ABSTRACT
The process of cellular differentiation is guided by molecular controls that regulate the expression of cell-specific genes. Clearly, these controls must be precise in both their specificity and temporal appearance. The signals that direct the molecular regulation of differentiation are intrinsic and extrinsic (extracellular) factors. Intrinsic mechanisms require a change in activity (e.g., by molecular modification) or concentration (by synthesis regulation or dilution due to cell division) of the effector molecules. Extracellular influences, whether circulating or insoluble elements, induce differentiation by interacting with receptive molecules on the target cell surface or, as with steroid-mediated induction, with intracellular receptors. Although little is known about the nature of regulatory factors or the mechanism of signal transduction involved in differentiation, research has demonstrated the importance of the extracellular environment in development. 1–4
