Ameloblasts responsible for tooth enamel formation are classified into two 
different phases: secretion and maturation. At the transition between these 
secretion and maturation stages, a considerable number of cells die. In this 
study, we examined the morphology of degenerating ameloblasts by conventional 
electron microscopy, and DNA cleavage in degenerating ameloblast nuclei by the 
in situ terminal transferase assay. The results suggest that apoptosis 
(programmed cell death) in ameloblasts, including DNA ligation is induced at the 
transitional stage. The nuclear fragments, chromatin condensation and DNA 
relocation in apoptotic nuclei were examined quantitatively by post-embedding 
anti-DNA immunogold electron microscopy and the in situ terminal transferase 
assay combined with electron microscopy. Numerical analysis revealed that 
immunogold labeling density in the condensed chromatin of apoptotic nuclei was 
comparable on the average to that in the perinuclear heterochromatin of normal 
nuclei, and that individual apoptotic nuclear fragments exhibited highly 
variable to that of normal heterochromatin, to fragments with densities twice as 
high as that of normal heterochromatin. The in situ terminal transferase assay 
combined with electron microscopy detected DNA ends exposed by ultrathin 
sectioning as well as DNA cleavage by a putative endonuclease. In conclusion, 
the state of the DNA, including its ligation and degeneration, changes gradually 
during chromatin condensation and nuclear fragmentation of apoptosis.