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 patt erns 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 odon toblasts along sites of dentinogenesis, with numerous cells and intens e staining near the pulp horn tip, and fewer cells and less staining i n mid-crown and cervical pulp. This gradient was well established in f irst molars of the one day old rats; it was first seen in second molar s 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 ha ve 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 e ach pulp horn closest to the gingival surface, i.e. anterior for maxil lary crowns and posterior for mandibular crowns; and the Fos/Fra-IR fi rst appeared next to those asymmetric sites. As the wave of dentinogen esis spread around the crown, it was accompanied by odontoblastic expr ession of Fos/Fra-IR that had decreasing intensity in mid- and cervica l 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 transie nt Fos/Fra, and it established a similar gradient that was most dense near the crown tips on anterior sides of maxillary molars and posterio r sides of mandibular molars. The non-secretory pre-ameloblast epithel ium at the molar enamel-free cusp tips also had prolonged Fos/Fra-IR d uring crown morphogenesis at the enamel-free cusp tips. Sensory innerv ation was concentrated there prior to eruption, and junctional epithel ium appeared to originate from those cells during eruption. The Fos/Fr a-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.