On the real structure of the cytoplasmic matrix: learning from the embedment-free electron microscopy.

The recent debate on the nature of the cytoplasmic matrix is reviewed on the basis of results obtained by electron microscopy of embedment-free materials, i.e., the critical point-dried whole mount-cell method, the polyethylene glycol (PEG)-embedding and subsequent de-embedding section method, and the freeze-etching method. Fine structural images obtained by these methods are carefully evaluated and close correspondence with electron microscopy regardless of these methods is demonstrated. Especially, 'novel' filamentous structures--the microtrabecular strands and the cross-linkers--correspond well to each other; they are structures which have been included in epoxy sections by conventional methods, but have been obscured simply because of a similar property of electron scattering between the filamentous structures and epoxy resin. Although this correspondence seems to support the existence of the microtrabeculae, the electron microscopy of serum albumin, when processed by the PEG-method, also exhibits filamentous networks resembling the microtrabecular lattice. This, together with the finding on the centrifugation of in situ cells, strongly suggests a possibility that most, if not all, microtrabecular strands and cross-linkers in cells without pretreatment by detergents do not represent actual in situ structures.