Yes, it’s absolutely a combination of chemical signals and physical pressure. An interesting specific example of these two signals working together during fetal development when the pre-neurons are growing their axons. There is both chemotaxis which steers the ameoba-like tip of the growing axon, and at the same time a substantial stretching force along the length of the axon. The stretching happens because the cells in-between the origin and current location of the axon tip are dividing and expanding. The long distance axons in the brain start their growth relatively early on in fetal development when the brain is quite small, and have gotten stretched quite a lot by the time the brain is near to birth size.
Yes, it’s absolutely a combination of chemical signals and physical pressure. An interesting specific example of these two signals working together during fetal development when the pre-neurons are growing their axons. There is both chemotaxis which steers the ameoba-like tip of the growing axon, and at the same time a substantial stretching force along the length of the axon. The stretching happens because the cells in-between the origin and current location of the axon tip are dividing and expanding. The long distance axons in the brain start their growth relatively early on in fetal development when the brain is quite small, and have gotten stretched quite a lot by the time the brain is near to birth size.