Which of the following statements about cells is true?

CONCEPT: A tissue is a group of cells that share a common origin, structure, and function. Because the cells within one tissue are specialized for the same job and descend from the same lineage, they are structurally similar to one another.

APPLICATION: Among the statements offered, "Cells of a tissue have similar structure" matches this definition directly — tissue cells are grouped precisely because they look alike and act alike.

CROSS-CHECK — why the other statements fail:

• Cell shape is not fixed to round — it varies with function; nerve cells are long and thread-like, red blood cells are biconcave discs, and epithelial cells can be flat or column-shaped.

• Cell size is not uniform within an organism — a human ovum (about 100–120 micrometres) is vastly larger than a red blood cell (about 7–8 micrometres), and a neuron's axon can extend over a metre.

• A nucleus is not present in every cell — mature mammalian red blood cells expel their nucleus during development, and prokaryotic cells such as bacteria have no membrane-bound nucleus at all.

So the only statement that holds true, among those given, is that cells of a tissue share a similar structure.