Crimp morphology in relaxed and stretched rat Achilles tendon

Abstract

Fibrous extracellular matrix of tendon is considered to be an inextensible anatomical structure consisting of type I collagen fibrils arranged in parallel bundles. Under polarized light microscopy the collagen fibre bundles appear crimped with alternating dark and light transverse bands. This study describes the ultrastructure of the collagen fibrils in crimps of both relaxed and in vivo stretched rat Achilles tendon. Under polarized light microscopy crimps of relaxed Achilles tendons appear as isosceles or scalene triangles of different size. Tendon crimps observed via SEM and TEM show the single collagen fibrils that suddenly change their direction containing knots. The fibrils appear partially squeezed in the knots, bent on the same plane like bayonets, or twisted and bent. Moreover some of them lose their D-period, revealing their microfibrillar component. These particular aspects of collagen fibrils inside each tendon crimp have been termed 'fibrillar crimps' and may fulfil the same functional role. When tendon is physiologically stretched in vivo the tendon crimps decrease in number (46.7%) ( P < 0.01) and appear more flattened with an increase in the crimp top angle (165 ° in stretched tendons vs. 148 ° in relaxed tendons, P < 0.005). Under SEM and TEM, the 'fibrillar crimps' are still present, never losing their structural identity in straightened collagen fibril bundles of stretched tendons even where tendon crimps are not detectable. These data suggest that the 'fibrillar crimp' may be the true structural component of the tendon crimp acting as a shock absorber during physiological stretching of Achilles tendon.

The elastic fibre network of the human lumbar anulus fibrosus: architecture, mechanical function and potential role in the progression of intervertebral disc degeneration

“…If fibres were to be maintaining crimp, it is reasonable to assume that they would initially be under tension and hence appear straight, but these observations suggest collagen crimp must straighten before elastic fibres become mechanically viable. Secondly, collagen crimp is present in other tissues which contain no documented elastic fibres, such as Achilles tendon, suggesting it is an intrinsic property determined by collagen ultrastructure [14,55]. Finally, the relatively small strains along the collagen fibre axes attributable to uncrimping [51,64] would seem to make elastic fibres redundant in consideration of their high extensibility [16].…”

The past, present and future in scaffold-based tendon treatments

“…During loading of collagen molecules, fibrils, and fibril bundles deform and finally fail by a process termed defibrillation. Up to a strain of 2% (toe region), stretching of the triple helix is the predominant mechanism of deformation [268][269][270][271] and corresponds to the gradual removal of a macroscopic crimp in the collagen fibrils [272][273][274][275][276][277][278][279][280][281]. This macroscopic crimp has been characterised as the shock absorber of tendons that permits non-damaging longitudinal elongation of fibrils within the tissue [261,282].…”

Polarization microscopy for characterizing fiber orientation of ocular tissues

“…Manual tracing is a relatively common method used to determine fiber orientation in soft collagenous tissues [33][34][35], and permits comparing with a reference set by an expert. We conducted three test cases: loaded tendon fibers, a single curved silk fiber, and a sample of cornea from an eye at high intraocular pressure.…”