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The monovalent ionophore monensin inhibits the secretion of both procollagen and fibronectin from human fibroblasts in culture . The distribution of these proteins in control and inhibited (5 x 10 -7 M monensin) cells has been studied by immunofluorescence microscopy . In control cells, both antigens are present throughout the cytoplasm and in specific deposits in a region adjacent to the nucleus, which we identify as a Golgi zone by electron microscopy . Treatment of cells with monensin causes intracellular accumulation of procollagen and fibronectin, initially in the juxta-nuclear region and also subsequently in peripheral regions. Electron microscope studies reveal that in such cells the juxta-nuclear Golgi zone becomes filled with a new population of smooth-membraned vacuoles and that normal Golgi complexes are not found. Immunocytochemically detected procollagen and fibronectin are localized in the region of these vacuoles, whereas more peripheral deposits correspond to the dilated cisternae of rough endoplasmic reticulum, which are also caused by monensin . Procollagen and fibronectin are often codistributed in these peripheral deposits . Accumulation of exportable proteins in Golgi-related vacuoles is consistent with previous analyses of the monensin effect . The subsequent development of dilated rough endoplasmic reticulum also containing accumulated proteins may indicate that there is an additional blockade at the exit from the endoplasmic reticulum, or that the synthesized proteins exceed the capacity of the Golgi compartment and that their accumulation extends into the endoplasmic reticulum.Experimental manipulation of the synthesis and secretion of procollagen has been achieved by the use of agents, such as inhibitors of proline hydroxylation (12), local anesthetics (9) and, for fibronectin as well, an inhibitor of protein glycosylation (18). Microtubule-disrupting drugs, such as colchicine, also inhibit secretion, apparently by impairing the transport of secretory vesicles to the cell surface (8). The use ofsuch agents is accompanied by dramatic changes in cell morphology (7) and, in some systems, the inhibition of procollagen synthesis (8). We have demonstrated (39) that the monovalent ionophore monensin reduces the rate of release ofprocollagen and fibronectin from human fibroblasts without seriously disturbing the synthesis of these macromolecules . Moreover, secretion returns to normal upon removal of monensin. Subsequent studies with pulse labeling in conjunction with subcellular fractionations have shown that the passage oflabeled procollagen and fibronectin through the cell is hindered at specific locations (38). Also, our studies of the effect of monensin on chondrocytes

The spicule primordium is formed in an intercellular cavity within a group of sclerocytes. This cavity contains organic material which ensheaths the growing spicule but does not appear to determine the nature of the mineral morph (magnesian calcite) or the crystallographic orientation of the spicule. The tip of each growing spicule ray is seated in a 'dense cup' in the cytoplasm of the sclerocyte concerned. Both ends of monaxons are initially inserted each into a dense cup. As rays elongate the sclerocyte membrane around the tip becomes invaginated and forms a system of 'converging spaces' that possibly indicate high secretory activity in that region. Spicule growth involves the displacement and expansion of the organic sheath by the enlarging spicule. Fully formed spicules which are exposed to the mesohyl become surrounded by collagen fibrils. However, these fibrils are in no way concerned with the process of mineral deposition and are never found within the spicule calcite.

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