FOCAL C-FOS EXPRESSION IN DEVELOPING RAT MOLARS - CORRELATIONS WITH SUBSEQUENT INTRADENTAL AND EPITHELIAL SENSORY INNERVATION

The purpose of this study was to analyze the temporal and spatial patterns of expression of the inducible transcription protein Fos and the Fos-related antigens (Fra) in developing rat teeth. Immunoreactivity (IR) for Fos/Fra was analyzed at postnatal ages 1-35 days. A transient gradient of Fos/Fra-IR was found in all the molars in the coronal odontoblasts along sites of dentinogenesis, with numerous cells and intense staining near the pulp horn tip, and fewer cells and less staining in mid-crown and cervical pulp. This gradient was well established in first molars of the one day old rats; it was first seen in second molars in the 2 day old rats; and it was found in third molars at 10 days. The Fos/Fra-IR was transient and faded after a few days. Rat molars have a tilted orientation so that maxillary molar crown cusps point in a posterior direction and mandibular crowns in the anterior direction. In each set of molars dentinogenesis was initiated along the side of each pulp horn closest to the gingival surface, i.e. anterior for maxillary crowns and posterior for mandibular crowns; and the Fos/Fra-IR first appeared next to those asymmetric sites. As the wave of dentinogenesis spread around the crown, it was accompanied by odontoblastic expression of Fos/Fra-IR that had decreasing intensity in mid- and cervical crown. Molar root pulp lacked Fos/Fra-IR, and incisor teeth only had odontoblastic and ameloblastic immunoreactivity in the 1 day old rats. Dentinal innervation developed in molars two weeks after the transient Fos/Fra, and it established a similar gradient that was most dense near the crown tips on anterior sides of maxillary molars and posterior sides of mandibular molars. The non-secretory pre-ameloblast epithelium at the molar enamel-free cusp tips also had prolonged Fos/Fra-IR during crown morphogenesis at the enamel-free cusp tips. Sensory innervation was concentrated there prior to eruption, and junctional epithelium appeared to originate from those cells during eruption. The Fos/Fra-IR in developing rat molar odontoblasts and ameloblasts had distinct sites of prolonged expression during crown morphogenesis that did not appear to involve apoptosis, and that matched later sites of sensory innervation.