Developmental comparisons of murine secretory amelogenesis in vivo, as xenografts on the chick chorio-allantoic membrane, and in vitro.

Abstract

The temporal, spatial, and cytological characteristics of secretory amelogenesis in developing mouse mandibular first molar tooth organs have been compared during in vivo odontogenesis (from the Cap Stage in Theiler stage 25 C57BL/6 embryos to 10-day-old postnatal mice), as xenografts on the chick chorioallantoic membrane (CAM) for periods up to 7 days, and as explants in chemically defined medium without serum or antibiotics for periods up to 21 days in vitro. Tooth morphogenesis and cytodifferentiation proceeded in each environmental condition in the same sequence albeit at different rates of development. In vivo and CAM xenografts were remarkably comparable in their respective expressions of dentinogenesis and amelogenesis, whereas those explants cultured in a chemically defined medium without serum or antibiotics developed at a much slower rate (e.g., 0.5 days in vivo is equivalent to 1 day in vitro). In each experimental group, secretory amelogenesis was typically first detected along the mesial-buccal cusp of the molar organ independent of which environment was evaluated. Tooth morphogenesis in vitro and as xenografts on the CAM was routinely smaller than in situ odontogenesis. In each environmental condition a “stippled” precursor ultrastructural form of enamel matrix preceded mineralization, except during in vitro cultures of tooth organs. In vitro secretory amelogenesis or dentinogenesis did not indicate morphological characteristics of mineralization; both dentine and enamel matrices did not mineralize under the permissive environmental conditions used in these experiments. Calcium hydroxyapatite crystal formations within dentine and along the dentinoenamel junction during initial enamel matrix formation were not observed during in vitro tooth organogenesis, even for periods up to 21 days in vitro. We conclude that cap stage mandibular first molar tooth organs (enamel organ epithelium and adjacent dental papilla mesenchyme) from Theiler stage 25 embryos contain all of the necessary developmental instructions to express morphogenesis and cytodifferentiation except cues for the serum-containing factors for mineralization.