The development of Golgi tendon organs.

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The fine structural development of Golgi tendon organs (GTOs) was studied in leg muscles of the rat from day 18 gestation until 2 months after birth. Small axons were found at the aponeuroses on day 18, but GTOs were first identified on day 20–21 gestation. Prenatally, they appear as discrete islets in the aponeurosis in which a single Ib axon branches and terminates among fibroblasts, collagen bundles and on myotubes inserting into the tendinous tissue. The receptor body is formed during week 1 postnatal as a result of several concurrent processes: (1) New terminals arise and form numerous contacts with 5–9 myotubes inserting into the GTO area. The innervated myotube tips gradually recede from the aponeurosis. (2) Schwann cells and fibroblasts proliferate and form longitudinal strands adjoining the receding myotubes and converging towards the attachment to the aponeurosis. (3) Axonal branches ensheathed by Schwann cells form terminal enlargements and small-sized terminals. Terminal enlargements are predominantly filled with mitochondria, whereas their peripheral projections and small-sized terminals mainly contain clear and dense core vesicles. (4) Concomitantly collagen fibrils assemble between Schwann cell processes and the developing terminals, and interconnect with collagen bundles, linking the muscle fibre tips to the aponeurosis. Thus collagen bundles of the GTO, which eventually spiral around the terminals and bring about their depolarization when stretched, are formed concurrently with the developing terminals. (5) The GTO becomes encapsulated from day 2 postnatal onwards. (6) Axon terminals loosen their contacts with muscle fibres and are only found among collagen bundles of the receptor from day 5 postnatal onwards. (7) Muscle-fibre tips recede from the GTO lumen by day 7 after birth. Subsequently, the GTO grows further in length, acquires its fusiform shape by tightening of the capsule at both poles, and becomes structurally mature 2–3 weeks after birth. The Ib sensory axon together with Schwann cells appear to be the main formative agents in the GTO development, the Ib terminals presumably inducing the retraction of myotubes from the aponeurosis, and the Schwann cells affecting the assembly of collagen fibrils and bundles surrounding the terminals.