Uhm, it looks like it's more than just the teeth that is missing? Did you check for flipped polygons? I believe Maya shows everything with 2 sided shading so it'd be hard to spot. you can turn off 2 sided lighting by going to the viewport menus Lighting > Two Sided Lighting. To actually spot if they are flipped you can go…
Well.. I cant really help but if you want people to help you should probably post images and describe things a little better.. like materials and things like that. Also what version of Maya are you using? EDIT: uuh.. only thing I can come to think of with disappearing geometry is flipped polygon normals.
Oh right. Display > Polygons > Backface Culling God.. my Maya kept rendering out the backface anyway and I couldn't find where to turn it off. Apparently it isn't under the materials but on the surface node instead. I.e. if you create a cube you can go to it's shape attributes and change it under the Render Stats tab…