Decreased muscle loading delays maturation of the tendon enthesis during postnatal development

Abstract

Physical environment influences the development and maintenance of musculoskeletal tissues. The current study uses an animal model to explore the role of the physical environment on the postnatal development of the supraspinatus tendon enthesis. A supraspinatus intramuscular injection of botulinum toxin A was used to paralyze the left shoulders of mice at birth. The supraspinatus muscles of right shoulders were injected with saline to serve as contralateral controls. The supraspinatus enthesis was examined after 14, 21, 28, and 56 days of postnatal development. Histologic assays were used to examine fibrocartilage morphology and percentage osteoclast surface. Micro-computed tomography was used to examine muscle geometry and bone architecture. At 14 days there were no differences between groups in fibrocartilage formation, muscle geometry, bone architecture, or osteoclast surface. When comparing groups at 21, 28, and 56 days, muscle volume was decreased, fibrocartilage development was delayed, mineralized bone was decreased, and osteoclast surface was higher at each timepoint in the botulinum group compared to the contralateral saline control group. Our results indicate that the development of the tendon enthesis is sensitive to its mechanical environment. A reduction in muscle loading delayed the development of the tendonto-bone insertion site by impeding the accumulation of mineralized bone. Physical factors did not play a significant role in enthesis maturation in the first 14 days postnatally, implying that biologic factors may drive early postnatal development. ß

Enthesis fibrocartilage cells originate from a population of Hedgehog-responsive cells modulated by the loading environment

“…BtxA was injected according to published protocols (Schwartz et al, 2013;Thomopoulos et al, 2007). Briefly, Gli1-Cre ERT2 ;mTmG animals were injected with 0.2 U in 10 μl sterile saline starting at P1, twice per week, until sacrifice, and the contralateral limb was injected concurrently with the same volume of saline (four or five animals were used for each time point of each assay).…”

What We Should Know Before Using Tissue Engineering Techniques to Repair Injured Tendons: A Developmental Biology Perspective

“…[54][55][56][57] Among these molecules, biglycan and fibromodulin are particularly interesting, because a recent study suggested that they are critical elements for maintaining a niche for tendon progenitor cells. 58 In the absence of biglycan and fibromodulin the organization of tendon fibers was disorganized, and the identity of tendon progenitors, for example, the expression of Scx and type I collagen, was lost. 58 These results demonstrate that proteoglycans play an essential role in tendon differentiation.…”

“…58 In the absence of biglycan and fibromodulin the organization of tendon fibers was disorganized, and the identity of tendon progenitors, for example, the expression of Scx and type I collagen, was lost. 58 These results demonstrate that proteoglycans play an essential role in tendon differentiation. Tenomodulin (Tnmd) is also important in tendon differentiation.…”

“…These dividing cells express stem cell characteristics such as clonogenicity, multipotency, and selfrenewal capacity. 58 However, whether these proliferating cells arose from stem cells, transit amplifying cells, or both is unknown. In addition, although stem cell characteristics were found in the in vitro cell culture system, it is unclear whether these distinctions exist in the tendon.…”

“…103 Although there has been no report showing the presence of MSCs in tendons, it has been shown that a slowcycling tendon-specific stem cell population might exist in the tendon. 58 In addition, when MSC collagen constructs were inserted into central-third defects of the rabbit patellar tendon, they produced repairs at 12 weeks that matched normal tangent stiffness up to 32% of normal failure force and 50% greater than peak in vivo forces recorded during activities of daily living. 104,105 Delivery of cultured MSCs to the injured Achilles tendon of rabbit resulted in the significant improvement of healing with larger cross-sectional area as well as better alignment of collagen fibers.…”