretically) more active type 2 cells in immature horses (short-term and foal studies), perhaps because the training regimens did not achieve certain threshold strain levels. In the foal study these findings can still be interpreted positively as evidence that the training regimen did not induce subclinical damage.     

Effect of exercise on age-related changes in collagen fibril diameter distributions in the common digital extensor tendons of young horses

OBJECTIVE: To determine whether specific treadmill exercise regimens would accelerate age-related changes in collagen fibril diameter distributions in the common digital extensor tendon (CDET) of the forelimbs of young Thoroughbreds. ANIMALS: 24 female Thoroughbreds. PROCEDURE: Horses were trained for 18 weeks (6 horses; short term) or 18 months (5 horses; long term) on a high-speed treadmill; 2 age-matched control groups (6 horses/group) performed walking exercise only. Horses were (mean +/- SD) 24 +/- 1 months and 39 +/- 1 months old at termination of the short-term and long-term regimens, respectively. Midmetacarpal CDET specimens were obtained and processed for transmission electron microscopy. Diameter and area of at least 1,000 collagen fibrils/specimen were measured by use of computerized image analysis. Mass-average diameter (MAD) of collagen fibrils and collagen fibril index were calculated for each horse. RESULTS: Collagen fibril MAD for the older horses was significantly less than that for the younger horses. Exercise did not significantly affect fibril diameter or distributions in either age group, and collagen fibril index did not differ significantly between groups. CONCLUSIONS AND CLINICAL RELEVANCE: Age-related reduction in collagen fibril MAD agreed with findings for other tendons and species. Training did not accelerate age-related change in the CDET in contrast to a reported decrease in collagen fibril MAD in the superficial digital flexor tendon of horses trained long term. Our results support the concept that the functionally distinct nature of the CDET and superficial digital flexor tendon in horses results in fundamentally different responses to high-speed exercise regimens.