Final answer:
Totipotent cells can be kept proliferating indefinitely in culture and yet retain unrestricted developmental potential.
Step-by-step explanation:
The cell type that, under appropriate conditions, can be kept proliferating indefinitely in culture and yet retain unrestricted developmental potential is totipotent cells.
Totipotent cells have the ability to differentiate into any type of cell in the human body and can give rise to all cells needed for the development of an organism.