The internal supporting framework of the cell is formed by:

It is a system of highly branching membranes, which almost fills the space inside the cell membrane. Some of the branches of endoplasmic reticulum are connected to the plasma membrane while some of them are connected to the nuclear membrane. An examination of endoplasmic reticulum under the electron microscope reveals two types of membranes: Smooth endoplasmic reticulum  and  Rough endoplasmic reticulum (RER) .Endoplasmic reticulum is absent in some cells such as embryonic cells and mature red blood corpuscles. It forms an internal supporting framework for the cell, called cytoskeleton.