Pongo's a 7-year old Newfoundland Retriever. His owner is concerned because one day when he was playing with him he noticed a "lump" on Pongo's belly. He brings the dog to you for examination.

The "lump" is soft and moveable, about 3 cm across, painless, and located under the skin. You take an aspirate and find the smear to contain what appear to be mature white adipose cells. Your diagnosis is that Pongo has a cutaneous lipoma. You tell the owner not to be too concerned, and that unless the lesion gets large enough to bother the dog in some way, to leave it alone.

Points to ponder:

1. What cell type(s) is (are) involved in the development of this growth?

2. Why is this abnormal behavior for these cells?

3. In general terms, what seems to have happened to precipitate the behavior?