The fine structure of human Golgi tendon organs as studied by three-dimensional reconstruction.

Human Golgi tendon organs (GTOs) found at the musculo-tendinous junctions of lumbrical muscles were studied using serial sections for light and electron microscopy as the basis for a three-dimensional reconstruction. These GTOs had a thick capsule and were filled with longitudinally arranged collagen bundles. The GTOs were divided into three small compartments by septal cells: the 'neuronal compartment' containing myelinated nerve fibres, the 'terminal compartment' having axon terminals, and the 'fibrous compartment' containing only collagen fibrils. The three-dimensional reconstruction demonstrated that myelinated fibres rotated spirally before losing their myelin sheaths, and ended as unmyelinated axon terminals in the 'terminal compartment'. Axons abruptly changed course at the beginning of the terminals. Axon terminals extended many thin processes between the collagen fibrils, partially encircling them. Part of the axolemma of such indentations lacked Schwann cell coverings. The intimate contact of collagen fibrils with the axon terminals, especially in the indentations of the axons, are the presumptive site that could transmit shearing stresses to the axolemma, especially in areas devoid of Schwann cell cytoplasm.