After the 1940s, a high-voltage electron microscope was used to observe the microbeam system composed of 1-10 angstroms thick fibers supporting various organelles, and various cell membranes were seen. Under the electron microscope, it is concluded that all the films are 75-100 angstroms thick three-layer structure (called unit film). Not only that, the membranes of all parts of a cell are connected, the plasma membrane is connected to the endoplasmic reticulum, and the endoplasmic reticulum is connected to the Golgi apparatus or nuclear membrane. The nuclear membrane is double-layered, composed of inner and outer membranes, and has a certain structure of nuclear membrane pores through which cytoplasmic materials and nuclear materials can communicate. Intercellular connections are also found on the plasma membrane: desmosomes, tight junctions and gap junctions. These structures are related to the combination between cells or the exchange of substances between cells; using freeze etching technology, they can be better observed.
