Figure 6. Intrinsic factors affecting the morphological differentiation of vertebrate neurons.

Figure 6

Intrinsic factors affecting the morphological differentiation of vertebrate neurons. The size of the neuronal soma dictates the total cross-sectional process area allowed to be partitioned into axons or dendrites (curving black arrow). The total cross-sectional area of the axonal domain may increase through increased branching proximally, or progressive proximo-distal enlargement of the axon (area symbol). In contrast, the cross-sectional area of dendrites may be conserved or instead decrease successively through partitioning into segments of decreasing diameter following the 3/2 power scaling law (area symbol). Neurite branching is constrained by the total cross-sectional process area, which may be partitioned into successive branch segments until a limiting size is met, representing the minimum number of microtubules required to maintain structural stability (blue circles). Developing neurites and their associated branches exhibit a characteristic projection orientation, termed the vector or branching angle, that results in the increased spread of the arbor through successive bifurcations along the neurite (angle symbol). Although the distance between branch points, or segment lengths, may vary for a given neuron, each neuronal cell-type exhibits a characteristic range of segment lengths for dendrites and the axon (segment length symbol). When the cross-sectional area decreases between branching segments, negative taper is produced, with positive taper instead resulting from increasing cross-sectional area between neurite branch points (gradient). The characteristic taper is established by the proportion of microtubules compartmentalized into successive branch segments, achieved in part through microtubule-associated protein binding and differential microtubule spacing or orientation (blue tubules). An additional form of intrinsic control over neuronal morphology during the development of polarity is produced through the background complement of genes expressed, independent of extracellular cues. Inset schematic represents an enlarged view of the typical cytoskeletal organization within a granule neuron dendrite. Figure reprinted with permission from K.M. Kollins, constructed from data presented in references 3, 8, 9, 26-29, 36, 49-54, 134, 164, 174-176, 189-191, 195, 196 and 343.

From: Branching Morphogenesis in Vertebrate Neurons

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