Tendon Anatomy: More Than Just Collagen
Hello guys - Here’s a summary of Chapter Three from Jake Turra’s Tendon Book. This is all about the anatomy of tendons. Enjoy!
Tendons have a complex anatomical structure made up of several different components, although most of a tendon is composed of collagen, a type of protein. Collagen has its own components, as well, and is arranged hierarchically. At the most fundamental level, there are short collagen molecules, which are amino acids–specifically glycine, proline, and hydroxyproline–structured together in a triple helix form. Collagen molecules connect to create collagen fibrils, which are very strong, supportive proteins. Groups of fibrils make collagen fibers, and fibers make the numerous collagen fascicles of a tendon.
These fascicles are held in a hydrated gel. Tendon gel also contains immature collagen (type 3 fibrils), “water attractors” (i.e., proteoglycans or PGs and glycosaminoglycans or GAGs), and a protein with elastic properties called elastin. Also, blood can move through the gel, and some nerves terminate there.
Cells serve a dynamic rather than structural purpose in tendons. Deep in the tendon around the collagen fibers, long, skinny tenocytes serve a tendon maintenance role. More superficially, in the gel, thicker, rounder tenoblasts and stem cells manage the building and cleaning up of tendon materials. There are also immune cells in the tendon gel.
The area at which tendon and muscle connect is called the myotendinous junction. There, muscle cells are present to contract the muscle, and there are “Golgi tendon organs,” which hinder further movement when there is too much tension. The enthesis is the point at which tendon and bone connect. The enthesis is composed of four areas: tendon, unmineralized fibrocartilage, mineralized fibrocartilage, and bone. There are more type 2 collagen or “cartilage-type proteins” and bigger PGs. These components manage the compressive forces on the tendon near the bone since tensile and compressive forces are high at the enthesis.
Some tendons are surrounded by double-layered synovial fluid tendon sheaths to limit friction (e.g., in the hands and ankles). Others (e.g., the Achilles and patellar tendons) have peritendons instead of tendon sheaths. Peritendons are made of loose, stretchy tissue that allows for greater movement.
