Engineering Fibrous Tissues and Their Interfaces with Bone - Structural Interfaces and Attachments in Biology - page 314

Fiber bundles gather into fascicles or secondary fiber bundles that form the
characteristic crimp pattern seen in many tendons and ligaments. Secondary fiber
bundles are surrounded by epitenon or epiligament, which serve as a loose connec-
tive layer for blood vessels, lymphatics, and nerves. Surrounding the connective
tissue layer is the paratenon that reduces friction and enables smooth movement
between neighboring tissues [ 15 ]. Long tendons and ligaments are sometimes
enclosed in a synovial sheath to further decrease friction [ 11 , 16 ]. The size of the
fiber bundles is related to the macroscopic size and function of the tendon and
affects the mechanical strength of the tissue [ 17 ]. Larger fascicles are found in big,
weight-bearing tendons such as the Achilles tendon, and smaller fibrils are gener-
ally found in the digital tendons.
Biochemically, the composition of tendon and ligaments is similar. In addition
to collagen that comprises the fiber bundles, these tissues contain water (55-70% by
weight), cells, and other extracellular matrix (ECM) components such as elastin,
proteoglycans, and glycoproteins [ 11 ]. Each component is discussed in more detail
in the following sections.
15.2.2 Cells
Cells compose only 20% of total tendon volume [ 18 ]. Fibroblasts, elongated and
spindle-shaped, are the main cell type found in fibrous tissues and lie parallel with
the collagen fibers [ 11 ]. Communication among neighboring cells occurs through
the formation of gap junctions [ 16 , 19 ]. The main action of fibroblasts is to secrete
molecules that form the ECM to provide structural support during mechanical
loading of the tissue. Thus, these cells have the ability to adapt to an environment
of increased mechanical loading through changes in gene expression and ECM
protein synthesis [ 3 , 10 , 20 ].
15.2.3 Extracellular Matrix
The ECM of tendon and ligament tissues consists of collagen, elastin,
proteoglycans, and glycoproteins. The ECM components comprise 80% by dry
weight of the total tissue volume [ 3 , 21 - 23 ]. The interactions of the ECMmolecules
provide the mechanical strength and properties of the fibrous tissue. Each compo-
nent is discussed in detail in the following sections.
15.2.3.1 Collagen
Besides water, the largest component of the ECM of tendons and ligaments is
collagen. In tendons, 75-85% of the dry weight of the tissue consists of collagen,