expression of the bone morphogenetic protein-6 in the dental follicle may also be necessary. It is also thought that signalling between the dental follicle and the reduced enamel epithelium exists. This signalling could be a plausible reason for the noteworthy regularity of eruption timings because the enamel epithelium is most possibly programmed as part of its functional life cycle. Signalling would also aid in explaining why radicular follicle, that is not related to reduced enamel epithelium, is involved in the formation of the periodontal ligament but does not experience degeneration. Periodontal ligament Cells of dental follicle will differentiate into collagen forming fibroblast of cementoblasts and periodontal ligaments, that produces and secretes cementum on the surface of the tooth roots. As the tooth roots break apart, some cells of the dental follicle will invade the roots of the tooth. Delicate fibres that appear along the developing roots near the cervical region of the crown will also be formed by some cells of the ligament. These are most likely the stem cell fibroblasts that form principle fibre groups that will surface as the roots lengthen. As fibres become implanted in the cementum of the root surface, the other end will be attached to the forming alveolar bone. Rejuvenation and development of the periodontal ligament have been taken as a factor in the eruption of the tooth due to the traction power possessed by fibroblasts and because of experimental results relying on the unceasingly erupting rat incisor. The case is not the same where the existence of a periodontal ligament does not always correspond with resorption in teeth that have limited growth period. Cases do however happen in which rootless teeth erupt and when a periodontal ligament is present, and the tooth does not erupt. One significant difference in the formation of fibres exists between teeth that have predecessors and teeth that do not have predecessors. For the former group of teeth (such as the permanent incisors, the canines and the premolars), the principal fibres group will develop later than in the latter group of teeth (such as the deciduous teeth and the permanent molars). It can be observed that the coronal half of the periodontal ligament is made up of well composed, obliquely orientated principal collagen fibre bundles when an erupting permanent molar enters into the oral cavity. The opposite is true too. A majority of the periodontal ligament of an erupting