This biofilm may calcify and harden, termed calculus (tartar). They can cavitate, calcify, or develop air-fluid levels. Their intervertebral disc material can calcify and become more brittle. With age, these lesions may calcify and appear as diffusely minute spots or stippled. A healthy, normal female guinea pig's pubic symphysis does not calcify. These nodules have a tendency to calcify as the patient ages. As such, they also have a tendency to calcify and are highly vascularized. Lesions have a tendency to calcify as they heal. They tend to calcify after menopause. In the skull, individual bones may calcify only at their centers. Plaque that remains in the oral cavity long enough will eventually calcify and become calculus. Maxillary canines begin to calcify by 4 months of age. A few species (of "Padina") calcify with aragonite needles. During the development of most tetrapods, teeth begin to calcify at their tips. It takes about 3 years for the adult premolar and its root to fully calcify.