Demonstrating collagen tendon fibril segments involvement in intrinsic tendon repair

Abstract

Severed tendons can undergo regenerative healing, intrinsic tendon repair. Fibrillogenesis of chick tendon involves "collagen fibril segments" (CFS), which are the building blocks of collagen fibers that make up tendon fascicles. The CFS are 10.5 micron in length, composed of tropocollagen monomers arranged in parallel arrays. Rather than incorporating single tropocollagen molecules into growing collagen fibers, incorporating large CFS units is the mechanism for generating collagen fibers. Is intrinsic tendon repair through the reestablishment of tendon embryogenesis? Gentamicin treated 10-day-old chick embryo tendons released CFS were fluorescently tagged with Rhodamine (Rh). Organ cultured severed 14-day-old embryo tendon explants received Rh tagged CFS. At day 4 auto fluorescent tagged CFS were identified at the severed tendon ends by fluorescent microscopy. Accumulation of fluorescent tagged CFS was exclusively localized to the severed ends of tendon explants. Parallels between collagen fiber growth during embryonic fibrillogenesis and tendon repair reveal CFS incorporation is responsible for collagen fibers growth. CFS incorporation into frayed collagen fibers from severed tendons is the proposed mechanism for intrinsic tendon repair, which is an example of regenerative repair.

Figure 1

A TEM of a fibril segment isolated from the supernatant of a long term incubation of a 10 day chick embryo tendon with 1 mg/mL gentamicin antibiotic. Released fibril segments were isolated by centrifugation and an aliquot subjected to TEM. The insert shows a higher power image with a banding pattern associated with a fibril segment.

Figure 2

Auto fluorescence micrographs from a 14 day severed tendon explant that was incubated with red fluorescent fibril segments for 4 days. Panels A and B show the accumulation of auto-fluorescence within the wound gap between organ cultured severed tendon explants. Panel C and D show DAPI blue stained fluorescence tendon fibroblast nuclei along with red auto-fluorescent fibril segments, which have amassed at the severed tendon wound gap. Note in panel D the absence of stained nuclei at the center of the wound gap, suggesting the accumulation of fibril segments on growing collagen fibers can be independent of direct association with tendon fibroblasts.