NJDS: Histology - Amelogenesis and Dentinogenesis

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How are enamel and dentin formed?: Reciprocal induction
DENTINOGENESIS The odontoblast: -dentin formed by odontoblasts derived from undifferentiated cells of the dental papilla in response to growth factors from the internal dental epithelium

-cessation of mitotic activity of the IDE --> change in polarity --> cells of superficial dental papilla become preodontoblasts and then odontoblasts

-have a nucleus polarized away from the IDE, increased cytoplasm that obliterates the acellular zone between the IDE and dental papilla
2 functional states of the odontoblast: 1) SECRETORY
-abundant cytoplasm, large basally located nucleus
-cytoplasm taken up by RER, Golgi and secretory granules
-junctional complexes, desmosomes connect odontoblasts

2) RESTING
DENTINOGENESIS Dentin formation - Mantle dentin: -first dentin formed

-organic matrix consists of ground substance and type I collagen deposited; several elongated apical processes emerge and matrix vesicles extruded from odontoblast cytoplasm (predentin deposited from neck area)

-as matrix deposition continues, a single large odontoblast process remains behind and hydroxyapatite crystals are deposited --> crystals increase in size and mineralize

-unmineralized (most recent) dentin matrix is predentin

**the odontoblast retreats towards the dental pulp, leaving the odontoblast process w/i newly formed dentinal tubule -- in coronal dentin, tubules form S shape, in root are straight
DENTINOGENESIS Dentin formation - Primary (circumpulpal) dentin, Secondary dentin, Tertiary (reparative) dentin: PRIMARY (CIRCUMPULPAL) DENTIN
-constitutes dentin that encircles the pulp chamber

-formed prior to completion of the apical foramen

**majority of dentin in tooth is primary**

SECONDARY DENTIN
-forms after establishment of apical foramen

-decreases root chamber and pulp

TERTIARY (REPARATIVE) DENTIN
-localized dentin in reponse to irritation or trauma

-abnormal, disorganized, rapidly produced dentin
DENTINOGENESIS Mineralization of Dentin: -shows a pattern of globular calcification
-small, isolated crystalline nuclei continue to grow radially and eventually coalesce with neighboring calcosperites

-interglobular dentin--> intervening uncalcified dentin adjacent to mantle dentin in areas of incomplete fusion
Amelogenesis: -the process of enamel formation that begins at the early crown stage of tooth development

-presence of predentin is necessary for amelogenesis to occur

-involves initial stage of deposition of partially mineralized enamel, then undergoes a subsequent stage of water and organic matrix removal and increased mineralization

-enamel derived from internal dental epithelium and assited by the stratum intermedium and stallate reticulum

-initiated at cusp tips and proceeds laterally along cusp inclines
AMELOGENESIS Life Cycle of the Ameloblast - Overview: -changes occur in the cells of the IDE beginning at the cusp tips down towards the cervical loop --> secretory function

-cells of IDE become tall columnar cells, nucleus is polarized towards the stellate reticulum
-at this point, superficial dental papilla cells differentiate into odontoblasts which secrete predentin-->enamel matrix deposition by ameloblasts

-ameloblasts migrate away from DEJ-->enamel matrix mineralizes and ameloblasts develop conical cellular projection--TOMES' PROCESS

-enamel organ then collapses until only several layers of cells on top of layer of post-secretory ameloblasts--reduced enamel epithelium
AMELOGENESIS Developmental Stages of the Ameloblast - Morphogenetic Stage: -the IDE cells comprise a cuboidal or low columnar epithelium with round, central nucleus and Golgi body toward stratum intermedium
AMELOGENESIS Developmental Stages of the Ameloblast - Differentiation Stage: -cells of IDE elongate, increase size in nucleus and Golgi; shift position so, nucleus towards stratum intermedium and Golgi encompasses much of central and apical cytoplasm

-junctional complexes at proximal and distal ends

**NOW CONSIDERED PRE-AMELOBLASTS**
AMELOGENESIS Developmental Stages of the Ameloblast - Secretory Stage: -enamel matrix produced and deposited against mantle dentin, immediately mineralizes -- structureless

-ameloblast migrates towards stellate reticulum forming the TOMES' PROCESS

-enamel matrix secreted from:
1) periphery of cell -- forms an encircling wall around a pit where Tomes' process fits
2) one surface of Tomes' process -- secretes enamel matrix into pit
AMELOGENESIS Developmental Stages of the Ameloblast - Maturation Stage: -occurs after enamel production is completeted

-ameloblast decreases in size and organelle content; water and organic components removed; inorganic material added to matrix
AMELOGENESIS Developmental Stages of the Ameloblast - Protective Stage: -closing of maturation

-ameloblasts secrete basal lamina against enamel surface and establish hemidesmosomes

-ameloblasts protect the enamel from cellular influences of the follicular connective tissue
AMELOGENESIS Maturation of Enamel: -early enamel organic matrix comprised mostly of low molecular weight proteins -- AMELOGENINS

-no matrix vesicles involved (differs from dentin)
-instead, crystallization begins w/i enamel matrix

-hydroxyapatite crystals from dentin at DEJ initiate process by serving as nuclei

-enamel proteins and water are removed and calcium is added as matrix matures

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