Marilyn A. Ludueña

Cell locomotion, nerve elongation, and microfilaments☆

Abstract A basic difference in locomotion between migratory cells and nerves correlates with a difference in distribution of certain microfilament systems. Lattice filaments are present where extension and movement of cell surface occur in both cell types. Bundles of sheath filaments which bind heavy meromyosin, are present in migratory cells, where displacement of the cell soma over the substratum occurs, but absent from nerves, where the cell body and axon remain fixed upon the substratum and “locomotion” is restricted to the axonal tip. It is proposed that the microfilament lattice is involved in the extension phase of locomotion, and the microfilament sheath in the contractile phase.

Nerve cell differentiation in vitro

The differentiation of cultured neurons from 4- and 8-day chick spinal ganglia has been studied when the nerve cell bodies rest on plastic, gelatin, glial cells, and heart fibroblasts, and when conditioned media are present. The percentage of neurons differentiated, time of axon initiation, total axonal length, rate of axon elongation, and level of 3H-leucine incorporation are employed as parameters of neuronal differentiation. Glia and heart fibroblasts increase the percentage of neuronal differentiation. Gelatin increases the percentage of neurons differentiated, as well as the rate of axon elongation and protein synthesis. Conditioned media also enhance the percentage of neuronal differentiation and the length of axon formed, but their effects are not equivalent to those of gelatin.

The growth of spinal ganglion neurons in serum-free medium

Abstract The presence of serum in the culture medium affects the morphology of spinal ganglion neurons. With serum present, neuronal cell bodies are rounded and axons are predominantly straight. In serum-free medium axons curve all along their lengths, while both cell bodies and growth cones are spread on the substratum. Such “curved” axons straighten if serum is added to the culture dish. Serum-free medium may increase the adhesion of neurons to the substratum. This can account for the curved morphology of axons in serum-free medium, since such axons may represent a “history” of the movement of the growth cone during axon elongation.

Thorotrast uptake and transit in embryonic glia, heart fibroblasts and neurons in vitro.

Abstract Thorotrast (colloidal ThO 2 ) is incorporated into coated vesicles, various agranular vesicles and sacs, and a surface-associated system of membranous channels in times as short as 1 min by single cultured glial and heart cells. Thorotrast appears in ‘C’-shaped bodies and in small, dense bodies of the lysosomal series within ca. 25 min. With longer chase periods, thorotrast ‘clears’ from all cytoplasmic organelles except the lysosomal series. The technique of applying thorotrast and using varying chase periods fails to distinguish a class of membranous organelles, located close to the cell periphery, that might serve as a source of new cell surface during locomotory activity. Similarly, thorotrast (colloidal ThO 2 ) is incorporated into almost all classes of membrane-bounded organelles of growth cones and axons of single nerve cells in vitro in times as short as 1 min. This includes elements of the smooth endoplasmic reticulum. No thorotrast enters the lysosomal granules in this short time. During various chase periods, the tracer disappears from the initial sites of incorporation and accumulates in dense bodies of the lysosome series within growth cones and axons. ‘C’-shaped bodies may be an intermediate in that process. No unique sites of endocytotic activity or of a complete absence of endocytosis were observed that could be correlated with growth cone function and axonal elongation, though the presence of the tracer in agranular sacs of the smooth endoplasmic reticulum in growth cones could reflect hypothesized cycling of cell surface (Bray, 1973).

Membrane fusion in the growth cone-microspike region of embryonic nerve cells undergoing axon elongation in cell culture

Abstract Fine structural analysis of embryonic nerve cells undergoing axon elongation in vitro has revealed structural evidence supportive of the time lapse cinematographic observation that extensive areas of active membrane fusion are present in the distal tip of the axon. Pre-fusion membrane alignment and post-fusion strings of vesicles characterize the putative fusions between microspikes, between microspikes and growth cones, and between growth cones and the distal axon. The restriction and possible significance of these apparent fusions are discussed.