part of part of hair outer root sheath integumentary system layer outer root sheath companion layer The innermost layer of the outer root sheath. BTO:0003970 UBERON:0035609 It contains cells that are morphologically and biochemically different from the inner root sheath and outer root sheath. The cells of the companion layer are more flattened than the other cells of the outer root sheath and do not contain glycogen. In addition, a novel human type II cytokeratin, K6hf, is specifically expressed in the companion layer of the hair follicle