Apoptosis in the early involuting stellate reticulum of rat molar tooth germs

When the enamel organ of the rat tooth germ is fully developed at the tip o f the prospective cusp, amelogenesis begins, and at this site the overlayin g stellate reticulum begins its involution. During the involution process, there is a gradual decrease in intercellular spaces, invasion by blood vess els, appearance of macrophage-like cells and reduction in the number of ste llate reticulum cells. Since reduction or disappearance of cells during emb ryonic development in organs and tissues has been shown to occur by apoptos is, we decided to examine early involuting regions of the stellate reticulu m in the hope of detecting apoptosis. For this purpose, upper first molars of Wistar newborn rats aged 1 and 3 days were fixed in formaldehyde for the TUNEL method and in glutaraldehyde-formaldehyde for light and electron mic roscopy. Paraffin sections revealed TUNEL-positive structures, i.e. brown-y ellow-stained bodies, in the central portion of the stellate reticulum, and next to the outer enamel epithelium and stratum intermedium. Examination o f ultrathin sections confirmed the TUNEL findings: some stellate reticulum cells showed nuclei containing crescent-like electron-opaque condensed mass es of peripheral chromatin, typical of apoptosis. Also, apoptotic bodies of various sizes and appearances were frequently observed within stellate ret iculum cells. We should like to suggest that apoptosis is associated with t he reduction in the number of cells during regression of the reticulum.