Typically, the word "interior" is used to mean "inside." For example, when we are discussing various cell organelle (specialized structures that carry out specific functions), we commonly use the term "cell interior" to refer to anything that is inside the cell. A place that is located away from the coast is commonly referred to as an interior place. Another word commonly used to mean inside is "within." For example, within the cell refers to something that is inside the cell. 

In life sciences, a commonly used term "in vivo" is generally used to describe something within a living organism. An example of its common usage is in vivo experimentation or surgery. This term is used for saying within something living; interior or within can be more generally used.

Hope this helps.