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A variety of distinctive cell types occur within tendons. Tenocytes are transformed from tenoblasts (immature tenocytes), which are rounded cells with large, ovoid nuclei (Borynsenko & Beringer, 1989). Tenocytes are flat tapered cells that appear spindle‐shaped in longitudinal tissue sections and stellate (with long extensions) in cross sections. Tenocytes, a common synonym for differentiated fibroblast‐like cells that lie within rows within the tendon proper, comprise about 95% of the cellular elements of a tendon. They lie sparingly in longitudinal rows between collagen fibrils (Figure 3.12) (Butler, Grood, Noyes, & Zernicke, 1978) and are aligned in parallel with the primary direction of force on the tendon. Tenocytes synthesize collagen and most other compounds of a tendon’s extracellular matrix (Evans & Barbenel, 1975; Kjaer, 2004). Tendons also contain low numbers of stem/progenitor cells (tendon stem/progenitor cells, which are also regarded as tendon‐derived stem cells). These cells show typical stem cell characteristics of self‐renewal capacity (Li, Wu, & Liu, 2021). In tendon fascial sheaths, there are also different subpopulations of fibroblasts with different roles in matrix synthesis and cell migration during wound healing (Jozsa & Kannus, 1997). The remaining 5–10% of cells in a tendon includes chondrocytes (located at entheses, where the tendon attaches to bone, Figure 3.12c), endothelial cells associated with blood vessels, and peripheral glial cells associated with nerve processes. In pathological conditions and during wound healing, inflammatory cells (neutrophils, macrophages, and lymphocytes) and myofibroblasts can be observed in tendon tissues (Barbe et al., 2021; Fedorczyk et al., 2010; Jozsa & Kannus, 1997; Kietrys, Barr, & Barbe, 2011).

Table 3.3 Summary of Cells, Extracellular Matrix (ECM), Subregions, and Function Of Tendons Under Normal Conditions

Characteristic	Description
Tissue type	Dense regular connective tissue
Cells	Main cell types: Tenocytes, tenoblasts, tendon stem/progenitor cells (TSPC)Additional cell types: Chondrocytes or synovial cells in tendon subregions, peripheral glial cells associated with nerve endings, fibroblasts in sheaths, endothelial cells associated with blood vessels in sheaths
ECM	Main composition: 65–80% collagen I, water, proteoglycans, glycoproteins, elastin, mucin
Subregions	Myotendinous junction, tendon midbody, enthesis
Function	Transfer of tensile forces created by muscles onto bone; absorbs sudden shocks to limit muscle damage

Figure 3.12 Tendon images. (a) Longitudinal tendon section, with examples of tenocytes (tendon fibroblast like cells) indicated. CT = loose connective tissue; cap = capillary; M = muscle fibers/cells. Hematoxylin and eosin (H&E) staining. (b) Cross‐section through a small tendon in the palm. It lacks individual fascicles, so just the outer epitenon (epi) and peritenon (peri) layers are visible. Masson’s Trichrome staining. (c) The tendon–bone intersection called an enthesis. Masson’s Trichrome staining.