High-resolution scanning electron microscopy of formative osteoblasts, with special emphasis on intracellular...

Formative osteoblasts in immature bone obtained from newborn rats were examined with a high-resolution scanning electron microscope. The three dimensional ultrastructures of intracellular membranous organelles in formative osteoblasts were revealed by the maceration and extraction of cytoplasmic soluble elements with dilute osmium tetroxide after DMSO (dimethyl sulfoxide) freeze-cracking in liquid nitrogen. Typical formative osteocytes covering self-produced unmineralized matrix were plumply ovoid or ellipsoid in shape. Their cytoplasms were filled with intracellular organelles related to matrix-formation such as Golgi apparatus, mitochondria, and well-developed rough endoplasmic reticulum (RER). The Golgi apparatus of formative osteoblast was composed of the three kinds of prominent elements, which were lamellar portions stacked several cisterns with some fenestrations, abundant vesicles, and vacuoles. The Golgi cisterns frequently extended continuing in a circle. A large number of spherical protrusions indicative of arising the vesicles and elongated tubular structures were usually observed in a part of the Golgi cisterns. The tubular structures which mediated between adjoining Golgi cisterns, or between the Golgi cisterns and RER in the Golgi area, presumably play an intercommunication role in secretion products. The RER was highly oriented in almost parallel rows from both the Golgi area and nucleus to the cell margins, while small in the Golgi area. The RER had enlarged fenestrations than that of the Golgi cisterns. Minute ribosomes of about 28 nm were associated closely with the membrane surface of the RER facing the cytoplasmic matrix.