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constructing tendon organoids, including cellular selection, the integrity of the ECM within the fiber space. 1,21,25 These
biochemical and mechanical factors, and bioengineering properties provide the structural foundation for tendons to
approaches. It will also explore their applications in disease transmit mechanical forces between muscles and bones and
modeling and drug screening. Finally, future directions to withstand high tensile stresses (Figure 1).
will be outlined, focusing on the integration of artificial Tendons are characterized by relatively low vascularity,
intelligence (AI) and standardization efforts to enhance the sparse cellularity, and reduced metabolic activity.
scalability and functionality of tendon organoid technology. A significant challenge in tendon repair is the post-injury
2. Tendon structure and microenvironment fibrotic response mechanism, which leads to tendon sheath
adhesion and excessive scar formation. This prevents the
2.1. Tendon tissue structure restored tendon from regaining the mechanical strength
26
Tendons serve as dense fibrous connective tissues that of uninjured tissue, often resulting in frequent reinjury.
anchor muscles to bones, thereby enabling the efficient The natural regeneration of tendons after injury involves
transfer of mechanical forces. Healthy tendons exhibit three primary phases: Inflammation, proliferation, and
1,2
20
a bright white appearance and are characterized by remodeling. During this process, fibroblast activity and
14
a low cellular content, primarily consisting of ECM the synthesis of Type III collagen significantly increase.
(55–70%). This structural specificity sharply contrasts However, healed tendons exhibit fewer cross-links and
with parenchymal organs (e.g., liver or kidney), where smaller collagen fibril diameters compared to healthy
cellular components dominate and ECM constitutes only tendons. Fibrotic tissue can develop between the tendon
a minor part. 15,16 The ECM-rich composition of tendons and its surrounding structures, resulting in adhesions and
provides both tensile strength and elasticity, enabling thereby increasing the risk of reinjury. 27
their unique biomechanical function in load transmission. 2.2. Tendon cell composition
The composition of tendons changes with age; as tendons
28
mature postnatally, cellular content decreases while ECM The primary cells in tendon tissue are tenocytes (90–95%)
20
content increases. The ECM is primarily constituted by and tenoblasts (resident tendon cells). Tenocytes
17
proteoglycans, glycosaminoglycans, glycoproteins (with are elongated, spindle-shaped fibroblasts with a low
a notable presence of small leucine-rich proteoglycans), nucleus-to-cytoplasm ratio and low metabolic activity,
and collagen fibers (60–85% of dry weight), 18-20 which are distributed among collagen fibers. Their main functions
organized into a network through the aligned arrangement include secreting ECM and releasing signals that regulate
of collagen fibers. Collagen fibers are mainly composed tendon formation and development. 21,29 Tenoblasts, the
19
of Type I collagen, accounting for 97–98% of the total immature form of tenocytes, differentiate into tenocytes
20
collagen content in tendons, while Type III collagen as the individual ages. Through single-cell transcriptomic
constitutes 1.0–1.5%, with minor amounts of collagen analyses, recent studies have illustrated the heterogeneity
21
Types V, XI, XII, and XIV. Type I collagen forms triple- of the tendon resident cell population 27,30-33 (Figure 1).
2
helical tropocollagen molecules, which aggregate into While tenocytes and their precursors form the core
microfibrils. These microfibrils further assemble into fibrils functional cellular component, tendon-resident cells can
that exhibit a periodic “crimping” pattern under unloaded be broadly categorized into three major subpopulations,
conditions. Fibrils coalesce to form fibers with diameters including functional fibroblasts that express high levels
22
ranging from 1 to 20 µm, which are grouped into fiber of ECM-related genes (e.g., COL1A1, COL3A1), tendon
bundles (150–500 µm) enveloped and separated by the stem/progenitor cells (TSPCs), and immune-regulatory
3,29,34-36
endotenon or interfascicular matrix (IFM). The epitenon cells that express cytokines and complement factors.
21
is a thin, dense connective tissue layer that closely envelops The study evaluates the expression of typical tendon
the tendon surface, functioning to provide lubrication and fibroblast markers, such as scleraxis (SCX), tenomodulin
1
minimize friction between the tendon and surrounding (TNMD), and Mohawk Homeobox, in each subpopulation,
tissues. The paratenon, located outside the epitenon, is identifying the presence of these markers in a subset of
composed of loose connective tissue rich in blood vessels fibroblasts. The phenotypic identification of tenocytes
and nerves. In tendons without a sheath, such as the Achilles relies on the expression of common markers in resident
tendon, the paratenon serves as the primary lubricating cell populations, including ECM proteins, such as Type 1
structure. Together, the epitenon and paratenon form the collagen (COLI) and Type 3 collagen (COLIII), or small
peritenon. 1,23,24 In addition to intermolecular cross-linking leucine-rich proteoglycans (SLRPs), such as decorin, within
1
between collagen fibers, non-collagenous elastic components the IFM, with emerging markers, including THBS4 and
31
of the ECM, such as proteoglycans, glycosaminoglycans, WNT10A, implicated in tendon development and repair.
and glycoproteins, play crucial roles in mitigating tissue Importantly, the described cell types (tenocytes,
deformation, enhancing viscoelasticity, and maintaining tenoblasts, functional fibroblasts, and TSPCs) represent the

Volume 1 Issue 3 (2025) 3 doi: 10.36922/OR025170016