the outside of the tooth and proceeds inward. cytoplasmic extensions are left behind as the odontoblasts move inward. the unique, tubular microscopic appearance of dentin is a result of the formation of dentin around these extensions. after dentin formation begins, the cells of the iee secrete an organic matrix against the dentin. this matrix immediately mineralizes and becomes the initial layer of the tooth's enamel. outside the dentin are the newly formed ameloblasts in response to the formation of dentin, which are cells that continue the process of enamel formation; therefore, enamel formation moves outwards, adding new material to the outer surface of the developing tooth. hard tissue formation sections of tooth undergoing development. enamel main article: amelogenesis enamel formation is called amelogenesis and occurs in the crown stage (advanced bell stage) of tooth development. "reciprocal induction" governs the relationship between the formation of dentin and enamel; dentin formation must always occur before enamel formation. generally, enamel formation occurs in two stages: the secretory and maturation stages. proteins and an organic matrix form a partially mineralized enamel in the secretory stage; the maturation stage completes enamel mineralization. in the secretory stage, ameloblasts release enamel proteins that contribute to the enamel matrix, which is then partially mineralized by the enzyme alkaline phosphatase. this mineralized phase occurs very early around the 3rd or 4th month of pregnancy. this marks the first appearance of enamel in the body. ameloblasts make enamel at the location of where the cusps of the teeth are located. enamel grows outwards, away from the center of the tooth. in the maturation stage, the ameloblasts transport some of the substances used in enamel formation out of the enamel. thus, the function of ameloblasts changes from enamel production, as occurs in the secretory stage, to transportation of substances. most of the materials transported by ameloblasts in this stage are proteins used to complete mineralization. the important proteins involved are amelogenins, ameloblastins, enamelins, and tuftelins. by the end of this stage, the enamel has completed its mineralization. a residue may form on newly erupted teeth of both dentitions that may leave the teeth extrinsically stained. this green-gray residue, nasmyth membrane, consists of the fused tissue of the reduced enamel epithelium and oral epithelium, as well as the dental cuticle placed by the ameloblasts on the newly formed outer enamel surface. nasmyth membrane then easily picks up stain from food debris and is hard to remove except by selective polishing. the child's supervising adults may need reassurance that it is only an extrinsic stain on a child's newly erupted teeth. patients with osteopetrosis display enamel abnormalities, suggesting that the a3 gene mutation found in v-atpases also plays a role in the development of hypomineralized and hypoplastic enamel. dentin main article: dentinogenesis dentin formation, known as dentinogenesis, is the first identifiable feature in the crown stage of tooth development. the formation of dentin must always occur before the formation of enamel. the different stages of dentin formation result in different types of dentin: mantle dentin, primary dentin, secondary dentin, and tertiary dentin. odontoblasts, the dentin-forming cells, differentiate from cells of the dental papilla. they begin secreting an organic matrix around the area directly adjacent to the inner enamel epithelium, closest to the area of the future cusp of a tooth. the organic matrix contains collagen fibers with large diameters (0.1–0.2 μm in diameter). the odontoblasts begin to move toward the center of the tooth, forming an extension called the odontoblast process. thus, dentin formation proceeds toward the inside of the tooth. the odontoblast process causes the secretion of hydroxyapatite crystals and mineralization of the matrix. this area of mineralization is known as mantle dentin and is a layer usually about 150 μm thick. whereas mantle dentin forms from the preexisting ground substance of the dental papilla, primary dentin forms through a different process. odontoblasts increase in size, eliminating the availability of any extracellular resources to contribute to an organic matrix for mineralization. additionally, the larger odontoblasts cause collagen to be secreted in smaller amounts, which results in more tightly arranged, heterogeneous nucleation that is used for mineralization. other materials (such as lipids, phosphoproteins, and phospholipids) are also secreted. secondary dentin is formed after root formation is finished and occurs at a much slower rate. it is not formed at a uniform rate along the tooth, but instead forms faster along sections closer to the crown of a tooth. this development continues throughout life and accounts for the smaller areas of pulp found in older individuals. tertiary dentin, also known as reparative dentin, forms in reaction to stimuli, such as attrition or dental caries. cross-section of tooth at root. note clear, acellular appearance of cementum.a: dentinb: cementum cementum cementum formation is called cementogenesis and occurs late in the development of teeth. cementoblasts are the cells responsible for cementogenesis. two types of cementum form: cellular and acellular. acellular cementum forms first. the cementoblasts differentiate from follicular cells, which can only reach the surface of the tooth's root once hertwig's epithelial root sheath (hers) has begun to deteriorate. the cementoblasts secrete fine collagen fibrils along the root surface at right angles before migrating away from the tooth. as the cementoblasts move, more collagen is deposited to lengthen and thicken the bundles of fibers. noncollagenous proteins, such as bone sialoprotein and osteocalcin, are also secreted. acellular cementum contains a secreted matrix of proteins and fibers. as mineralization takes place, the cementoblasts move away from the cementum, and the fibers left along the surface eventually join the forming periodontal ligaments. cellular cementum develops after most of the tooth formation is complete and after the tooth occludes (in contact) with a tooth in the opposite arch. this type of cementum forms around the fiber bundles of the periodontal ligaments. the cementoblasts forming cellular cementum become trapped in the cementum they produce. the origin of the formative cementoblasts is believed to be different for cellular cementum and acellular cementum. one