What should a diagram of tendons include?

Comprehensive Tendon Diagram Components

A diagram of tendons should illustrate the hierarchical structural organization from molecular to macroscopic level, showing collagen molecules assembling into fibrils, fibers, and bundles, along with the surrounding connective tissue sheaths and the tendon-bone insertion point. 1, 2

Essential Structural Elements to Include

Hierarchical Organization (Micro to Macro)

• Collagen molecules - the fundamental building blocks that assemble into larger structures 1, 2
• Microfibrils - initial aggregation of collagen molecules 1
• Collagen fibrils - visible under electron microscopy with characteristic 67 nm D-period banding, showing large diameter in mature tendons 1
• Collagen fibers - bundles of fibrils representing the basic structural unit, with fibrils oriented longitudinally, transversely, and horizontally within each fiber 1, 2
• Primary fiber bundles - groups of collagen fibers bound together 2
• Secondary fiber bundles - aggregations of primary bundles 2
• Tertiary fiber bundles - the largest organizational unit comprising the complete tendon structure 2

Connective Tissue Sheaths

• Endotenon - fine connective tissue sheath investing each individual collagen fiber and binding fibers together 2
• Epitenon - outer connective tissue sheath surrounding the entire tendon 2
• Peritendon sheaths - external covering structures 1
• Synovial sheaths - where applicable for tendons requiring gliding motion 1
• Fibrous sheaths or retinaculae - restraining structures 1
• Tendon bursae - fluid-filled sacs reducing friction 1

Cellular Components

• Tenocytes and tenoblasts - elongated fibroblasts positioned between collagen fibers that produce the extracellular matrix 1, 2
• Endothelial cells - supporting vascular structures 1
• Chondrocytes - present in specialized regions, particularly near insertions 1

Matrix Components

• Type I collagen - comprising 65-80% of tendon dry mass, the dominant structural protein 2, 3
• Elastin - approximately 1-2% of dry mass, providing elastic properties 2
• Proteoglycans - primarily decorin and hyaluronan embedded in the water matrix 1
• Non-collagenous proteins - present in small amounts but functionally important 3

Structural Features

• Crimp pattern - the characteristic wavy appearance of collagen bundles visible under polarized light microscopy, showing alternating dark and light transverse bands 1
• Fibrillar crimps - knots where individual collagen fibrils suddenly change direction within each crimp, serving a functional role in force absorption 1
• Spiral arrangements - longitudinal fibers crossing each other and forming spiral-type plaits, not running strictly parallel 2
• Three-dimensional fiber orientation - showing fibrils oriented not only longitudinally but also transversely and horizontally to withstand multidirectional forces 2

Functional Zones

• Tendon-bone enthesis - the transitional interfacial zone between compliant fibrous tendon and rigid mineralized bone 4
• Muscle-tendon junction - the proximal attachment point where muscle fibers connect to tendon 1, 5

Pathological Features for Clinical Context

Normal vs. Damaged Tendon Comparison

• Normal tendon histology - highly organized parallel collagen fiber arrangement 6
• Tendinosis histology - collagen disorientation and fiber separation characteristic of degenerative changes, not inflammatory infiltrates 6, 7

Common Pitfalls to Avoid in Diagram Design

• Do not oversimplify the three-dimensional structure - tendons are not simple parallel cables but have complex spiral and crossing fiber arrangements that buffer forces from multiple directions 2
• Include the hierarchical levels clearly labeled - confusion about the organizational scheme from molecule to whole tendon is common 1, 2
• Show the relatively hypovascular nature - particularly proximal to insertion points, which predisposes to degenerative pathology 7
• Distinguish between acute inflammatory changes and chronic degenerative changes - the vast majority of tendon pathology is degenerative tendinosis with collagen disorientation, not inflammatory tendinitis 6, 7