Here's why:

* Eukaryotic cells are characterized by having their genetic material (DNA) enclosed within a membrane-bound nucleus. This nucleus is a distinct compartment within the cell, giving it a well-defined shape and structure.

* Prokaryotic cells, on the other hand, lack a true nucleus. Their DNA is located in a region called the nucleoid, which is not enclosed by a membrane.

Therefore, all organisms that are made up of eukaryotic cells, including animals, plants, fungi, and protists, have cells with well-defined nuclei.