The teeth are living tissue
and are covered with enamel, the hardest substance in the dog's body. Dentine is the next layer, and the core is called the pulp. The pulp contains blood vessels and nerves which nourish the teeth.
Defective enamel of the permanent teeth generally is the result of a disruption in the tooth developmental process. Any disease to which the fetus or newborn puppy is exposed could cause a defective development in the tooth enamel. Canine distemper is a severe viral disease, which can cause defects in the enamel of the recovered puppy. This is often called 'enamel hypoplasia.'
Some drugs, most notably tetracycline derivatives including oxytetracycline, tetracycline, doxycycline, and minocycline, if administered to puppies during tooth development, can cause a discoloration of the permanent enamel. This discoloration is usually a yellow to brown and is the result of the tetracycline bonding with the calcium of the tooth. The staining is permanent. It is best if these drugs are not administered to pregnant mothers or to puppies less than six months of age.