Histogenesis and Morphology of the Flexor Tendon Pulley System in the Human Embryonic Hand

The number, position, structural and ultrastructural features of the flexor tendon pulley system in six human embryonic hands, aged from 6 to 12 weeks, were studied by light and electron microscope. The pulley system can be recognized from the ninth week; later, at 12 weeks, the structures are easily identified around the flexor tendon in positions closely correlated to those found during post-natal growth and in the adult hand. Structurally and ultrastructurally the pulleys are not simply thickened portions of the sheath. They are formed by three layers: an inner layer, one or two cells thick, probably representing a parietal synovial tendon sheath; a middle layer formed by collagen bundles and fibroblasts whose direction is mainly perpendicular to the underlying phalanx; and an outermost layer consisting of mesenchymal tissue with numerous vessels which extends dorsally in an identical layer, forming a ring that includes flexor and extensor tendons and the cartilaginous model of the phalanx. The pulley does not have a semicircular shape but a much more complicated one, owing to the middle layer which in part runs dorsally and in part ventrally, under the flexor tendons.