Dendritic development of newly generated neurons in the adult brain

Abstract

Ramon y Cajal described the fundamental morphology of the dendritic and axonal growth cones of neurons during development. However, technical limitations at the time prevented him from describing such growth cones from newborn neurons in the adult brain. The phenomenon of adult neurogenesis is briefly reviewed, and the structural description of dendritic and axonal outgrowth for these newly generated neurons in the adult brain is discussed. Axonal outgrowth into the hilus and CA3 region of the hippocampus occurs later than the outgrowth of dendrites into the molecular layer, and the ultrastructural analysis of axonal outgrowth has yet to be completed. In contrast, growth cones on dendrites from newborn neurons in the adult dentate gyrus have been described and this observation suggests that dendrites in adult brains grow in a similar way to those found in immature brains. However, dendrites in adult brains have to navigate through a denser neuropil and a more complex cell layer. Therefore, some aspects of dendritic outgrowth of neurons born in the adult dentate gyrus are different as compared to that found in development. These differences include the radial process of radial glial cells acting as a lattice to guide apical dendritic growth through the granule cell layer and a much thinner dendrite to grow through the neuropil of the molecular layer. Therefore, similarities and differences exist for dendritic outgrowth from newborn neurons in the developing and adult brain.

Fig. 1

Light micrograph of DCX-immunolabeled cells in the granule cell layer (GL) at the border with the hilus (H). Two labeled cells are shown with their apical dendrites (arrowheads). They display both lamellipodia (asterisks) and filipodia (arrows). Note that the cell on the right has a branching apical dendrite, and one of its branches has trifurcating filipodia (top three arrows). Scale bar = 8 μm. (Reprinted with permission from Ribak et al., 2004)

Fig. 2

Confocal micrograph of BrdU-immunolabeled and DCX/BrdU double-labeled cells at 72 h after a single BrdU injection from the adult dentate gyrus. Three BrdU-labeled cells (asterisks) are shown and two of them are double-labeled for DCX (arrows) while the third cell only has a nearby DCX-labeled process. Note that the two double-labeled cells have apical dendrites (arrowheads) that extend through the granule cell layer and enter the molecular layer (ML). Scale bar = 15 μm. (Reprinted with permission from Shapiro et al., 2006)

Fig. 3

Electron micrograph of a DCX-labeled newborn neuron (NN) and an astrocyte cell body (GC) at the border (dashed line) between the granule cell layer (GL) and subgranular zone (SGZ). The DCX-labeled NN has immunoreaction product within its thin shell of perikaryal cytoplasm (arrowheads) and its rudimentary apical dendrite (arrows). Note that the dendrite is directed into the GL and arises from the pole of the DCX-labeled cell that is opposite to the pole apposed by the astrocyte cell body. Mature granule cells are found on each side of the DCX-labeled cell. Scale bar = 2 μm. (Reprinted with permission from Shapiro et al., 2005)