Complex sentence: Ultrastructure (or ultra-structure) is the detailed structure of a biological specimen, such as a cell, tissue, or organ, that can be observed by electron microscopy; the cellular structures that are too small to be seen with an optical microscope.
Split into simpler sentences: Ultrastructure (or ultra-structure) is the detailed structure of a biological specimen, such as a cell, tissue, or organ, that can be observed by electron microscopy. It refers in general to the study of cellular structures that are too small to be seen with an optical microscope.