LECTURE IV 08-02-2020
1 First Year BDS Session 2019-23
Subject: Oral Biology Chapter # : 03 Topic: Development and Growth of Teeth Sub Topic: Developmental (Morphological) Stages – Bell Stage Date : 08-02-2020 Presenter: Dr. Munawar Aziz Khattak
2 Learning Objective
• At the end of this lecture, the students will be able to know about the;
Developmental Stage - Bell
Early Bell Stage
Late (Advanced) Bell Stage
3 DEVELOPMENTAL STAGES
4 Early Bell Stage
5 Early Bell Stage Cont’d…
• The enamel organ assumes a bell shape • Crown shape is determined • The shape of the crown is due to the pressure exerted by the growing dental papilla cells on the inner enamel epithelium • The folding of enamel organ to cause different crown shapes is shown to be due to; Differential rates of mitosis Differences in cell differentiation time
6 Early Bell Stage Cont’d…
• The inner enamel epithelial cell which lie in the future cusp tip or incisor region stop dividing earlier and begin to differentiate first. • Cell proliferation and differentiation occurring gradually; from cusp tips to the depth of the sulcus proceeds gradually cervically • The determination of crown shape (tooth morphogenesis) is under the control of; genes and their signaling molecules growth factors
7 Early Bell Stage Cont’d…
8 ENAMEL ORGAN
Inner enamel epithelium • Consists of a single layer of cells • Differentiate prior to amelogenesis into tall columnar cells called ameloblasts • 4 to 5 micrometers (μm) in diameter and about 40 μm high • Cells attached to one another by junctional complexes laterally • Attached to cells in the stratum intermedium by desmosomes • Cells exert an organizing influence on the underlying mesenchymal cells in the dental papilla, which later differentiate into odontoblasts
9 Early Bell Stage Cont’d…
Stratum Intermedium • A few layers of squamous cells • between the inner enamel epithelium and the stellate reticulum • Cells are closely attached to one another by desmosomes and gap junctions • Cells are attached to stellate reticulum and inner enamel epithelium by desmosomes • Cells indicate a high degree of metabolic and alkaline phosphatase activity • Work as a single functional unit
10 Early Bell Stage Cont’d…
11 Early Bell Stage Cont’d…
Stellate Reticulum • Expands further, mainly by an increase in the amount of intercellular fluid. • Are star shaped, with long processes that anastomose with those of adjacent cells • Desmosomal junctions are observed with stratum intermedium and outer enamel epithelium. • Before enamel formation begins, the stellate reticulum collapses • Its cells then are hardly distinguishable from those of the stratum intermedium. • This change begins at the height of the cusp or the incisal edge and progresses cervically
12 Early Bell Stage Cont’d… Outer enamel epithelium • The cells flatten to a low cuboidal form • The formerly smooth surface of the outer enamel epithelium is laid in folds. • Between the folds the adjacent mesenchyme of the dental sac forms papillae that contain capillary loops
• Is seen to extend lingually • The enamel organs of deciduous teeth in the bell stage show successional lamina and their permanent successor teeth in the bud stage
14 Early Bell Stage Cont’d…
15 DENTAL PAPILLA
• Is enclosed in the invaginated portion of the enamel organ • The peripheral cells of the mesenchymal dental papilla differentiate into odontoblasts First, they assume a cuboidal form Later they assume a columnar form and acquire the specific potential to produce dentin Basement Membrane • The basement membrane that separates the enamel organ and the dental papilla just prior to dentin formation is called the membrana preformativa.
16 DENTAL SAC
• Before formation of dental tissues begins; the dental sac shows a circular arrangement of its fibers and resembles a capsular structure • With the development of the root; the fibers of the dental sac differentiate into the periodontal fibers that become embedded in the developing cementum and alveolar bone
17 Cont’d…
18 Cont’d…
19 Late/ Advanced Bell Stage • Is characterized by the commencement of mineralization and root formation • The boundary between inner enamel epithelium and odontoblasts outlines the future dentinoenamel junction (DEJ) • The formation of dentin occurs first as a layer along the future dentinoenamel junction in the region of future cusps and proceeds pulpally and apically • After the first layer of dentin is formed, the ameloblast lay down enamel over the dentin in the future incisal and cuspal areas proceeds coronally and cervically
20 Early Bell Stage Cont’d…
• In addition, the cervical portion of the enamel organ called the cervical loop gives rise to the epithelial root sheath of Hertwig The junction between inner and outer enamel epithelium is called cervical loop and it is an area of intense mitotic activity • The Hertwig’s epithelial root sheath (HERS) outlines the future root shape, length, size, and number of roots.
21 Cont’d…
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