Z-disk
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The Z-disk, a dark line separating adjacent I-bands in muscle cells, acts as the border of the sarcomere and is composed of connectin protein that anchors actin filaments. A sarcomere is a segment of a myofibril between two Z-disks. Synonyms for the Z-disk include Z-line, Z-band, and Z-plate. Related terms include myofibril, sarcomere, actin, and connectin.