Odontocetes possess teeth with cementum cells overlying dentine cells. Dentin is the substance between enamel or cementum and the pulp chamber. The other major tissues are dentin, cementum, and dental pulp. The cementum and the enamel invaginate into the thick layer of dentin. It consists of the cementum, periodontal ligaments, alveolar bone, and gingiva. These cause damage to the periodontal ligament (PDL), cementum, or pre-dentine. Periodontal ligaments connect the alveolar bone to the cementum. Planing often removes some of the cementum or dentine from the tooth. They are composed of dentin and coated with a thin layer of cementum. Fibroblasts develop the periodontal ligaments which connect teeth to the alveolar bone through cementum. Acellular cementum only contains extrinsic collagen fibres. Wear-resistant, they were held together with cementum and dentin. The first cementum to be formed during tooth development is acellular extrinsic fibre cementum. These were quite wear-resistant and kept together by cementum and dentine. Two types of cementum form&colon; cellular and acellular.