Which characteristic indicates that enamel is nonliving?

Enamel is classified as nonliving primarily because it does not have a blood supply. This is a fundamental characteristic that distinguishes living tissues from nonliving ones. Living tissues require a blood supply for nutrient delivery, waste removal, and oxygen transport, all of which are essential for the health and maintenance of living cells. Since enamel lacks these features, it cannot sustain life or regeneration in the same way that living tissues can.

In contrast, the ability to heal itself, the presence of living cells, and porosity are characteristics that may suggest a tissue is capable of some degree of metabolic function or regeneration, which enamel does not possess. Enamel cannot repair itself once damaged, does not contain living cells, and while it can exhibit some degree of porosity, this does not negate its nonliving status. Overall, the absence of a blood supply is the most definitive indicator of enamel being nonliving.