Practice question

Permissive induction occurs when responding tissue is already specified and merely requires a permissive microenvironment to execute its intrinsic program, rather than instructional patterning information. Epithelial cells pre-determined to differentiate need extracellular matrix components like laminin, collagen IV and entactin to support survival, polarity and transcription. Laminin binding to integrins activates FAK and ILK pathways enabling expression of milk proteins in mammary culture. In contrast, optic vesicle inducing lens or mesenchyme regionally instructing epithelium to form distinct structures exemplifies instructive interactions where new positional information is imposed upon competent responder.