Ultrastructure and cytochemical detection of alkaline phosphatase in long-term cultures of osteoblast-like cells from rat calvaria

Two methods of collecting osteoblast-like cells from newborn rat calvaria were tested, either placing individual glass fragments or tipping dense glass beads onto the endocranial surface of periosteum-free bone. Inoculated at high density, cells collected by using these two methods form large mineralized plates after three weeks of culture. The main purpose of our investigation was to analyze the progressive formation of this mineralized structure and to localize alkaline phosphatase activity. At the beginning of the culture, flattened cells gathered into multilayers and synthesized collagen fibers. Cells in the upper layer became rapidly cuboidal in shape and continued to secrete collagen at their basal pole, whereas other cells became progressively embedded in the extracellular matrix. The upper cells featured ultrastructural characters of osteoblasts, whereas the embedded cells resembled osteocytes. After two weeks, the matrix began to mineralize: crystals appeared on collagen fibers, on matrix vesicles, and on cell debris. During the first days of the culture, the alkaline phosphatase activity was localized on the plasma membranes and on the collagen fibers. Thereafter, only the upper cells and collagen fibers that were juxtaposed to these cells showed alkaline phosphatase activity. In addition, the presence of mineralized matrix prevented the reaction product from being visualized on collagen fibers. The ultrastructural analysis reveals large mineralized plates with a structure resembling that of bone in vivo. This culture appears to be an appropriate model to study bone formation and regulation.

Flow-stream waveguide for collection of perpendicular light scatter in flow cytometry

We report a new physical configuration for the detection of perpendicular light scatter or fluorescence in flow cytometry when using a fluid stream in air. This configuration increases the signal-to-noise ratio and narrows the coefficient of variation for uniformly sized latex spheres when compared to using a microscope objective to collect such light. The new technique views the scattered light that is trapped within the optical waveguide that is naturally formed by the flow stream in air. One efficient and simple way to detect the light trapped within this optical waveguide is to place one end of a fiber optic, with a conically polished tip and sufficiently large-core diameter, directly into the flow stream and to place an optical detector at the fiber's other end. For perpendicular light scatter, the flow-stream waveguide achieves high collection efficiency (NA = 0.88) as well as high efficiency of optical through put due to lack of surfaces between the light scatterers and the detector. We obtained 10-fold higher signals with this technique than with a long-working-distance microscope objective. The flow-stream waveguide is also much easier to align than traditional microscope-lens-based systems.

Morphologic characterization of osteoblast-like cell cultures isolated from newborn rat calvaria

Two methods for harvesting osteoblast-like cell populations from newborn (10 days) rat calvaria were compared. The first one consisted in culturing the periosteum-free bones and then trypsinizing the cells on the bone surface. The second one involved the migration of the osteoblasts on glass fragments before trypsinization. Since the plating efficiency, the proportion of alkaline phosphatase-positive cells, the population doubling time, and the calcium deposition were more adequate, the second method was used to further characterize the behavior of the cultures. During the first week of culture, the cells featured shapes similar to those observed in vivo on the surface of periosteum-free calvaria. They formed multilayers and, in the presence of ascorbic acid, synthetized an organic matrix containing exclusively type I collagen. Later, small amounts of type III collagen appeared. The cells were embedded in the matrix and progressively acquired the morphologic phenotype of osteocyte-like cells. The matrix mineralized in the presence of beta-glycerophosphate. The technique of drop-inoculation (high concentration of cells in a small volume of medium) promoted the multilayer formation and the achievement of large mineralized plates (about 1 cm2) in 3 weeks of culture.

Cell interactions during the in vitro neoformation of fetal rat pancreatic islets

As shown by scanning electron microscopy, transmission electron microscopy and membrane labeling analysis, the in vitro neoformation of rat pancreatic islets arose from two main processes: a budding from explants containing duct cells, and a competition between endocrine monolayers and fibroblasts on the culture substratum. The stronger cytoskeleton of fibroblasts and their higher adhesive properties, probably related to their more homogeneous distribution of membrane charges, may explain the spherization of the islets. The pure endocrine cell population of neoformed islets was composed mainly of insulin-secreting cells, and the other types of endocrine cells were distributed in the periphery. Preformed extracellular matrices of osmotically disrupted fibroblasts enhanced the yield of the cultures by increasing the anchorage of endocrine cells and slowing down the fibroblastic growth.