Patricia L. Hinds

Early ganglion cell differentiation in the mouse retina: An electron microscopic analysis utilizing serial sections☆

Abstract The retina of a mouse embryo on day 13 of gestation, the first day when ganglion cells with axons are detectable, has been studied both qualitatively and quantitatively by reconstructing a large number of cells (more than 100) from an electron microscopic serial section series. Direct evidence has been obtained for migration of prophase nuclei of ventricular cells to the ventricle within an intact process which spans the thickness of the retinal wall. At metaphase most of the vitreal process appears to be pinched off, and the cell completely rounds up. After cytokinesis, cells take one two courses: (1) regrowth of their vitreal process to the vitreal surface while keeping their ventricular process attached at the ventricular surface by a junctional complex; these cells will undergo another round of DNA synthesis and division; (2) regrowth of their vitreal process only so far as the marginal layer with detachment of their ventricular process from the junctional complex and beginning migration of their centrioles and cilium away from the ventricle. These changes represent the earliest detectable quantitative or qualitative changes undergone by cells that will subsequently differentiate into ganglion cells. The sequence of events for the formation of unipolar ganglion cells from these early bipolar cells involves transformation of the simple vitreal process ending in the marginal layer into an axonal growth cone insinuating itself between the tangential axons of the marginal layer and growing toward the optic stalk; at the same time the Golgi complex and centrioles migrate to the perikaryon, and the ventricular process completely withdraws. Usually, but not always, both daughter cells of a mitotic division appear to have the same fate, either both remain ventricular cells or both become ganglion cells. This result is used to construct a simple hypothesis explaining some of the apparently contradictory results of neuronal development, both in the retina and in the rest of the central nervous system.

Reconstruction of dendritic growth cones in neonatal mouse olfactory bulb

SummaryAn improved and simplified method of serial sectioning for electron microscopy was used to prepare material from which were reconstructed dendritic growth cones of mitral cells in neonatal mouse olfactory glomerulus. Growth cones are characterized by: (1) one or more filopodia, each approximately 0.2 μm in diameter, projecting from an expanded dendritic terminal or pre-terminal region; (2) a polygonal array of microfilaments approximately 50 Å in diameter, filling the enlarged terminal and filopodia; (3) an absence of microtubules and a paucity of mitochondria; (4) a few profiles of smooth endoplasmic reticulum or vesicles; (5) occasional axodendritic synapses. These characteristics are compared with those of other growing processes in the nervous system and a consistent picture of the appearance of growing neuronal processes obtained, suitable to use as a guide in a search for additional growing processes in developing and mature brains.

Extent of stain penetration in sections prepared for electron microscopy.

Tissue embedded in Araldite 502 was sectioned for electron microscopy and stained in alcoholic uranyl acetate and lead citrate. Sections were then reembedded and resectioned perpendicularly. Such resulting preparations showed that these heavy metal salts completely penetrated through these thin sections for structures were stained throughout the section depth. Resectioning also showed the two surfaces of a thin section to have different textures. With sections cut by glass knives, the surface that touched the water of the knife trough was smooth, and the upper one was rougher. The upper surface was less rough when a diamond knife was used. The consequences of difference in texture of section surfaces and the extent of heavy metal staining are discussed in respect to the images produced by biological structures embedded in Araldite.