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Module 1: Figure formation and action of cell stimuli

Formation and mode of action of cell stimuli.

Cell stimuli are released from cells through different mechanisms: membrane-anchored stimuli are released by ectoderm shedding; stimuli formed in the cytoplasm pass out across the plasma membrane; stimuli packaged into vesicles are released by exocytosis. Such stimuli have four main modes of action. Membrane-anchored stimuli on the surface can activate receptors on neighbouring cells directly (juxtacrine). Stimuli that are released from the cell can feed back to activate receptors on the same cell (autocrine); they can diffuse to neighbouring cells (paracrine); or they enter the blood stream to act on cells further a field (endocrine).