Talk about the periosteum.

Covers outer surface of the bone,, in adults has a thin outer layer of dense CT (tediniform or fibrous layer) and a predominantly cellular and well vascularized inner layer (osteogenic fibro-elastic layer) of loose CT, containing stem cells, there are bundles of non-oriented collagen fibers penetrating deep into bone tissue, Sharpey's fibers that attach periosteum to the bone.