Textbook cell biology is derived largely from cultured cells — cells grown in dishes under controlled laboratory conditions. The cytoplasm of cultured cells is relatively dilute, with ribosomes and other macromolecules distributed at concentrations that allow straightforward biochemical analysis. This is convenient. It is also 50 times less crowded than reality.
A study comparing the cytoplasm of C. elegans (roundworms) with cultured cells found that the worm cytoplasm contains roughly 50 times more ribosomes per unit volume. The cytoplasm is not a dilute solution of macromolecules in water. It is a dense, crowded gel where molecular motion is constrained by collisions with neighboring molecules at every step. The diffusion constants, reaction rates, and binding kinetics measured in cultured cells are not wrong — they are measurements of a different physical regime than the one inside a living organism.
The regulatory implications are direct. mTORC1 — a central pathway that controls cell growth, protein synthesis, and metabolism — functions as a crowding sensor. When the cytoplasm is appropriately crowded, mTORC1 signals for growth and protein production. When it is too dilute, the pathway quiets. The pathway is not just responding to nutrient availability or growth factors. It is reading the physical density of its environment. Cultured cells, at 50 times lower density, may be operating mTORC1 in a regime that doesn't reflect its in vivo behavior.
This means that decades of cell biology research have been conducted in cells that are physically different from the cells they are supposed to represent. Not genetically different — the DNA is the same. Not nutritionally different — the growth media is carefully formulated. Physically different. The interior of the cell is the wrong density. The macromolecules are too spread out. The collision rates are too low. The reaction kinetics are measured in conditions that the cell never experiences in the body.
How much of cell biology needs to be repeated at the correct density? The honest answer is: unknown. Some findings will be robust — if a protein binds another protein in dilute conditions, it will bind in crowded conditions. But the rates, the competition between binding partners, the spatial organization, the phase behavior — all of these are density-dependent. Liquid-liquid phase separation, a phenomenon that has generated intense recent interest, is particularly sensitive to crowding: condensates that form or dissolve in cultured cells may behave entirely differently at physiological density.
The connection to expansion microscopy is structural: both are cases where a technical limitation — here, the difficulty of doing biochemistry in dense cytoplasm; there, the resolution limit of light microscopy — created an invisible barrier that was mistaken for a knowledge gap. The field didn't know it was studying a dilute artifact because the artifact was the only version of the cell it had access to. The worm data reveals the artifact by providing a comparison. The question the comparison raises — what do our results look like at the correct density? — does not yet have a systematic answer.