Final answer:
Protrusion is limited to the region of the cell closest to the stimulus because cells prefer rigid substrates and tend to move towards stiffer regions. This behavior is known as durotaxis. Cells actively sense the physical properties of their environment by generating and transmitting force.
Step-by-step explanation:
The region of a cell closest to the stimulus is where protrusion is limited because cells prefer rigid substrates and tend to move towards stiffer regions. When a cell is close to a boundary between soft and rigid regions of a substrate, it orients parallel to the boundary line to maximize substrate deformation. This behavior has been observed experimentally and is known as durotaxis.
Cells actively sense the physical properties of their environment by generating and transmitting force. They strain their surroundings to probe the elastic properties of the substrate. The size of the contacts between cells and their environment grows with physical force. Overall, cells prefer stiff substrates because they minimize the elastic energy invested in deforming the matrix.