Which signaling mode requires direct cell-to-cell contact through membrane-bound signals?

• A. Endocrine
• B. Paracrine
• C. Autocrine
• D. Juxtacrine

Answer: (D)

Direct cell-to-cell contact signaling uses signals that stay attached to one cell’s membrane and bind receptors on a neighboring cell, so the communication happens only when the two cells touch. This is juxtacrine signaling. An example is the Notch-Delta system, where a membrane-bound ligand on one cell engages a membrane receptor on the adjacent cell to trigger a response, with no secreted messenger traveling through the extracellular space.

In contrast, endocrine signaling relies on hormones released into the bloodstream to reach distant targets; paracrine signaling involves secreted factors that affect nearby cells but still diffuse through the local environment; autocrine signaling occurs when a cell’s own secreted signals act back on the same cell or nearby cells of the same type.