Which of the following calcified tissues have minimum lag phase between matrix formation and mineralization?

  # Which of the following calcified tissues have minimum lag phase between matrix formation and mineralization?

A. Enamel

B. Cementum

C. Bone 

D. Dentin

The correct answer is A. Enamel.

The formation of the enamel can be described as the secretory phase (primary mineralization or matrix formation) where only 30 percent is mineralized and the secondary phase or maturation characterized by the crystal growth and complete mineralization. There is no time lag between these phases and hence the mineralization of enamel is a continuous process. This is the reason why structure equivalent to that of predentin or osteoid as seen in dentin and bone is not found in enamel. 

The way mineral is introduced into the organic matrix of enamel differs from its introduction in other hard tissues. In these tissues matrix vesicles provide a closed environment to initiate crystal formation in a preformed organic matrix. No matrix vesicles are found in association with the initial mineralization of enamel protein. 



Non keratinized epithelium is found over:

  # Non keratinized epithelium is found over:

A. Attached gingiva

B. Free gingiva

C. Interdental papilla

D. Gingival sulcus

The correct answer is D. Gingival sulcus.

The gingival tissue is made up of 75% Para keratinized epithelium, about 15 % keratinized and only 10% non keratinized epithelium.

The keratinized area of the gingiva includes the attached gingiva and some portions of the outer gingival epithelium. 

The para keratinized gingiva includes the marginal gingiva and portions of the outer gingival epithelium.

The non keratinized portion of gingiva includes the gingival col, junctional epithelium and sulcular epithelium.



The gene that defines the localization of the tooth germs is:

  # The gene that defines the localization of the tooth germs is:

A. FGF-8

B. BMP-2

C. Dlx-5

D. Pax-9

The correct answer is D. Pax-9.

During tooth development, several mesenchymal molecules and their receptors act as mediators of epithelial - mesenchymal interactions. Bone morphogenetic proteins (BMPs) 2,4,7 mRNAs shift between epithelium and mesenchyme during regulation of tooth morphogenesis. Fibroblast growth factor (FGF8, FGF9) have been localized in the dental mesenchyme and act via downstream factors MSX1 and PAX9. Muscle specific homeobox genes MSX-1 and Msx-2, appear to be involved in epithelial mesencymal interactions and craniofacial development, particularly in the initiation of developmental position (Msx-1) and further development (Msx-2) of tooth buds.Pax-9 is transcription factor necessary for tooth morphogenesis. 



Salivary gland aplasia is seen in:

 Salivary gland aplasia is seen in:

A. Hemifacial microsomia

B. LADD syndrome

C. Mandibulofacial dysostosis (Treacher Collins)

D. All of the above

The correct answer is D. All of the above.

Aplasia occurs for unknown reasons as an isolated finding or in conjunction with other developmental defects such as hemifacial microsomia, the LADD syndrome and mandibulo-facial dysostosis (Treacher Collins). In the more severe cases, the ensuing xerostomia causes clinical problems. Salivary loss leads to increased caries, burning sensations, oral infections, taste aberrations and difficulty with denture retention.