Animal cells contain a single centrosome that nucleates and organizes a polarized array of microtubules which functions in many cellular processes. In most cells the centrosome is composed of two centrioles surrounded by an ill-defined ''cloud'' of pericentriolar material. Recently, ␥-tubulin-containing 25-nm diameter ring structures have been identified as likely microtubule nucleation sites within the pericentriolar material of isolated centrosomes. Here we demonstrate that when Spisula centrosomes are extracted with 1.0 M KI they lose their microtubule nucleation potential and appear by three-dimensional electron microscopy as a complex lattice, built from 12-to 15-nm thick elementary fiber(s), that lack centrioles and 25-nm rings. Importantly, when these remnants are incubated in extracts prepared from Spisula oocytes they recover their 25-nm rings, ␥-tubulin, and microtubule nucleation potential. This recovery process occurs in the absence of microtubules, divalent cations, and nucleotides. Thus, in animals the centrosome is structurally organized around a KI-insoluble filament-based ''centromatrix'' that serves as a scaffold to which those proteins required for microtubule nucleation bind, either directly or indirectly, in a divalent cation and nucleotide independent manner.

In the past, centrosome maturation has been described as the change in microtubule nucleation potential that occurs as cells pass through specific phases of the cell cycle. It is suggested that the idea of centrosome maturation be expanded to include gain of functions that are not necessarily related to microtubule nucleation. Some of these functions could be transient and dependent on the temporary association of molecules with the centrosome as cells progress through the cell cycle. Thus, the centrosome may best be viewed as a site for mediating macromolecular interactions, perhaps as a central processing station within the cell. The centromatrix, a relatively stable lattice of polymers within the centrosome's PCM, could serve as a scaffold for the transient binding of mediator molecules, as well as allow the dynamic exchange of centrosome constituents with a soluble cytoplasmic pool. New evidence adds support to the idea that centrioles are crucial for the maintenance of PCM structure. However, significant evidence indicates that aspects of centrosome structure and function can be maintained in the absence of centrioles. In the case of paternal centrosome maturation, sperm centrioles may not contain an associated centromatrix. It is proposed that regulation of paternal centrioles or centriole associated proteins could mediate centriole-dependent centromatrix assembly following fertilization. Thus, regulation of centromatrix-centriole interactions could be involved in maintaining the integrity of the centrosome's PCM and play an important role in centrosome disassembly during cell differentiation and morphogenesis.