tooth development 2.pdf

Development of teeth  Stages of Tooth Development: initiation stage – 6th to 7th week bud stage – 8th week cap stage – 9th to 10th weeks bell stage – 11th to 12th weeks apposition stage (dentinogenesis and amelogenesis) – vaires per tooth maturation stage – varies pe...

Development of teeth  Stages of Tooth Development: initiation stage – 6th to 7th week bud stage – 8th week cap stage – 9th to 10th weeks bell stage – 11th to 12th weeks apposition stage (dentinogenesis and amelogenesis) – vaires per tooth maturation stage – varies per tooth Human teeth: 20 primary and32 permanent teeth develop from the interaction of the oral epithelial cells and underlying mesenchymal cells each tooth is a anatomic distinct unit but basic developmental processes are similar for all teeth Initiation of tooth development: epithelial cells form enamel organ, mesenchymal cells form dental papila – develops to dental pulp. At the 6th week the stomatodeum is lined with ectoderm – outer portion is the oral epithelium. Interaction of these epithelial and mesenchymal cells is vital to the initiation and formation of the teeth. Mechanisms remain unknown. Primary epithelial bands: Horseshoe-shaped bands that appear approximately around the 37th day of development, one for each jaw. -there are two subdivisions: vestibular lamina and dental lamina -the dental lamina grows deep into the mesenchyme -develops in the future spot for the dental arches -the ingrowths represent the future sites for each deciduous tooth -the vestibular lamina cells rapidly enlarge and then degenerate – forms a cleft that becomes the vestibule of the oral cavity The initiation of tooth formation starts around the 37th day of gestation.  Bud Stage marked by the incursion of epithelium into the mesenchyme period of extensive proliferation and growth of the dental lamina each tooth bud is surrounded by the mesenchyme buds + mesenchyme develop into the tooth germ and the associated tissues of the tooth 1. Tooth bud 2. Oral epithelium 3. Mesenchyme  Cap Stage characterized by continuation of the ingrowth of the oral epithelium into the mesenchyme. tooth bud of the dental lamina proliferates unequally in different parts of the bud – forms a cap shaped tissue attached to the remaining dental lamina this stage marks the beginning of histodifferentiation (differentiation of tissues) the tooth germ also begins to take on form – start of morphodifferentiation a depression forms in the deepest part of each tooth bud and forms the cap or enamel organ – produces the future enamel (ectodermal origin) below this cap is a condensing mass of mesenchyme – dental papilla – produces the future dentin and pulp tissue (mesenchymal origin) the basement membrane separating the enamel organ and the dental papilla becomes the future site for the dentinoenamel junction (DEJ) remaining mesenchyme surrounds the dental/enamel organ and condenses to form the dental sac or the dental follicle  Cap Stage together the enamel organ + dental papilla + dental follicle is considered the developing tooth germ these germs are found in the developing dental arches and will develop into the primary dentition Enamel organ Dental papilla Dental follicle  Cap Stage ©Copyright 2007, Thomas G. Hollinger, Gainesville, Fl  Bell Stage Continuation of histodifferentiation and morphodifferentiation cap shape then assumes a more bell-like shape differentiation produces four types of cells within the enamel organ – 1. inner enamel epithelium – 2. outer enamel epithelium – 3. stellate reticulum – 4. stratum intermedium the dental papilla undergoes differentiation and produces two types of cells: 1. outer cells of the DP – forms the dentin- secreting cells (odontoblasts) 2. central cells of the DP – forms the primordium of the pulp dental sac/follicle increases its collagen content and differentiates at a later stage than the EO and DP  Differentiation of the Enamel organ outer enamel epithelium (OEE) – cuboidal shape – protective barrier during enamel production – may also be called the outer dental epithelium inner enamel epithelium (IEE) – short, columnar cells – differentiates into the enamel secreting cells = OEE ameloblasts – separated from the dental papilla below it by a basement membrane – may also be called the inner dental epithelium – the IEE and OEE are continuous – region where they connect – curved rim of the EO = cervical loop stellate reticulum – star-shaped cells in many layers – center of the enamel organ IEE – forms a network = reticulum – supports production of enamel cervical loop stratum intermedium – inner layer of compressed flat to cuboidal cells – supports production of enamel  Oral cavity Outer dental epithelium Dental lamina Enamel knob Stellate reticulum Stratum intermedium Inner dental Epithelium Dental papilla Structures Seen at Bell Stage  Bell stage – early crown formation the dental papilla is separated from the enamel organ by a basement membrane immediately below this BM is a region called the acellular zone – this is where the first enamel proteins will be laid down the dental lamina begins to break up into discrete islands of epithelial cells (epithelial pearls)– separates the oral epithelium from the developing tooth  Cap and Bell stages & Permanent teeth during the cap stage the development of the permanent dentition begins the primordia for these teeth appears as an extension off the developing dental lamina its site of origin is called the successional dental lamina Tooth development  Ameloblasts and Odontoblasts ameloblasts – the cells of the IEE – columnar shape – differentiate into pre-ameloblasts – the pre-ABs induce the cells of the dental papilla to differentiate also odontoblasts – differentiation by the mesenchyme of the dental papilla – occurs after differentiation of pre-ABs begins – results because the pre-ABs induce differentiation of the mesenchymal cells after differentiation – the ODs start dentinogenesis  Dentinoenamel junction after OD differentiation and the initiation of dentinogenesis – the BM between the pre-ABs and ODs disintegrates this allows direct contact between the pre- ABs and ODs – results in the completion of pre-AB differentiation to mature ABs ABs then begin amelogenesis – apposition of enamel matrix upon contact of the enamel matrix and dentin – the disintegrating BM begins to mineralize – forms the dentinoenamel junction (DEJ) mineralization of the developing dentin and enamel is distinct for each type of tissue  Appositional stage secretion of enamel, dentin and cementum these tissues are initially secreted as a matrix that is partially calcified – serves as a framework for later calcification Dentinogenesis: when the odontoblasts become columnar cells they form the matrix of collagen fibers – predentin after 24 h matrix calcifies and become dentin calcification of the matrix: deposition of hydroxyapatite crystals in matrix, growing and spreading of the crystals until matrix is calcified, Dentinogenesis  Amelogenesis ameloblast begin enamel deposition after a few micrometers of dentin have been deposited at the dentinoenamel junction. first enamel deposited on the surface of a dentin establishes the dentinoenamel junction substances needed to enamel production arrive via the blood vessels and pass through the stellate reticulum and stratum intermedium cells and ameloblasts when amelogenesis is completed and amelogenin is deposited matrix begins to mineralize – small crystals are deposited and begin to grow in length and diameter  ROOT FORMATION takes place as the crown is completely shaped and the tooth begins to erupt root formation is through the formation of a cervical loop the CL is the most cervical portion of the enamel organ – two layers consisting of IEE and OEE the CL begins to grow down into the dental sac it forms a Hertwig's root sheath this sheath shapes the root and induces dentin formation in the root area by the ODs of the dental papilla this sheath lacks the stellate reticulum and stratum intermedium is capable of differentiating into ODs BUT NOT ABs  Root Formation Cervical loop forms the epithelial root sheath (of Hertwig) Cervical loop Dental papilla ©Copyright 2007, Thomas G. Hollinger, Gainesville, Fl  Root Dentin The root of the tooth is composed by dentin and cementum dentin forms when the outer cells of the dental papilla are induced to differentiation into ODs similar to what occurs at the crown area influenced by Hertwig’s root sheath the ODs then undergo dentinogenesis and secrete predentin after dentin formation – the BM disintegrates along with the Hertwig’s sheath  Cementum and Pulp formation cementogenesis in the root area also occurs upon degradation of the H. root sheath the degradation allows contact of the dental sac cells with the dentin surface – induces the formation of cementoblast cells the CBs cover the root dentin and undergo cementogenesis – laying down cementoid only upon mineralization of the cementoid can it be called cementum- tissue similar to the bone the region of contact between cementum and root dentin = dentinocemental junction (DCJ) acellular cementum creates thin layer on the dentin cellular cementum with cementocytes while the cementum is forming - the central cells of the dental papilla form the pulp Cellular Cementum Acellular cementum Cellular cementum Hyaline layer (of Hopewell Smith) Granular layer of Tomes Dentin (with tubules)  Timetable for tooth development Entire primary dentition initiated between 6 and 8 weeks of embryonic development. Successional permanent teeth initiated between 20th week in utero and 10th month after birth permanent molars between 20th week in utero (first molar) and 5th year of life (third molar)  Periodontal ligament and alveolar bone the surrounding tissues of the tooth also develop as the crown and root form the mesenchyme of the dental sac condenses to form the periodontal ligament forms adjacent to the new cementum ends of these fibers insert into the outer layer of cementum and surrounding alveolar bone the cells of the disintegrating H. root sheath develop into discrete islands of epithelial cells become epithelial rests of Malassez no known function they can be identified in the periodontal ligament and are responsible for the development of radicular cysts. As the teeth develop, so does the alveolar bone, which keeps pace with the lenghthening roots Alveolar bone is composed of alveolar bone proper, which lines the socket, and supporting bone, which consists of spongy or cancellous compact bone.