This description as well as his other work led him to postulate the Neuron Theory and argue against the Reticular Theory. In fact, his description of process outgrowth has been confirmed in many other neural systems and brain regions.

His Golgi preparations revealed details of the growing tips of axons in the spinal cord of chicks, including aspects of the growth cone with its thin appendages (filopodia) and wider ones (lamellipodia). Ramon y Cajal was the first to study process outgrowth from newly generated neurons in the fetal brain and spinal cord ( Ramon y Cajal, 1960). It is now a well-established phenomenon that neurogenesis occurs not only during development of the mammalian central nervous system but also in the adult ( Altman and Bayer, 1965, 1990 Altman and Das, 1965 Bayer et al., 1982 Cameron and McKay, 2001 Eriksson et al., 1998 Kaplan and Hinds, 1977 Kempermann et al., 2003 Kornack and Rakic, 1999 Palmer et al., 2000 Van Praag et al., 2002). Therefore, similarities and differences exist for dendritic outgrowth from newborn neurons in the developing and adult brain. 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, some aspects of dendritic outgrowth of neurons born in the adult dentate gyrus are different as compared to that found in development. However, dendrites in adult brains have to navigate through a denser neuropil and a more complex cell layer. 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. 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. 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. However, technical limitations at the time prevented him from describing such growth cones from newborn neurons in the adult brain. Ramon y Cajal described the fundamental morphology of the dendritic and axonal growth cones of neurons during development.

Neuron dendrite development