Ribosomes are very small, spherical, membrane-free cell organelles made of ribosomal RNA and globular proteins with a diameter of 10 - 25 nm. They are found in all living cells. Under the light microscope, ribosomes are not visible. In the electron microscope they appear as small dots (see figure). Several million ribosomes exist in metabolically active cells. Since ribonucleic acids bind basic dyes, the basophilia of a cell of organelles that are not visible under the light microscope indicates a high ribosome content. Ribosomes occur singly or are located one behind the other on messenger ribonucleic acids (messenger RNAs) as polyribosomes (= polysomes = ergosomes). They are also found embedded in the outer surface of the membranes of the rough endoplasmic reticulum and the outer nuclear membrane associated with it. Mitochondria also contain ribosomes.