Tendon and ligament fibrillar crimps give rise to left-handed helices of collagen fibrils in both planar and helical crimps

Abstract

Collagen fibres in tendons and ligaments run straight but in some regions they show crimps which disappear or appear more flattened during the initial elongation of tissues. Each crimp is formed of collagen fibrils showing knots or fibrillar crimps at the crimp top angle. The present study analyzes by polarized light microscopy, scanning electron microscopy, transmission electron microscopy the 3D morphology of fibrillar crimp in tendons and ligaments of rat demonstrating that each fibril in the fibrillar region always twists leftwards changing the plane of running and sharply bends modifying the course on a new plane. The morphology of fibrillar crimp in stretched tendons fulfills the mechanical role of the fibrillar crimp acting as a particular knot/biological hinge in absorbing tension forces during fibril strengthening and recoiling collagen fibres when stretching is removed. The left-handed path of fibrils in the fibrillar crimp region gives rise to left-handed fibril helices observed both in isolated fibrils and sections of different tendons and ligaments (flexor digitorum profundus muscle tendon, Achilles tendon, tail tendon, patellar ligament and medial collateral ligament of the knee). The left-handed path of fibrils represents a new final suprafibrillar level of the alternating handedness which was previously described only from the molecular to the microfibrillar level. When the width of the twisting angle in the fibrillar crimp is nearly 180 degrees the fibrils appear as left-handed flattened helices forming crimped collagen fibres previously described as planar crimps. When fibrils twist with different subsequent rotational angles (< 180 degrees ) they always assume a left-helical course but, running in many different nonplanar planes, they form wider helical crimped fibres.

Fig. 1

SEM picture of sagittal section of the tendon of flexor digitorum profundus muscle of rat. Two fibres of mostly large collagen fibrils running parallel and densely packed bend forming crimps. At the top angle of these crimps the fibrils first twist leftwards and then bend forming a fibrillar knot or fibrillar crimp. Bar = 1 μm.

Fig. 2

(A-B) SEM pictures of sagittal section of Achilles tendon of gastrocnemius muscle of rat. Both A) and B) show a tendon crimp composed of fibrillar crimps. Note that both small and large collagen fibrils first twist leftwards changing the plane of their running and then bend changing their direction in the new plane. Bar = 1 μm.

Fig. 3

SEM picture of sagittal section of rat tail tendon. Collagen fibrils of a collagen fibre run densely packed and form a crimp. At the top angle of the crimp the collagen fibrils twist leftwards and then bend forming many parallel fibrillar crimps. Bar = 1 μm.

Fig. 4

SEM picture of a sagittal section of rat patellar ligament. Collagen fibrils forming a fibre twist leftwards and then bend in the fibrillar crimp regions. Bar = 1 μm.

Fig. 5

TEM picture of fibrillar crimps in the tendon of flexor digitorum profundus muscle of rat. Parallel collagen fibrils in the fibrillar crimp region twist leftwards and bend changing both their direction and plane of coursing. No D-period banding is observable in the fibrillar crimps but a microfibril swelling is clearly evident. Bar = 500 nm.

Fig. 6

SEM picture of a frontal section of rat medial collateral ligament of the knee. Collagen fibres run intertwined and fibrils are not running as parallel and densely packed as in tendons. When fibrils form crimps they always show fibrillar crimps. In fibrillar crimp regions all fibrils first twist leftwards and then bend changing first the plane and then the direction of running. Bar = 1 μm.

Fig. 7

SEM picture of a frontal section of Achilles tendon of gastrocnemius muscle of rat. Many collagen fibres are composed of fibrils forming fibrillar crimps at the top of each crimp. The collagen fibres and fibrils running in parallel planar planes form “planar crimps”. Bar = 10 μm.

Fig. 8

SEM picture of a frontal section of rat medial collateral ligament of the knee. Two collagen fibres are composed of fibrils left-twisting and bending to form fibrillar crimps at the top of each crimp. The fibres show a helical array because the fibrils course in different non planar planes. Crimps of these fibres appear as helical crimps. Bar = 1 μm.

Fig. 9

Isolated collagen fibrils from medial collateral ligament of rat. A) A large collagen fibril forming different fibrillar crimps assumes a flattened helical array (compare it with Fig. 4); B) a large collagen fibril coursing straight intertwines with a smaller fibril showing a flattened helical array (compare it with Fig. 6). Bar = 500 nm.

Fig. 10

In vivo stretched rat Achilles tendon at SEM. In a completely flattened crimp the stretched collagen fibrils run straight and parallel still showing knots similar but different to the fibrillar crimps observed in relaxed tendons: fibrils still twist leftwards, running straight in two different parallel planes, but not bending. Bar = 1 μm.

Fig. 11

A) Single subfilaments (tropocollagen) arranged in a left helical array form single rope filaments (microfibrils composed by left-handed helices of tropocollagens) which twisting rightwards form a rope (single fibril composed by microfibrils arranged in a rightward helix). When the rope (fibril) twists leftwards of about 180° in a local region, the single rope filaments (microfibrils) appear swelled also changing their handedness. B) When the rope (fibril) is stretched at the ends the portion of the rope, twisting leftward, acts like a knot (fibrillar crimp). (Compare it with tendon fibrils stretched in vivo in Fig. 10).

Fig. 12

A) Model of a single flattened left-handed helix simulating a flattened collagen fibril helix forming “planar crimps” (compare it with collagen fibrils in Figs 7 and 9B). B) Model of a wider lefthanded helix simulating the collagen fibrils forming helical crimps (compare it with collagen fibrils in Fig. 8).