Homing of hemopoietic progenitor cells to the marrow

The recognition of hemopoietic stem cell after intravenous transplantation of marrow cells occurs initially by a lectin moiety on the surface of marrow sinus endothelium. The cell is then transported across the endothelial cytoplasm much in the way that a soluble ligand, such as transferrin, is transported. In the extravascular compartment, the cell binds to lineage-specific stromal cells. This mechanism, known as homing, is mediated by a lectin-glycoconjugate interaction, the lectin being on the surface of progenitor cell with specificity for galactosyl and mannosyl residues. The binding is subsequently stabilized by membrane-bound proteoglycans, integrin-like receptors, and fibronectin.     

Polarization of membrane glycoproteins during monocyte chemotaxis

Distribution of membrane glycoproteins was studied in chemotactic monocytes using ferritin-conjugated lectins. The cells became polarized forming a pseudopodia at a leading head. Membrane glycoproteins were redistributed at the head. This phenomenon was not observed in chemokinetic or non-chemotactic cells suggesting that membrane glycoproteins may have a role in recognition of the chemoattractant.     

In vivo development of adipose tissue following implantation of lipid-depleted cultured adipocyte

The monolayer culture of isolated and disaggregated adipocytes from rat omental and perirenal sites, gave rise to a population of fibroblast-like cells, usually devoid of lipid inclusion. Similar fibroblast-like cells have been obtained in cultures of adipose tissue stromal cells and are thought to be undifferentiated adipocyte stem cells. Although the adipocyte-derived fibroblasts were morphologically indistinguishable from culture-derived fibroblasts of other origins, upon autoimplantation into the splenic bed they regained the lipid inclusion and developed again into adipose tissue. The findings suggest that the transformation of adipose cells into fibroblast-like cells is reversible modulation and not a dedifferentiation into the adipose tissue stem cell. This work also substantiates the increasingly recognized heterogeneity of fibroblasts.     

A modified technique to obtain uniform precipitation of lanthanum tracer in the extracellular space.

A method for obtaining a uniform, dense precipitate of lanthanum nitrate to delineate extracellular space is described. Improvement of the previous technique is achieved by phosphate precipitation of lanthanum in the tissue carried out at low temperature. This method has been successfully applied to bone marrow.     

Plasma membrane—endoplasmic reticulum contact sites regulate phosphatidylcholine synthesis

Synthesis of phospholipids, sterols and sphingolipids is thought to occur at contact sites between the endoplasmic reticulum (ER) and other organelles because many lipid‐synthesizing enzymes are enriched in these contacts. In only a few cases have the enzymes been localized to contacts in vivo and in no instances have the contacts been demonstrated to be required for enzyme function. Here, we show that plasma membrane (PM)—ER contact sites in yeast are required for phosphatidylcholine synthesis and regulate the activity of the phosphatidylethanolamine N‐methyltransferase enzyme, Opi3. Opi3 activity requires Osh3, which localizes to PM–ER contacts where it might facilitate in trans catalysis by Opi3. Thus, membrane contact sites provide a structural mechanism to regulate lipid synthesis.     

Receptors for transferrin and transcobalamin II display segregated distribution on microvilli of leukemia L1210 cells

Simultaneous addition of uniform latex particles derivatized with transferrin (0.532 micron) and transcobalamin II (0.345 micron) to leukemia L1210 cells resulted in segregated binding to individual microvilli as demonstrated by scanning electron microscopy. This segregated distribution suggests that individual microvilli are endowed either transferrin or transcobalamin II receptors but not both. Intracellular sorting and segregation of newly synthesized or recycling receptors probably occur prior to expression on the plasmalemma microvilli.