Figure 2. Mechanisms of Nuclear Body Formation.

(A) Biological systems are thought to be governed by the principle of self-organization (Camazine et al., 2001), which is distinct from the concept of self-assembly (Worrall et al., 2007). Self-assembly involves formation of stable complexes that essentially reach thermodynamic equilibrium (left). In contrast, self-organization operates on steady-state systems—those that are far from equilibrium (right). As outlined by Misteli (2001), in cell biological terms, self-organization can be defined as: “the capacity of a macromolecular complex or organelle to determine its own structure, based on the functional interactions of its components.” Through this mechanism, which requires a continuous exchange of materials, the cell is capable of generating a stable (steady-state) structure from a set of dynamic components. In the cartoon, the steady-state approximation is met because a constant flux of components is maintained. Factors enter the body from the newly synthesized pool and can exit the structure, perhaps in a modified form (sunbursts). Note that the modifications do not necessarily preclude a given component from rebinding to the structure.

(B) The assembly of a nuclear body can follow a hierarchically ordered assembly pathway (top), or components can assemble stochastically by a number of individual pathways (bottom). Note that components can enter singly or as large complexes. Although the order of assembly is random in the stochastic model, it is still predicated on molecular interactions. Thus, loss of a given component could lead to failure to incorporate another component or complex.