Questions about tooth formation

We were discussing dentistry today. None of us could answer these questions:

  1. Are we born with both sets of teeth? How do they form in the gums?
  2. If permanent teeth push the baby teeth out, what pushes the baby teeth through the gums?

Question:

When we are born, our primary teeth are still developing. Except for the first permanent molar, no permanent teeth have begun the calcification process. Just like certain cells in your body are programmed to develop certain tissues, the teeth also have cells which are programmed to develop each layer of tooth structure. This development takes place within the jaw bones.

The primary teeth all begin formation in utero, usually between 14-19 weeks. Crown formation of the primary teeth is not complete until several weeks to several months after birth. Complete root formation takes even longer.

Permanent tooth development can begin shortly after birth with the incisors (front teeth) and 6 year molars. Several years after birth, especially for the wisdom teeth or 3rd molars, the canines and posterior teeth begin their calcification process. Again, root formation will be completed after the tooth has erupted in the oral cavity.

The forces of eruption are still being studied; however, it appears that the cells of the periodontal ligament are responsible for eruption of both the primary and permanent teeth. The periodontal ligament surrounds the root of the tooth and is actually the structure which attaches the tooth to the bone surrounding the tooth. As the root of the tooth develops, the periodontal ligament is also forming, and the mobility of these cells appears to cause the tooth to erupt. A tooth normally begins to erupt when 1/2 to 2/3 of the root is developed. Eruption of the tooth is generally complete when about 3/4 of the root is developed. Then, it can take about 3 years to complete root formation. Eruption of permanent teeth causes resorption of the roots of the primary dentition. This root resorption causes the primary tooth to loosen and eventually exfoliate, thus allowing for complete eruption of the permanent tooth.

Answer:
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