Fig. 2.

Fig. 2.

Morphogen gradient formation mechanisms. (A) Morphogen gradient concept. (B) Gradient formation via active transport mechanisms: cytonemes (top) and migrasome/transcytosis (bottom). Cytonemes are cellular projections which can emanate from cells towards the morphogen source. Cytonemes carry Type II Bmp receptors which can take ligand back to the cell where they can signal in a receptor complex. Migrasome/transcytosis shows vesicle-based transport of ligand away from the source. (C) Gradient formation by free (passive) diffusion. Ligand diffuses from areas of high concentration near the source to areas of lower concentration. Pre-steady state ligand concentration is depicted. (D) Gradient formation via regulated diffusion. Extracellular matrix, immobile regulators and diffusible extracellular regulators all act to regulate diffusion. Note: receptors and ligands not to scale.