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Let me try to give a picture of my understanding of neuritic and synaptic growth and their interplay.

Pick two neurons in corresponding areas of opposite hemispheres which are connected by a synapse (and a rather long axon).

Both neurons were originally born at approximately the same time t during brain development in a given distance d, without already having neurites.

They necessarily have a latest common ancestor (stem cell), which doesn't have to be the same parent cell, but maybe the same grandmother or great-grandmother cell.

Now several processes start with different delays and at different speeds:

  • the two neurons wander away from each other

  • they specialize thus expressing specific "markers" on their membranes

  • their axons grow

  • their dendritic trees grow

Case 1: If neuritic growth is fast enough, the two neurons can meet and build a synapse early (recognizing each other by markers they have in common due to their common ancestor). While wandering away from each other, the neurites have to grow accordingly.

Case 2: If neuritic growth is too slow, and the neurons wander away from each other too fast, the growth of axons and dendrites at least has to be guided somehow (by glia cells or by molecule gradients?) such that later they have a chance to find each other. Finally they recognize each other by markers expressed eventually during specialization.

I could imagine that both cases do occur purely, but even that they can be somehow intermingled.

Question: Is this picture essentially correct or flawed? (A yes/no answer would suffice!)

Furthermore, I am looking for a concise presentation of (resp. gentle introduction to) these processes (and maybe others?) and their interplay that lead to specific synaptogenesis.

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  • $\begingroup$ Not an expert on this but Case 1 is mostly wrong. Have you read this: en.wikipedia.org/wiki/Axon_guidance. Also, interhemispheric connections are rather rare, at least in the cortex. $\endgroup$ – vkehayas Oct 13 '17 at 9:09
  • $\begingroup$ What do the commissural fibers connect? (See biology.stackexchange.com/questions/65351/…) $\endgroup$ – Hans-Peter Stricker Oct 13 '17 at 9:20
  • $\begingroup$ The Wikipedia article describes Case 2. But does it explicitly rule out Case 1? $\endgroup$ – Hans-Peter Stricker Oct 13 '17 at 9:21
  • $\begingroup$ I cannot believe that all neurons have to be at their final positions before synaptogenesis starts. It must be (in my humble opinion), that at least some neurons already connect during their travel to their final positions, so there must also be axon growth just by lengthening, not only guided. But of course I may be wrong. $\endgroup$ – Hans-Peter Stricker Oct 13 '17 at 9:50
  • $\begingroup$ For example, it might be the case that some "pioneer" axons connect rather early to their target neuron, then follow it (during its travel) by lengthening, and thus help somehow to guide the following axons (somehow like the emigrant trails led by pioneers in North America in the 19th century), which connect to their targets only later. $\endgroup$ – Hans-Peter Stricker Oct 13 '17 at 10:12
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Short answer
Both short and long-distance synapse formation can occur.

Background
Short-distance connections can be made between adjacent cells (Van den Pol et al, 1988). Your 'common ancestor part' is a bit obscure to me. In the end, every cell is born out of a single one :)

Long-distance connections can also be made. For example after neuroneogenesis either during development or even in the adult brain, whole neurons can migrate from their origin to a new part (Kaneko et al., 2017), or axons can grow to find a new target distally, for example after neural injury (Purves et al., 2001).

References
- Kaneko et al., J Neurochem (2017); 141(6): 835-47
- Purves et al. (eds), Neuroscience (2001), 2nd ed. Sunderland (MA): Sinauer Associates
- Van den Pol et al., J Comp Neurol (1998); 399(4): 541-60

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  • $\begingroup$ But the OP is asking about connections that are formed when cells are very close to one another with axons that later stretch as the neurons migrate to their target area. Can that happen in vivo in the case of neurons so far as to be in different hemispheres? $\endgroup$ – vkehayas Oct 13 '17 at 15:03
  • $\begingroup$ @vkehayas - I don't see that particular question framed as such in the question $\endgroup$ – AliceD Oct 13 '17 at 15:14

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