Ribosomes are spheroid or ellipsoid particles about 15 x 25 nm in diameter, consisting of rRNA and protein. They are formed by two subunits. Single ribosomes are inactive. When linked into polyribosomes (polysomes), they read and translate from mRNA a message transcripted from nuclear DNA and synthesize peptides or proteins from amino acids according to this genetic instruction (Fig. 1). In electron micrographs, ribosomes should be strictly differentiated from glycogen beta-particles of nearly the same diameter (15 - 30 nm) (Fig. 2).
|Fig. 1: Ribosomes synthesize proteins (red) according to instruction translated from mRNA (yellow).|
|Fig. 2: Free polyribosomes (red arrow) and glycogen beta-particles (blue arrow) in a process of astrocyte (A). Cisterns of granular endoplasmic reticulum are present in the lower part of figure. (Rat, hippocampus, CA1)|
Polyribosomes either lie free, releasing new peptides into the cytoplasm, or are attached to the membrane of endoplasmic reticulum (rough, granular e.r.), releasing new peptides into its cisternal or tubular lumen (Weiss 1988).
The synapse is constructed from receptors, ion channels, adhesion molecules, second messenger systems (e.g., protein kinase C which triggers a variety of protein phosphorylations, protein kinase II which participates in Ca2+ signaling in tandem with calmodulin and inositol 1,4,5-trisphophate which mobilizes Ca2+ from stores such as smooth endoplasmic reticulum), and cytoskeletal and associated proteins (such as actin, myosin, tubulin, MAP2, etc.). Most of these components are highly specialized proteins. They are not transported into thousands of synaptic sites from the neuronal soma but particular mRNA molecules are differentially distributed via selective intracellular transport from the nucleus to the postsynaptic microdomains. Here they form the machinery which allows a local synthesis of specific synaptic proteins (Steward and Banker 1992). Subsynaptic regions are thus typical representatives of proteosynthesis independent of neuronal soma.