Tenocyte

Tenocytes are cells of the fibroblast lineage located within tendons. Viewed under a microscope, they appear as thin, elongated cells arranged between parallel bundles of collagen fibers. The cells contact one another, forming a network of connected rows. Young, immature cells are called tenoblasts; they are round and actively mobile, becoming thin, elongated tenocytes as they mature.

Tenocytes have three main functions.

• Synthesizing matrix components — They synthesize major extracellular matrix components, including type I collagen, which supports tendon structure, and water-retaining proteoglycans.
• Breaking down and replacing old tissue — They break down aged or damaged matrix proteins and replace them with new components, maintaining the physiological balance of tendon tissue.
• Aligning collagen fibers — They guide newly synthesized collagen fibers into an orderly arrangement along the longitudinal axis where tension acts, maintaining and increasing tendon stiffness.

In addition to tenocytes, tendons are known to contain cells with stem cell-like properties. These cells are thought to contribute to tendon maintenance and repair.

A key characteristic of tenocytes is their ability to convert external physical forces into biochemical responses within the cell. This is called mechanotransduction, and the model proposed by Khan and Scott in 2009 describes it in three stages.

1. Detecting force — Tension applied to a tendon is transmitted to the cytoskeleton through deformation of the cell membrane or receptors such as integrins. 2. Passing the signal to neighbors — Stimulated cells spread signals to neighboring cells, including those that have not directly received the force. 3. Rebuilding tissue — Cells that receive the signal promote the synthesis of collagen and extracellular matrix proteins, reinforcing tendon tissue.

In effect, regular physical exercise acts as a signaling system that instructs cells to strengthen tissue. These biological mechanisms occur not only in tendons but also in skeletal muscle, articular cartilage, and bone tissue. Conversely, when prolonged immobilization removes mechanical loading, cellular synthesis declines and the tissue atrophies.

• Exercise conditions tendons, too. Progressive loading stimulates tenocyte synthesis activity, allowing tendons to adapt to withstand external forces. This cellular response underlies the tendon-care principle of “gradually increasing a manageable load.” See Tendon adaptation to loading for details of the process.
• Tissue adaptation needs sufficient time. The metabolic process through which tenocytes synthesize collagen fibers and align them into a firm structure is slower than gains in muscle strength. Tendon metabolic cycles therefore need to be considered when increasing exercise volume.
• Stopping completely leads to weakening. Prolonged disuse reduces cellular activity. Even when mild discomfort is present, maintaining movements that provide loading within a range where pain is not severe is understood to be beneficial for tendons.
• Cells retain their capacity to respond even with aging. Although the physical properties of tendons change with age, the basic physiological mechanisms through which cells respond to mechanical loading remain intact. Slow, consistent loading exercise has value.