Ectoplasm, also called exoplasm, is the clear, gel-like, and smooth outer part of the cytoplasm in protists. It is found just below the cell membrane. In other eukaryotes, this region is called the cell cortex. Ectoplasm contains actin filaments, which help the cell move and change shape. In contrast, endoplasm is the inner part of the cytoplasm, located between the ectoplasm and the nuclear envelope. It holds most of the cell's organelles and is active in chemical processes. The ectoplasm's ability to change shape, made possible by interactions between actin and myosin, helps with processes like forming the spindle during cell division, amoeboid movement, and slime mold flow networks. The separation of cytoplasm into ectoplasm and endoplasm is considered an important step in the evolution of cells.

The word "ectoplasm" comes from the Ancient Greek words ektos, meaning "outside," and plasma, meaning "anything formed." In most eukaryotes, the term "ectoplasm" has been replaced by "cell cortex."

History and terminology

In the 1800s, German biologist Ernst Haeckel first used the term "ectoplasm" to describe the clear outer part of cytoplasm. He studied cytoplasm in single-celled organisms, such as amoebas, during their movement. Haeckel separated ectoplasm from the inner part of the cell, called endoplasm. He also observed that ectoplasm helps amoebas form pseudopodia, which are temporary projections used for movement.

In the early 1900s, spiritualists used the term "ectoplasm" to describe a gel-like substance they believed was connected to spiritual energy. They claimed this substance supposedly came out of people during spiritual activities. However, there is no scientific proof to support this idea, and it is not related to the ectoplasm found in cells.

Structure and function

Ectoplasm is a clear, gel-like fluid found just below the cell membrane. It contains many actin and microfilament structures, and the way actin and myosin interact creates its gel-like texture. Ectoplasm surrounds the endoplasm, forming a protective layer that shields the endoplasm and its organelles. It is a stiff structure with actin spread throughout, helping maintain the cell's shape and flexibility.

Actin and myosin work together in the ectoplasm to provide structure and allow the cell to move. Straight actin filaments create force that causes contraction, while branching actin spreads the force evenly. These filaments work together to enable efficient movement and support. Actin and heavy meromyosin combine to form actomyosin complexes, which create tubes in the ectoplasm. These tubes help move the cell's contents through a process called cytoplasmic streaming. Actomyosin complexes often attach to the cell membrane, allowing movement on the membrane side of the ectoplasm and providing structural support. This interaction between actin and myosin is similar to how actin and myosin slide past each other during muscle contraction, but it occurs on a much smaller scale in the ectoplasm. Spindle fibers used during cell division (mitosis and meiosis) are also made of actin filaments from the ectoplasm.

Ectoplasm changes shape by switching between two forms of actin: globular (G-actin) and filamentous (F-actin). Low levels of ions help form G-actin, while metal ions and ATP help form F-actin. The change from G-actin to F-actin causes the ectoplasm to shift from a liquid (sol) state to a gel-like (gel) state. G-actin represents the sol state, and F-actin represents the gel state.

Examples of ectoplasm in cellular processes

Amoeboid movement, which involves the extension of temporary structures called pseudopodia, depends on two parts of the cell: ectoplasm and endoplasm. A change in the acidity or alkalinity of the environment causes the ectoplasm to alter the direction of the pseudopodia. Changes in ion levels and osmotic pressure lead the ectoplasm to shift between a gel-like state and a liquid state. When actin fibers break apart, the ectoplasm becomes liquid, and this liquid flows forward, extending the pseudopodium. Osmotic pressure then pushes the liquid endoplasm into the extended pseudopodium. As actin and myosin proteins reassemble, the endoplasm changes back to a gel-like state, solidifying the pseudopodium. At the opposite end of the cell, the ectoplasm changes from a gel to a liquid, moving cellular parts toward the extension. Throughout this process, ectoplasm and endoplasm continuously change forms. Movement repeats as endoplasm becomes ectoplasm at the tip of the pseudopodium, while ectoplasm becomes endoplasm at the back of the cell. When food is present, ectoplasm forms a tube that allows the amoeba to take in the food and transform the tube into a food vacuole. In humans, immune cells called macrophages also use amoeboid movement to travel through the body.

Slime molds use a network of tubes made of ectoplasm (a gel-like substance) to move and transport endoplasm (a liquid substance). These tubes are formed by interactions between actin and myosin proteins. The fibers in the tubes contract in waves, pushing the endoplasm through the network. Changes in the size of the tubes influence the slime mold’s movement, allowing it to change direction. This network also helps move nutrients and signals throughout the slime mold, making it able to adapt to its environment. This adaptability helps slime molds navigate mazes and create efficient pathways in their surroundings.

Sertoli cells, found in the testes of mammals, form specialized connections called ectoplasmic specializations with sperm cells to support sperm development. These specializations are tight junctions made of actin located between the cell’s outer membrane and the endoplasmic reticulum. The correct formation of the blood-testis barrier depends on these ectoplasmic specializations, which also help move sperm cells through the seminiferous epithelium during sperm production. Toxins like cadmium can damage these specializations, preventing proper sperm development and weakening the blood-testis barrier.

Evolutionary significance

In the 1930s, Dr. Ernest E. Just suggested that the separation of cytoplasm into ectoplasm and endoplasm helped cells change and develop over time. The ectoplasm is the outer part of the cell, not including the cell membrane. He believed that the development of ectoplasm changed how cells interact with their surroundings. Specifically, ectoplasm allowed organisms to contract and conduct. The contraction of cytoplasm led to movements like cytoplasmic streaming in amoebas and the formation of cilia in protozoans. Conduction helped fertilization happen and allowed egg cells to protect themselves from being fertilized twice. Dr. Just concluded that organisms evolved more quickly when their cytoplasm was more divided into ectoplasm and endoplasm because ectoplasm enables important cellular processes. Modern science agrees that ectoplasm plays a key role in how cells move and maintain their structure.