of the major current hypotheses is that cells producing cellular cementum migrate from the adjacent area of bone, while cells producing acellular cementum arise from the dental follicle. nonetheless, it is known that cellular cementum is usually not found in teeth with one root. in premolars and molars, cellular cementum is found only in the part of the root closest to the apex and in interradicular areas between multiple roots. histologic slide of tooth erupting into the mouth. a: tooth b: gingiva c: bone d: periodontal ligaments formation of the periodontium the periodontium, which is the supporting structure of a tooth, consists of the cementum, periodontal ligaments, gingiva, and alveolar bone. cementum is the only one of these that is a part of a tooth. alveolar bone surrounds the roots of teeth to provide support and creates what is commonly called a "socket". periodontal ligaments connect the alveolar bone to the cementum, and the gingiva is the surrounding tissue visible in the mouth. periodontal ligament cells from the dental follicle give rise to the periodontal ligament (pdl). specific events leading to the formation of the periodontal ligament vary between deciduous (baby) and permanent teeth and among various species of animals. nonetheless, formation of the periodontal ligament begins with ligament fibroblasts from the dental follicle. these fibroblasts secrete collagen, which interacts with fibers on the surfaces of adjacent bone and cementum. this interaction leads to an attachment that develops as the tooth erupts into the mouth. the occlusion, which is the arrangement of teeth and how teeth in opposite arches come in contact with one another, continually affects the formation of periodontal ligament. this perpetual creation of periodontal ligament leads to the formation of groups of fibers in different orientations, such as horizontal and oblique fibers. alveolar bone as root and cementum formation begin, bone is created in the adjacent area. throughout the body, cells that form bone are called osteoblasts. in the case of alveolar bone, these osteoblast cells form from the dental follicle. similar to the formation of primary cementum, collagen fibers are created on the surface nearest the tooth, and they remain there until attaching to periodontal ligaments. like any other bone in the human body, alveolar bone is modified throughout life. osteoblasts create bone and osteoclasts destroy it, especially if force is placed on a tooth. as is the case when movement of teeth is attempted through orthodontics using bands, wires, or appliances, an area of bone under compressive force from a tooth moving toward it has a high osteoclast level, resulting in bone resorption. an area of bone receiving tension from periodontal ligaments attached to a tooth moving away from it has a high number of osteoblasts, resulting in bone formation. thus, the tooth or teeth are slowly moved along the jaw so as to achieve a dentition that works in harmony. in this way, the width of the space between the alveoli and the root is kept about the same. gingiva the connection between the gingiva and the tooth is called the dentogingival junction. this junction has three epithelial types: gingival, sulcular, and junctional epithelium. these three types form from a mass of epithelial cells known as the epithelial cuff between the tooth and the mouth. much about gingival formation is not fully understood, but it is known that hemidesmosomes form between the gingival epithelium and the tooth and are responsible for the primary epithelial attachment. hemidesmosomes provide anchorage between cells through small filament-like structures provided by the remnants of ameloblasts. once this occurs, junctional epithelium forms from reduced enamel epithelium, one of the products of the enamel organ, and divides rapidly. this results in the perpetually increasing size of the junctional epithelial layer and the isolation of the remnants of ameloblasts from any source of nutrition. as the ameloblasts degenerate, a gingival sulcus is created. nerve and vascular formation frequently, nerves and blood vessels run parallel to each other in the body, and the formation of both usually takes place simultaneously and in a similar fashion. however, this is not the case for nerves and blood vessels around the tooth, because of different rates of development. nerve formation nerve fibers start to near the tooth during the cap stage of tooth development and grow toward the dental follicle. once there, the nerves develop around the tooth bud and enter the dental papilla when dentin formation has begun. nerves never proliferate into the enamel organ. vascular formation blood vessels grow in the dental follicle and enter the dental papilla in the cap stage. groups of blood vessels form at the entrance of the dental papilla. the number of blood vessels reaches a maximum at the beginning of the crown stage, and the dental papilla eventually forms in the pulp of a tooth. throughout life, the amount of pulpal tissue in a tooth decreases, which means that the blood supply to the tooth decreases with age. the enamel organ is devoid of blood vessels because of its epithelial origin, and the mineralized tissues of enamel and dentin do not need nutrients from the blood. tooth eruption main article: tooth eruption tooth eruption occurs when the teeth enter the mouth and become visible. although researchers agree that tooth eruption is a complex process, there is little agreement on the identity of the mechanism that controls eruption. some commonly held theories that have been disproven over time include: (1) the tooth is pushed upward into the mouth by the growth of the tooth's root, (2) the tooth is pushed upward by the growth of the bone around the tooth, (3) the tooth is pushed upward by vascular pressure, and (4) the tooth is pushed upward by the cushioned hammock. the cushioned hammock theory, first proposed by harry sicher, was taught widely from the 1930s to the 1950s. this theory postulated that a ligament below a tooth, which sicher observed under a microscope on a histologic slide, was responsible for eruption. later, the "ligament" sicher observed was determined to be merely an artifact created in the process of preparing the slide. the most widely held current theory is that while several forces might be involved in eruption, the periodontal ligaments provide the main impetus for the process. theorists hypothesize that the periodontal ligaments promote eruption through the shrinking and cross-linking of their collagen fibers and the contraction