Signals for determining the shape of the tooth are given by:
These are the densely packed transitory cluster of nondividing epithelial cells seen in the cap stage of tooth development. Enamel knots are induced by the signals from the surrounding dental mesenchyme. They are thought to be the cells from inner enamel epithelium and can be histologically seen during the embryonic day 14.0 and 14.5. These compacted areas of cells express many signaling molecules belonging to the four main families like: Bmp, Hh, Fgf, and Wnt.
Functions of enamel knot:
1. Enamel knot acts as signaling centers where many important growth factors are expressed which control the cusp morphology and placement of cusps. Hence they play a key role in morphogenesis of the tooth.
2. The cells of the enamel knot function as the reservoir of cells for the fast-growing enamel organ.
3. Further, the signaling center of the enamel knot is responsible for the formation and maintenance of the dental papilla.
Primary and secondary enamel knot:
Enamel knot which are formed during the initial stage are known as primary enamel knot which has only a limited life span and undergoes apoptosis during the cap stage. This apoptosis of primary enamel knot corresponds to the embryonic day 16.5. The primary enamel knot is seen in the incisors and molars. This primary enamel knot is necessary for the transition from bud to cap stage. Further the signals from the primary enamel knot help in the initiation of secondary enamel knots. These secondary enamel knots are seen during the early bell stage of tooth development and show the signals similar to that of primary enamel knot. Secondary enamel knot develops only in molar tooth germ and sits at the site of the future cusp. Secondary enamel knots maintain interaction with the already formed dental papilla and allow further differentiation and aids in the completion of tooth morphogenesis. Enamel knot disappears before the enamel formation begins.
It is the vertical extension of the enamel knot. It is also a temporary structure which disappears before the formation of enamel.
It is a transient structure formed when the enamel cord extends and meets the outer enamel epithelium.
It the area of depression seen at the point of meeting of outer enamel epithelium with the enamel cord.
The exact function of the enamel cord, septum and enamel navel is unknown.